Blue Flower

LIPC certifications are reviewed by a group of Linux experts whom are not bound to any particular distro or company, meaning that if you certified as an LPIC, your knowledge (the whole point of getting certified, to prove your knowledge) can be in theory exported to any other distro running a Linux kernel... that includes Red Hat. So, start small but solid, and continue relentlessly: let's get LPIC-1 certified! :) Modularity and Flexibility are features of the command line, where one tool does one task and does it well.

For this article I have at the end followed the structure of the exams objectives for each of the LPIC-1 exams. Here we go for exam 101: 

  • 101.1 Determine and configure hardware settings (weight index number)
  • 101.2 Boot the System
  • 101.3 Change runlevels / boot targets and shutdown or reboot system
  • 102.1 Design hard disk layout
  • 102.2 Install a boot manager
  • 102.3 Manage shared libraries
  • 102.4 Use Debian package management
  • 102.5 Use RPM and YUM package management
  • 103.1 Work on the command line
  • 103.2 Process text streams using filters
  • 103.3 Perform basic file management
  • 103.4 Use streams, pipes and redirects
  • 103.5 Create, monitor and kill processes
  • 103.6 Modify process execution priorities
  • 103.7 Search text files using regular expressions
  • 103.8 Perform basic file editing operations using vi
  • 104.1 Create partitions and filesystems
  • 104.2 Maintain the integrity of filesystems
  • 104.3 Control mounting and unmounting of filesystems
  • 104.4 Manage disk quotas
  • 104.5 Manage file permissions and ownership
  • 104.6 Create and change hard and symbolic links
  • 104.7 Find system files and place files in the correct location 

Those are the objective for exam 2, you can find a deep description of them here:

  • 105.1 Customize and use the shell environment
  • 105.2 customize or write simple scripts
  • 105.3 SQL data management
  • 106.1 Install and configure X11
  • 106.2 Setup a display manager
  • 106.3 Accessibility
  • 107.1 Manage user and groups accounts and related system files
  • 107.2 Automate system administration tasks by scheduling jobs
  • 107.3 Localisation and internationalisation
  • 108.1 Maintain system time
  • 108.2 System logging
  • 108.3 Mail Transfer Agent (MTA) basics
  • 108.4 Manage printers and printing
  • 109.1 Fundamentals of internet protocols
  • 109.2 Basic network configuration
  • 109.3 Basic network troubleshooting
  • 109.4 Configure client side DNS
  • 110.1 Perform security administration tasks
  • 110.2 Setup host security
  • 110.3 Securing data with encryption 

101.1 Determine and configure hardware settings Determine and configure hardware settings (2)

The kernel is the mediator between the applications and hardware. To find out which kernel you're running on your system, executes uname -r  (or uname -a), you can also search /etc/ for *.release files. Visit /proc/cpuinfo for the processor info. Execute cat /etc/os-release on any distro to see where it comes from



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The procfs (/proc folder) contains processes information (like ACPI power, CPU and devices information) so that all other programs can refer to, etc; with time the /proc folder started accumulating system information which is not process information, therefore the /proc folder became messy and confusing

  • From kernel 2.5; the folder sysfs (/sys folder) was introduced to contain information specifically about the system (devices, cpu, etc), which before was stored in /proc
  • From kernel 2.6; the udev system was introduced, which populates the /dev folder.; udev provides persistent device names for all devices in the Linux system, it supports hotplugging; the dbus was also introduced which send signals to desktop application from the system /dev folder, so that devices that are plugged-in are shown and interact with the desktop. The correct order that udev will use to read rules of the system (rules that you may want to modify yourself) is:
    1. /etc/udev/rules.d
    2. /run/udev/rules.d
    3. usr/lib/udev/rules.d

In Linux, just like in Unix, everything is treated as a file 

dmesg   #list the event that occurs when the system loads or something is plugged in
lsmod   #list the installed modules (drives) running in the system
lsusb   #shows all the USB devices currently connected on the system
usb-devices  #gives you a lot more information than lsusb
lspci        #shows that pci controllers that exist on the system, the mobo primarely
modprobe modname #modprobe add/remove modules on the kernel (loads a driver)
modprobe lp      #loads a printer driver
modprobe -r lp   #will remove the printer module driver

sudo lshw -C network  ;will list your network adapter, to see if they have been loaded; lshw list hardware on the Debian (Ubuntu) systems

sudo rmmod uvcvideo ;remove the video module of the webcam, sudo modprobe uvcvideo will add it again, hopefully the webcam will work this time!  

101.2 Boot the System (3)

After the POST (Power On Self Test) the system runs GRUB (Grand Unified Bootloader) from the MBR (Master Boot Record, the first 512KB of the drive). Back on the day, I remember very well, it used to be LiLo (Linux Loader) the default loader, which was really clunky (it uses the /etc/lilo.conf file, remember that if you make changes to this file, you need to run the lilo utility, else changes won't be effective). Whichever bootloader you use, it always launches the initrd (initial ram disk) that loads a little kernel into ram to detect all hardware and load only the modules needed for the system to run efficiently; it is the initrd tiny kernel the one that mounts the hard drive. Once initrd loads, it then load the beginning of the Linux kernel called "init" (which has a process number of 1, being the first process that runs), it is then init the one who initializes the Linux kernel. The file /proc/cmdline passes the parameters from the bootloader (grub) to the kernel.

Modern Linux distributions have these 3 different types of init programs (there is a fourth one called "BSD init" but not many distro use this one, because is not very flexible)

  • SysVinit; around for a long time; it uses numerical order to start the systems one at a time; it can run at level 1 (single user mode), run level 3 (console mode) or run level 5 (GUI mode) etc; the first program SysVinit usually starts is /sbin/init. To disable the crtl-alt-delete configuration edit the file /etc/inittab. On systsemd systems, this settings is configured by usr/lib/systemd/system/
  • Upstart; developed by Ubuntu, start the system by working out dependencies, therefore it loads faster with this mode because different programs can start at the same time
  • Systemd; the new guy, use by Ubuntu and RedHat, compatible with SysVinit but not compatible with other systems, and it may need "systemctl enable____" or "systemctl start _____" to load the systems; Systemd uses binary code, and not just scripts like "SysVinit" and "Upstart"

To disable a service, depending on the system you're using, you can use the following:

For SysV:

chkconfig ServiceName off     #chkconfig is ususally found in Red Hat
sysv-rc-conf Servicename off  #ususally found in Ubuntu

For Upstart:

update-rc.d ServiceName remove #this command does remove the system from the startup procedure

For Systemd:

systemctl disable ServiceName

"SuperServers" used to be files on the Linux distributions that used to control everything (it was in the /etc/inetd.d or the /etc/inetd.conf file). What we still have is the /etc/xinetd.d/ where they are lots of files (daemons) all run by the /etc/xinetd.conf which basically says to run all files inside the xinetd.d directory

sudo service xinetd restart  ;to restart the parent proccess once you enable any of its services, then run netstat -tuna to verity the services are listening like FTP servers, etc. Check the /etc/hosts.allow or /etc/hosts.deny to add or remove IP addresses (allow takes precedence) and then restart xinetd to prevent certain computers from accessing the relevant services; this method of allowing/denying IP address is called TCP Wrappers, so it is not in use anymore

initramfs ; At boot time, the boot loader loads the kernel and the initramfs image into memory and starts the kernel. The kernel checks for the presence of the initramfs and, if found, mounts it as / and runs /init. The init program is typically a shell script. Note that the boot process takes longer, possibly significantly longer, if an initramfs is used.  

101.3 Change runlevels / boot targets and shutdown or reboot the system (3)

Here is a table comparison of the run levels usage between different systems; just to note that run level 4 doesn't exist, and that Debian/Ubuntu have put all-the-run-levels on level 2

Run Level (Boot target) CentOS / Suse Debian/ Ubuntu Description  
0 (power off) Halt Halt The system is completed stop systemclt isolate
1 (rescue) Single User Mode Single User Mode Only one user can logon, to change the root passwd for example systemctl isolate
2 Mutli User, no network Full, multiuser, GUI is available if installed To troubleshoot the system ; for Debian and Ubuntu everything is at run level 2  
3 (multi-user non-gui) Multi User, with network  n/a On CentOS, level 3 is console with net access systemctl isolate
4 n/a n/a Not use at all  
5 (GUI) Multi user with GUI n/a With the fully GUI loaded systemctl isolate
6 (reboot) Reboot Reboot

When the system is rebooting

systemctl isolate

To change run level use "telinit" or "init" following by the level you need, though note that "init" is not the right command to use, stick to "telinit" instead

su runlevel    #shows at what run level you're running
3 5            #this means that before you were on runlevel 3 but now you are on 5
su telinit 3   #switches to run level 3

The default running mode can be determine by examining the file /etc/inittab; (on a distro running systemd, that file probably doesn't exist, and you need to examine the folder /etc/systemd/system instead). The init command read the /etc/inittab file to logon on to single-user mode; use the command cat to examine it or the command vi to edit it (on the screenshot below notice that we are using an operating system with systemd, therefore the command "systemctl" must be use to change the boot target):


To explore the different target that the system has, explore these locations:

cat etc/systemd/system/*.target        #shows available target levels
cat usr/lib/systemd/system/*.target    #shows available target levels
systemctl isolate    #the "isolate" string is used to switch target
systemctl get-default                  #shows the current running level
systemctl set-default #set running level 5 as default
telinit 3                              #exactly the same as the 'systemctl isolate' command

The telinit q instruct the system to read the /etc/inittab file again. To mess-up your colleague, and enhance his/her troubleshooting techniques, you can set a system to "reboot" as the default running level. This table maps the different runlevels with their corresponding target strings:

Run level Boot target Command
0 poweroff telinit 0
1 rescue, no network telinit 1 OR telinit s
3 multi-user, with network  
5 graphical  
6 reboot telinit 6

How to shutdown your Linux box. Oh well, just as a start, please don't pull the plug! To shutdown your system, switch to one of these run levels:

telinit 0    #the same as:   #systemctl isolate
telinit 6    #the same as:   #systemctl isolate

If your system is frozen, you may try "reboot -f now" (not recommended, unless, of course, system is frozen); by using the -f flag you force the reboot, obviously all data and integrity is lost and compromise, and there is not warranty at all that the system may boot up normally again. You can also use these different variations to shutdown your system:

shutdown -r now     #inmediately reboot now
shutdown -h +3      #halt the system (h) in 2 minutes time  
shutdown -P 17:30   #it powers off (P) the system at the specified time
shutdown -P +3 "System is shutting down"  #displays a message in console

To display a message to all your buddies who are connected to the console, use the "wall" command with the pipe (|) operator

#echo "The system is shutting down now, please save all your work!" | wall

102.1 Design hard disk layout (2)

/var ;keep it if possible on a separate hard drive or partition, with fast access, as this is folder is in constant use and it tends to be filled quickly with logs, etc

/swap ;this is a partition that acts as the page file in windows

LVM (Logical Volume Manager) adds all drives together on a pool, where you can expand it by adding drives as you go along, giving you in that way lots of flexibility. LVM provides no redundancy at all, therefore underneath the LVM some kind of RAID1 (mirror) or RAID5 (parity) must be established if you want to provide resilience to your system. LVM uses the following acronyms:

  • PV ;represent Physical Volumes
  • VG ;represent Volume Groups, collection of 1 or more PV
  • LV ;represent Logical Volumes
  • For example: sudo vgcreate VolumeG1 /dev/sda3 /dev/sda4 ;then create a new LV from the VG by doing sudo lvcreate -n Volume1 VolumeG1 ;and finally scan for changes by doing lvscan

Remember that on a MBR drive only 4 partitions are possible, if you need more than 4 partitions you need to start using extended partitions

 Directories that may need their own partition during installation are:

  • /usr ;because it contain all the commands, libraries, man pages, documentation and static files
  • /tmp ;because every user can write to this directory

The FHS (File Hierarchy Standard) is organized as follows:

/     #base level                                /proc  #virtual filesystem
/bin  #system binaries                           /root  #the root  home
/boot #boot loader                               /sbin  #essential system binaries  
/dev  #devices                                   /tmp   #temporary storage location
/etc  #text base config files                    /var   #temp, log files, mail
/home #user files
/dev  #contains a list of files that are pointing to actual hardware devices of the system
/media #removable storage, usb
/mnt   #mount temporarily filesystem

/lib  #system code libraries (NOT libray for apps)required at system startup are here
/usr            #application files, non-essential programs of the system, firefox, etc
/usr/code       #contains the source code for the Linux kernel
/usr/bin        #user binary fiels,firefox, etc, normally mounted read-only
/usr/sbin       #contains the system binary files of the Linux system
/usr/lib        #libraries use by the system internally
/usr/local      #store the scripts created by the users
/usr/local/bin  #same as above, not a part of the distribution of Linux
/usr/local/lib  #libraries for downloaded program that are not part of Linux distribution

102.2 Install a boot manager (2)

Grub vs Grub2: They both stand for "GRand Unified Boot system", and both application work by inserting themselves on the MBR of the drive, then loading from there the boot sector of the Linux OS. The /etc/grub.conf gives the location of GRUB stage 2 to GRUB stage 1

Grub Grub2

To install it: grub-install

Located at: /boot/grub/menu.lst

To modify just edit the *.lst file

To install it: grub2-install

Located at: /etc/grub2/grub.cfg

To modify you need to use "grub2-mkconfig -o" (for output) to edit the *.cfg file; the grub.cfg file pics the configuration from the folders "etc/default/grub" and from "/etc/grub.d/"

 Enterprise Linux 6 and before use only grub

Debian/Ubuntu has been running grub2 since 2009, and furthermore they are backward compatible

Enterprise 7+ and beyond uses grub2, and on these distros, notice that the "grub.cfg" file is located at /boot/grub2 (instead of the /etc/grub2 folder), making it no script backward compatible

In Debian/Ubuntu you need to type "grub-mkconfig -o" , while on Enterprise Linux you need to type "grub2-mkconfig -o". It is very rare to find a Debian/Ubuntu OS running legacy grub, as since 2009 where grub2 was adopted by these distros it's a long time, but potentially you can find some Enterprise Linux 6 in production, and those guys will run legacy grub

To make changes to the grub2 configuration of a system, do as follows:

  1. Edit the file /etc/default/grub, and make the adjustment that you need (for example, extend the default time out)
  2. To activate the modified configuration, you need to re-generate the .cfg file by running: "grub-mkconfig -o /boot/grub/grub.cfg"

Adding rw init=/bin/bash to the boot parameters in GRUB will allow root access to the system upon booting, scary ah?

102.3 Manage shared libraries (1)

The command "ldconfig" read the file /etc/ and created the, looks into the folder "" and populate the cache with all the configuration files in that folder and the location of the share libraries. The "LD_LIBRARY_PATH" environment variable ensures that a certain set of directories are always searched when trying to use shared libraries.

See this diagram of the file system, of the Linux share libraries:

  • etc (folder)
    •  #this file tells the system where to look for library files. This file normally points to the folder underneath ( where different program can use the share files
    •  #this is a binary file (not editable at all) that is created by the ldconfig command, this file is used for programs to load the share files quickly
    • (folder)
      • libc.conf
      • linux.conf
      • vmware.conf

The ldd application is used to determine which share libraries ("so" for Share Object, meaning it is a dynamic, or "a" for static) a determine program needs in order to function, for example, typing "ldd /bin/ls" outputs how many share libraries the 'ls' program is using; ldd is mostly use when you're troubleshooting a program, etc

Share Libraries work the same across different distros

ls | grep  #displays all files that contain the string

If you add a modification to any of the conf files (or add a new conf file) under the "" folder, you need to recreate the file by running: sudo ldconfig, this is so that the different programs know and are aware of the share libraries available in your system

You can use the "export" command to add an specific share library to your local path

102.4 Use Debian package management (3)

Debian package build is determine by a "control" file, which makes the creation process relatiely easy. In the other hand, the creation of a debian (APT) repository is more difficult, it needs a web/ftp server and the use of certain tools such as Debian Archive Kit. There are different Linux Philosophy between Debian and Red Hat packages:

RPM Debian
Doesn't autoconfigure Autoconfigures
Doesn't autostart services Autostarts services
Doesn't prompt the user Prompt the users
Used on the majority of commercial Linux server installations Used on the majority of Linux desktop installations
Packages complex to create Packages easy to create
Repository easy to create and mirror Repository complex to create and mirror

dpkg/rpm ;installs and removes software

APT/YUM ;searches, downloads softare from repository and manage dependencies


To find out what online repositories your system is using, visit the file /etc/apt/sources.list (some Ubuntu distros also includes the folder etc/apt/sources.list.d to hold information about the repos; aptitude is the package manager that provides a front-end for Advance Package Tool (apt) infrastructure in Debian/GNU Linux systems. Use the following commands to interact with apt:

#To interact with the repos, you'd only use these 3 commands:
apt-get    #this command contains lots of switches for customisation, like -update, -upgrade, -dist-upgrade, -install, etc;
#The -remove switch will remove a package while the -purge switch not just remove the package too but also all its configuration files
apt-cache   #used mainly in Ubuntu, is the same thing as the aptitude program
aptitude    #used mainly in Debian systems

The first thing to do is to run "sudo apt-get update", which will sync your repo with the online repos to ensure you have the latest packages entries in your cache, basically update your system. The dpkg configuration file is kept in /etc/dpkg/dpkg.cfg

  • apt-cache search apache2  ;searches the cache for applications that contains the word apache2
  • apt-cache show apache2  ;shows information about the apache2 program
  • sudo apt-get install apache2  ;will install apache2 program and any dependencies that it may have, as well as suggested packages
  • sudo aptitude remove apache2 ,it will remove the app, we could have used the apt-get tool if we wanted to
  • sudo dpkg-reconfigure tzdata  ;reconfigure existing installation of packages by running the start wizard if applicable
  • sudo dpkg -s vim ;shows you details of the vim package

A nice utility to install is tree, issue sudo apt-get install tree ; if you don't know the name of the package you'd like to install, use the GUI utility called Synaptic

dkpg ;package utility for Debian systems, -P to purge the application and delete configuration files, -p display package details including version number (OMG, -P is purge and -p is info, so close!), -R to install a package recursively, -r remove the package but not its configuration files,

102.5 Use RPM and YUM package management (3)

The distros that use RPM are Red Hat, Fedora, CentOS and openSUSE; Zipper is used by SuSe Linux and Mandriva has Urpmi. A "spec file" determines how to complie and build an RPM package, the process is complex but could be make easier by macros. In the order hand, creating a YUM repository package is very easy, all you need is a Web/FTP server and the use of the createrepo command. YUM is a smart application that check for dependencies whenever you install a RMP package, checking the Internet and your local cache before actually installing anything, always keeping it updated. RPM mostly manages and can query installed packages or files thanks to its database rpm -qa | sort will -query -all packages in the whole system.

yum search firefox #it searches for the firefox package on your system
yum info firefox
yum install firefox
yum remove firefox
yum provides /etc/any-given-file.conf   #if your spedify a file, the 'provides' switch of yum will tell you which rpm install it
yum update *    #with the * it means it will update all packages in your system
yum -y update  #before installing any packages, update your system with this command
yum install --downloadonly --downloaddir=/tmp/packages httpd #try this out, it's nice!

 To configure YUM edit these two files:

  • /etc/yum.conf (set keepcache=1 if you want to keep the packages locally, not needed though), ensure gpgcheck=1 to ensure packages are md5 verified;
  • /etc/yum.repos.d/

yum update --security ;only updates the security packages

yum update-minimal --security ;these commands in red hat works, but on CentOS it will download the buggy ones too

yum changelog updates ;shows all changes logs or yum changelog firefox shows only for firefox

yum install -y epel-release ;add the epl repository, once added install yum install -y yumex which gives you a

rpm ;also install packages, -ql query and list the all the files of a package, qi query and display name, version and description of a package

  • rpm -I --nodeps package1.0.3.rpm  ;notice the double dash (--) for the no dependencies option
  • rpm -qc -dump firefox ;display information regarding the configuration files of firefox; -c display configuration files; -dump parameter must be use with either -l -c or -d (the -q must support the -dump too)
  • rpm -qd yum ;query yum for the location of documentation files
  • rpm -qa Group="System Environment/Shells" ;query for packages that fall in this category
  • rpm -qa --last ;query for packages that were installed recently
  • rpm -q --changelog firefox ;will show you all the changes that have happened to the package
  • rpm -qpi httpd-2.4.centos.x64.rpm ;to query a packages for info you must use the -p switch, otherwise it will query the rpm database instead
  • To upgrade an installation use the -F string (freshen)
  • To verify an installation use the -V specifying only the name of the package
  • To perform an MD5 checksum use the -K switch
  • Capital -F (Freshen) will only updates the packages that are already installed
  • To find information about a package use rpm -qi bash
  • rpm -qi centos-release ;shows you the exact version you're running

rpm tags ;you can issue this command to query tags rpm --querytags and you'll see all the tags that you can use on your commands, then you can be creative and constructs nice queries like, need to surrounder in curly braces and percents:

  • rpm -qa --queryformat "%{NAME} %{VERSION}\n" | sort ;notice that double dash for --queryformat and the curly braces
  • rpm -q --queryformat "[%{FILENAMES} %{FILESIZE}\n"] firefox ;put the expression between square brackets and will list you the files
  • rpm -q --queryformat "%{NAME} %{INSTALLTIME:date}\n" ;tells you the installation date of firefox
  • rpm -q --queryformat "[%{FILEMODES:perms} %{FILENAMES}\n]" firefox ;show you the permission of each of the files

rpm2cpio ;extracts cpio archives from rpm packages

yumdownloader ;download rpm packages from yum repositories, use rpm -ivh httpd2.4xxxx.rpm to install a package

If a file get damage and you need to extract the config file again, do as follows:

  • rpm -qcp httpd xxx.rpm ;the -q for query, the -c for contents and the -p for package, this will list the .conf files of the package
  • rpm2cpio httpdxxx.rmpm | cpio -id ;this will extract the contents of the package, run ls to verify

The package database contain the attributes of the package, which are SM5DLUGTC ;to verify a package use rpm -V firefox if nothing is shown it means the package has not been modified, use rpm -Vv firefox with the lower -v to verbose mode so it can verify all files of the package; the rpm -Va will verify your whole OS

  • S ;file size
  • M ;file permission mode
  • 5 ;file digest or checksum
  • D ;device major or minor numbers
  • L ;link path
  • U ;user ownership
  • G ;group ownership
  • T ;last modified time
  • C ;and capabilities

rmp -K --nosignature httpd-2.4xxxx.rpm ;you should get an ok saying signature is okay

Sometimes the RPM database may become corrupt, to recover it first do an archive of it by visiting /var/lib and typing tar -czvpf rpmdb.tar rpm ;once the backup is created delete the current database using rm /var/lib/rpm/__db* and do after a rebuild by rpm --rebuilddb ;you can also use rpm to restore permissions to files by rpm --ssetperms firefox and will put the permissions as default

yum --showdubplicates list samba-test-libs ;it will show you duplicates samba packages that you have

yum -showduplicates list available ;it will show you on colour code

yum list installed ;shows all the installed packages, showing you in the 3rd column the repository (anaconda is for the CD) the color code of the packages listed is as follows: bold (update in repository), bold and underlined (current kernel) red (package doesn't exist in repository) and yellow (newer than package in repository)

  • yum group install system-admin-tools --setopt=group_package_types=mandatory,default,optional

yum groups hidden ;shows you the hidden installed packages; yum groups list ids ;will show all groups

yum groups info "Development Tools" ;shows you info of the packages group; to properly remove a group (all of its packages) use yum autoremove development-tools

yum localinstall whoishxxx.rmp ;this is how to use yum to install already downloaded rpm packages, the yum reinstall whatever.rpm will reinstall the package

yum reinstall --skip-broken whateverxxx.rpm ;will skip the dependencies packages that are faulty or can't be downloaded, use yum remove package.rmp to re-install a package

yum install -y yum-utils ;install utility for yum, from where you can issue package-cleanup --leaves ;so this will tell you which packages are no longer needed

Repositories ;all repos have to end in .repo otherwise yum will ignore them. To disable a repository add "enabled=0" to the repo file; yum repolist shows you what you have yum repolist disable ; yum repoinfo ;shows you the size of the repos; visit here to add more repositories if needed: The command yum --disablerepos"*" --enablerepos="centosplus" list available disables all repos but the centosplus

103.1 Work on the command line (4)

Here there are some tricks that you can use while operating a Linux machine, note that you can invoke the | by pressing Shift + backslash 

apropos "string" ;use the apropos command to find out what other commands can do stuff in Linux

stat ;find out the statistics of your file, eg stat file.txt

Login Shell ;after you logon the system reads the following configuration files and path variables

The /etc/bashrc loads the /etc/profile (this is where the #path is located), where one of the lines will load any scripts files found in /etc/profile.d/* ; then it will go the user home folder and executes the relevant files on this 'specific' order:

  • /home/user/.bash_profile
  • /home/user/.bash_login
  • /home/user/.profile
  • /home/user/.bashrc

Interactive Shell ;in that shell you're not prompted for username/password, and the only loaded on this mode are:

  • /etc/bash.bash.rc
  • /home/user/.bashrc

You can create your own scripts under /etc/profile.d so they don't get overwritten when there is a system update. The two main files system-wide locations to look for are "/etc/profile" and "/etc/bash.bashrc"

If you want to do customization (aliases, etc) you will do it in the /home/user/.bashrc

export PATH=/home/manuel/sbin:$PATH  #append that folder to the PATH variables,
#but only for the session that you are on, the setting is delete it when you exit the terminal
unset PATH  #remove all the path!!! to restore the paths just re-open the terminal

export ;the export command makes the value of a varialbe available across varios subshells

unset ;destroy a previously exported variable

env ;see a list of current variables (bash shell only), NOT local variables values or functions

set ;display the values of shell variables, INCLUDING local variables and functions

  • set +o history ;turn off history
  • set -o history ;turn on history

setenv ;similar to env but for tsch sell 

.bash_history ;this file keep your history, check it out by cat ~/.bash_history The command !! recalls the entire last line which is quite useful if you have entered a command and then realised that needed to put "sudo" at the front, you can just do sudo !!, the commnad !-4 will execute the command numbered 4 beginning from the bottom

reverse-i-search ;you can enter this mode by typing crtl-r , and it will call from the history a matching of the commands you enter; if you keep pressing crtl-r the reverse-i-search will jump to the next line in the history file

HISTCONTROL ;you can use this variable to hide some commands from the history, first of all type export HISTCONTROL=ignorespace and then all the commands that you enter with a space at the front won't be recorded, cool ah? If you type HISTCONTROL=ignoredups the history won't record duplicated entries, and if you type HISTCONTROL=erasedups it will delete from the history the duplications, to do both use HISTCONTROL=erasedups:ignorespace . To keep certain lines from being recorded you can keep it in the history ignore variable, e.g. the following will not record the history or ls commands itself export HISTIGNORE="history*:ls" .If you would like to add date and time to your history command type export HISTTIMEFORMAT="%h %d %H: %M: %S:> " Type man stftime to see other posibilities for customisation. Type export HISTSIZE=10000 to increase the number of lines in the history entries, if you set the HISTSIZE to zero it will not record anything and if you set it to a negative value it will be infinite

pwd  #displays your current directory, present working directory
cd - #does the same thing as above, displays your current directory, and if you type it again it takes you to the previous directory you were working on, cool ah?
history     #have a look at your history
history -c  #clears the history
cat .bash_history  #shows history of the bash log, which is deleted at log off

103.2 Process text streams using filters (3)

man ;the famous manual pages, use grep to find a particular item, use its switches (man 5 passwd) to find a particular category of the enquired command, as follows: 1 (user commands), 2 (system calls), 3 (subroutines), 4 (services), 5 (file formats), 6 (games), 7 (miscellaneous), 8 (system administration), l (local) and n (new). The -k switch is a similar to apropos, so by using it the man will display all manual related to the entered string man -k passwd

cd ;takes you to the folder you wish. if you type only "cd" it takes you to your home folder (cd ~ also takes you back home), once at home do less .bash_history

  • cd - ;takes you to the previous folder you were working on
  • cd ../../folder1 ;takes you to that folder1

cat ; short for concatenate, use to display the file on the screen and to merge more than 2 files

cut ;on this example use the delimiter as: to chop the file, thereafter only print the first field of every line cut -d: -f1 /etc/passwd (-d is use to change the delimiter from TAB designation to the DELIM designation, while -f specify the column, the -c specify the characters to be displayed, while -b specify the bytes)

less ;similar to cat, use the "f" and "b" keys to travel around, press "h" to see the command available

more ;display file page by page, but you can only move forward

fmt ;format files in different way, this will start a new line every 60 character fmt -w 60 file1

pr ;modifies the file before display, the -d adds a double space while -l x add maximum number of lines pr -d -l 10 /etc/passwd

sed ;allows you to edit a file on the fly, without actually interacting with it; sed most use string is "s" for substitute where you can replace a given string with a different one, remember to use the "g" for global string at the end, so that the replacement occurs recursively in the whole file

  • The way it works is like this: sed 's/string1/string2/g' file1.txt ;this will replace string1 with string2 inside file1.txt, don't forget the 'g' for global
  • You can also replace more than 1 string at the same time: sed 's/string1/string2/g' file1.txt | sed 's/string3/string4/g'
  • You can also use regular expressions, on this example it will look for a capital/lower case S on string1: sed 's/[Ss]tring1/string2/g' file1.txt
  • In this example ^ symbol means that the substitution will occur only if string1 is at the beginning of the line: sed 's/^String1/String2/g' file1.txt

tr ;translate command, use to delete or replace characters in a file, eg echo 'Hello World' | tr -d aeiou will actually display Hll Wrld

head ;give you the first 10 lines only by default of a file, handy to see the first entries on a log file for example. To modify the number of lines that you want to see use the -n switch head -n 15 file1.txt

tail ;same as "head", but only gives you the last 10 lines tail file1.txt

split ;create several files from a single one, depending on how many lines (-) you want on each file split -l 5 file1.txt file_ ;this will create file_aa, file_ab, file_ac, etc, where each file contains only 5 lines in sequential order from the parent file

nl ;number of lines, very useful command that helps you create a file of a given file listing the number of lines this file had, very handy when a script it has an error in line number 273 and you had no idea what line is that

  • nl -b a context.xml > context.lines ;this creates a file called 'context.lines' that contains all lines (a switch) of all the body (-b switch) of the context.xml file

paste and join ;they both work by merging data together from 2 x files, but they do it on different way paste/join file1.txt file2.tx > file3.txt

  • paste ;copy the files line-by-line into a single file, separating them by tabs; with the -s will print them sequentially paste -s file1 file2
  • join ;combine the lines depending on the fields identifier, so you can bind line 1 with line 3 as long as you identify these two lines with the same number (let's say 2)

expand ;convert a tabbed-file into a spaced-file, so the lines won't have any tabs but spaces instead; expand file1.txt > expanded.txt or you can use the other way around unexpand -a unexpanded.txt

sort ;sort lines of text files, input sort -k2n file to organise the data by column 2 (k2) and in numerical order

  • sort -u file ;remove the duplicated lines

uniq ;only print lines that are unique on the file, with the -u will only display lines that are never repeated uniq -u /etc/passwd

od ;the od command displays the contents of non-ASCII files

wc ;provides lines, words and number of character (LWC), don't worrry for the number of characters if they don't match, wc always add one extra character to them, the \n line character.

103.3 Perform basic file management (4)

ls ;list of the files on current directory

  • ls -lh * ;list of files and its properties, with the size ready for humans understanding. oh yeah!
  • ls -a   ;list hidden files and directories, in Linux a file/directory is hidden when it starts with a dot (.)
  • ls -R   ;recursive mode, list all contents of all stuff, a kind of similar to the tree command
  • ls --color=always ;to see the colour of the files
  • ls -1 | wc -l ;notice the number 1 after ls and the "l" after wc, that command will tell you the number of files you have in a directory

mkdir ;creates a directory, use it with the -p to also create the parent directory at the same time: mkdir -p folder3/folder2/folder1

rm ;deletes files, use * for all; if you type rm file? it will delete all files called file1, file2, file3, etc, the question mark stands for 1 character. The combination rm -rf is very dangerous as it deletes everything without asking; rm *.txt deletes all these extensions while rm * deletes all files

rmdir; removes folders only if they are empty

touch ;creates an empty file, touch {apple, banana, cherry, durian} try this one too touch file_{01...100}

cp ;copy files, use the combination copy -R to copy all contents inside the folder

mv ;move files and folders, you can use this command to rename a file mv file1.txt file2.txt

dd ;copy files at the block level, it uses the structure input file > output file > block size (4k normally)

  • dd  if=/dev/sda out=/dev/sdb ;it copies all of the blocks from the drive sda (input file, if)  to the drive sdb (output file, of), byte by byte

tar ;copy files at the file level, it stands for tape archive, always use the switches -cvf (c f or create, v for verbose and f for file, always put f last)

  • tar -cvf file1.tar /home/user1/stuff ;that will includes all the contents of the "stuff" folder into the file1.tar
  • tar -xvf file1.tar ;extract the contents of the file1.tar to the current directory you are working on, notice the -x switch that stand for extract

gzip ;older compression command, for better compression use bzip2, they work exactly the same

  • gzip file1.txt ;compresses file1.txt by deleting that file and leaving it file1.txt.gz
  • gunzip file1.txt.gz ;uncompressed the file

bzip2 ;the newer compression command

  • bzip2 file1.txt ;it created the compressed file with the extension bz2
  • bunzip2 file1.txt.bz2 ;uncompress the file

Both gzip and bzip2 can only compress files, that's why you have to use tar to compress folders into a file, and then gzip/bzip2 that file

With gzip do as follows:

  • tar -zcvf file1.tar.gz /home/user1 ;use the -c (compress) will create the specified tar file that will contain all stuff into the user1 folder
  • tar -zxvf file1.tar.gz ;this will extract (use the -x for extraction) the contents of file1.tar.gz (could also have the extension tgz) into your current folder

With bzip2, do as follows, it uses the -j to say you are using the bzip2 command

  • tar -jcvf file1.tar.bz2 /home/user1 ;to compress
  • tar -jxvf file1.tar.bz2 ;to extract

xz ;similar compression program as gzip and bzip2 (.xz format), the *.lzma is a legacy format

cpio  ;(copy in copy out) an older version of the tar command, rarely use nowadays, because is not as flexible as tar, you actually have to pipe the data into cpio in order to user it. To uncompress a cpio archive, you need to use the gunzip utility. The cpio program copy all the files and put them on a kind of zip file with the extension .cpio, then that .cipo file is package with a description and dependencies of the files to create the .rmp file. The way to extract the contents of a .rpm package is therefore as follows:

  • ls  | cpio -ov > file.cpio ;gathers the data from ls and pipe into the cpio program, who outputs it (the o switch) to a file
  • cpio -idv < file.cpio ;does the opposite, input (i) the data from the file and creates any relevant directories (d) 
  • rmp2cpio package.rpm ; this will extract all the files that are inside the rpm package, one of those files would be a .cpio file
  • cpio -idv packagefile.cpio ;this will extract the original file structure
  • cpio is useful mostly when you use the find command to search for files that you want, and then pipe them into a single .cpio file

find ;search for stuff in real-time, (#find / -name ping)

  • find / -type f -perm u+rwx -ls ;find and display all the files whose owner has rwx permissions
  • fin . ;return files on current directory, no filter being applied 
  • find /home/user1 -size +50M   ;find files that are bigger than 50MB in size
  • find . -mtiime +1 ;show files that have been modified more than one day ago (yesterday)
  • find . -mtime -2 ;show files that have been modified less than 2 days ago
  • find . -size +50M -mtime +1 ;show files bigger than 50MB that were modified yesterday

file ;determines the file type in use,  file * in any directory to see

rev ;for reverse, prints the file on the screen the other way around

tac ;prints it out from the bottom up, it is the reverse of cat

103.4 Use streams, pipes and redirects (4)

/ ;use it to cancel a space/special character in a folder/file name, for example: ls "folder 2" or ls folder/ 2

Operators ; and $$

  • ls ; ls  ;the semicolon allows to enter 2 commands independently
  • ls && ls  ;the double ampersand executes the 2nd command only if the 1st one succeed

All Linux commands have 3 streams opened for them: STDIN (numerical value of 0), STDOUT (1) and STDERR (2)

 Redirects the to STDIN, standard input which most of the time the keyboard. You can actually create a variable using the read command to store input that you entered on the keyboard, for example:

  • read VAR ;this command will stop at the cursor and wait for you to type anything, and once you type it the string will be stored in the variable VAR
  • echo $VAR ;using the echo command, you will be able to recall the string you stored into the VAR variable
  • read < VAR file1.txt ;this command uses the < operator, which configures the input to be "file1.txt", which will be stored on the VAR variable instead

>  Redirects (and careful! > always override the file) to STDOUT, standard output, normally the screen or a file, for example:

  • ls > test-out-pipe ;this redirects the output by creating a file called "test-out-pipe", that will contain the result of the command ls

2>  Redirects to STDERR

  • ls myfolderA 2> error  ;this command will try to list the contents of "myfolderA", but because it doesn't exist will give an error through the STDERR pipe, which in this case will be captured by the directected parameter "2>" and put into a file called "error"
  • ls > out 2> error ;this example create the "out" file from the STDOUT pipe and the "error" file from the STDERR pipe
  • tar cvf backup.tar /dev/sda1 2> errors.txt ;this will keep all the error of trying to backup a hard drive, handy ha?

>>  Append all standard output to a file that already contain data

&>  This operator creates a new file that contain both standard output and standard error

&1 ;this operator redirects the input into the output, like a look, for example:

  • ls myfolderA > results.txt 2>&1 ;the error generated by the lack of existence of this folder will be take from the STDERR and put it in STDIN by &1

2>&1 This operator redirects standard error as standard output to the same place 

1>&2 This operator redirects standard output to standard error

<> use the same file as both standard input and standard output

With the concept of standard input/output, we can manipulate data so that the output of an application becomes the input of another, for example

  1. ls > file1.txt ;will create a file called file1.txt with the listing result from ls
  2. grep is file1.txt ;will search for the string "is" on the file1.txt and display the result on the screen
  3. You can do this command, with the pipe operator, to get the output of ls and piped into the input of grep, cool ah? ls | grep is

xargs ;is a program that listens in STDOUT and executes the data as an input to another command, for example

  • ls | xargs rm -rf ;this get the STDOUT from ls and pipe it to xargs, which executes the rm command, just like if you typed: rm -rf
  • sudo find. -maxdepth 4 -name "*.txt" | xargs tar cvf textfiles.tar ;this will find all *.txt files within 3 directory levels and compress them on a file

tee ;similar command to xgars, get the output of a command and pipe it into another, plus shows in the screen the output

  • echo "print this" | tee file.txt ;the tee command will get the output from the each and put it in the screen, creating in addition the file.txt

103.5 Create, monitor and kill processes (4)

Every single process in Linux has a number (process ID - PID - that identifies itself; these are the different signal commands with their signaling number

Name Number Description
SIGHUP 1 Hang-up, instruct the program to reinitialize itself, including reading configuration files
SIGINT 2 Interrupt, this is done also by Crtl + C
SIGKILL 9 Kill the process immediately
SIGTERM 15 Terminates a program, it is doesn't terminate you can kill it by using the "9" signal


sleep 1000 &   #sends the sleep process to the background
pgrep -a sle   #gives you the process ID and for processes starting with "sle"
kill -15 23456 #kill the process id 23456
killall sleep  #kill ALL sleep processes, the process name has to be exact
pkill sle      #same as above, kill ALL process starting with "sle", careful with this command!!
killall -i sleep #kill all instances of the "sleep" command and prompts you before executing
ps -aux | grep sleep  #list of processes that contain the word sleep
ps -eaf | grep sleep  #same as above

& ;by putting the ampersand at the end of a command it sends the job to the background but they are only there in the terminal window that you are working on, the minute you close that terminal all background commands are stopped; to stop jobs just use the popular Crtl+Z

  • sleep 60 & ;
  • jobs ;allows you to see what jobs are running in the background
  • fg %2 ;allows you to bring to the foreground (fg) the job id number 2; when you see the + next to a job number that indicates that this is the most recent command you sent to the foreground, meaning that if you just type "fg" it will bring back the one with the plus

nohup ;it sends jobs to the background too, but they will stay running in the system even if you close the terminal window

  • nohub sleep 60 & ;creates a background job just like before, but it will stay on even if you close the terminal window
  • ps aux | grep sleep ;allow you to see the background jobs on a different window

disown ;detach from a terminal window a background job 

pgrep ;look up for a signal process base on its name or other attributes

103.6 Modify process execution priorities (2)

In the Linux world, the priority level of a particular process is refer to as the "nice" level, and a process can have a priority level anywhere from -20 (takes a lot of the pizza of the CPU) to +19 (only takes a tiny slice of the CPU pizza), being 0 (zero) the default "nice" level of a process. The reason that the high priority level is in negative, is because the process that fall into negative are not "nice" to the system because they use lots of CPU


nice --10 sleep 999 #start the process "sleep" to priority level -10 (needs to be root)
nice -10 sleep 999  #start the process to positive 10

renice ;it changes the already assigned priority level of a process that was started with nice.  A user DO NOT have the access to renice any process to higher priority, they only can lower the priority of the given processes

renice -15 -p 1779 #reassigned process ID 1779 to priority level -15

103.7 Search text files using regular expressions (2)

 grep ;search for string inside a file, here we have some examples of using the grep command:

grep ^This file #the caret symbol (Shift+6)indicates the beginning of a line
grep ^.pp file  #same as above, where the dot represents any symbol, this search will display "apple" if that word is at the line-beginning
grep y$ file #the dollar symbol indicates the end of a line, this command will find all words ending in y in a line
grep -in "the" file #displays the with lines number (n) and regardless of capitalisation, lower or capital case (i) for indiferente
ls | grep 1 #the results of ls are piped into grep, and it will show the results of any file having the string "1"

egrep ;extender version of the grep command

  • egrep '^[a-m]' file1.txt ; find all words at the beginning of a line that start with either "a" or "m"
  • egrep '^[P|p]' file1.txt ; find the words at the beginning of the line that start with either capital P or lower p

fgrep ;similar to grep but faster (for f) as it doesn't evaluate any expressions like $$ or special character

regex (7) ;that is for regular expressions use some examples here:

  •     . (dot) - a single character
  •     ? - the preceding character matches 0 or 1 times only
  •     * - the preceding character matches 0 or more times
  •     + - the preceding character matches 1 or more times
  •     {n} - the preceding character matches exactly n times
  •     {n,m} - the preceding character matches at least n times and not more than m times.
  •     [agd] - the character is one of those included within the square brackets.
  •     [^agd] - the character is not one of those included within the square brackets.
  •     [c-f] - the dash within the square brackets operates as a range. In this case it means either the letters c, d, e or f.
  •     () - allows us to group several characters to behave as one.
  •     | (pipe symbol) - the logical OR operation.
  •     ^ - matches the beginning of the line.
  •     $ - matches the end of the line.

Character Sets ;they are included within square brackets [], if you put an hyphen it means that is range [0-9] otherwise it will treat the character individually. If you put an exclamation mark inside the set, that means not to match whichever character will folow after [!0-9] ;the character classes matches one character of a certain type [:digit:], [:lower:],[:alpha:], etc, meanint that [0-9] is equal to [:digit:]

Extended globs ;ensure extglob is enable by typting shopt -s extglob, you can created more defined queries and group matches like this !(+(photo|file)*(.jpgl.gig)) = all files that do no have photo or file and don't end with  jpef or gif.

Single quote ' tell the shell to present the character inside literally, and all special characters inside ' have no meaning

Double quote " tell the shell to interpret waht  is insiss

103.8 Perform basic file editing operations using vi (3)

vi ; use the keys H-J-K-L (K for kites go up) to navigate through vi if needed. When starting vi you always start in Command mode, press INSERT to switch to Insert mode, you can also press "i" and vi will allow you to insert text just before the cursor; shift + i let you insert text at the beginning of the line. Refer to this for more info:

  • :wq = write and quit
  • :wFile2.txt = write to a new file called File2.txt
  • :q!  ;quit without saving
  • :e!  ;reads the original file back again and starts all over
  • :sh ;open the shell so you can enter commands while the file is open, press Crt+D to return to the file
  • :g/heart/p ;displays all lines containing the word heart
  • :!ls -l ;this will run the command ls -l while maintaining the file open
  • ZZ  ;writes the file if it has been modified and then exit the application 
  • /string   ;that will search for the string "string"  forward in the document
  • :%s/Love/Dragoste   ;substitute the string "Love" for "Dragoste" in the entire document (because you have used the % operator); if you add the "/gic" at the end it will do g (global, will find all matches), i (makes case in-sensitive) and c (will confirm for each replacement)
  • .  ;the dot command repeat the last command
  • a ;insert text After the character at the current position
  • b ;navigates to the Beginning of a word
  • c ;insert a character Before the cursor
  • B ;navigates to the beginning of a White-Space delimiting word
  • dw ;deletes the word immediately after the cursor
  • d$ ;deletes from the insertion point to the end of the line
  • dl ;deletes a character
  • d5l  ;deletes 5 characters
  • dd ;deletes the entire current line
  • D ;deletes the rest of the current line from the cursor position
  • cw ;deletes the current word, and automatically enter INSERT mode so you can type a replacement word
  • e ;navigates to the End of a word
  • E ;navigates to the end of a White-Space delimiting word
  • u ;undo the most recent command in the vi editor
  • U ;(capital u) redo the last recent operation
  • h ;moves left one character
  • j  ;moves down one line
  • J ;join two lines
  • k ;moves up one line (kites go up!)
  • l ;moves right one characterq
  • 0 ;(zero) moves to the start of the current line
  • o ;opens a new line under the current line
  • p ;inserts the text deleted in the last deletion operation after current cursor location
  • yy  ;copy one single line to clipboard (yank) the current line, if you want to copy 4 x lines use 4yy
  • y5y  ;yank five lines, if you press "p" after it will copy whichever lines/s you copied
  • x ;deletes a character at the cursor
  • X ;deletes a character before the cursor 
    • I = navigate right one character
    • $ = navigate to the end of the current line
    • H = navigate to the first line of the text
    • M = navigate to the middle of the text
    • L = navigate to the last line of the text
    • G = move to a specific line to perform an editing operation
    • :sh allows you to go back to the shell to write commands, use Crtl+D to return to the file, so cool!
      • Crtl + U = moves half a screen up 
      • Crtl + D  = moves half a screen down 
      • Crtl + F  = moves down almost a full screen
      • Crtl + B = moves up almost a full screen
      • Crtl + E = scroll down one line and Crtl + Y scroll up one line

nano ;user friendly text editor, crtl+a moves you to start of a line, crtl+e moves you at the end

104.1 Create partitions and filesystems (2)

 Extended file systems are these:

  • ext2 ;max partition 32TB, no journal but good for lower write actions (like USB drives)
  • ext3 ;max partition 32TB, journaled
  • ext4 ;max partition 1EB, journaled
  • BTRFS  ;snapshot, compression and auto-defrag
  • ReiserFS  ;very space efficient with small files, but the developer apparently is in jail so the filesystem is not in use that much
  • XFS  ;good for large partitions
sudo fdisk /dev/sdb   #takes you to the wizard to format the drive sdb, and create partitions accordingly for MBR

When running fdisk, do first "o" to create an MBR, then n to start creating partitions. Press "L" to list the code (83 = Linux partition, 82 = Linux swap partition)

sudo gdisk /dev/sdb   #convert MBR to GPT or creates a new GPT partition

mkswap ;creates a swap partition

mkfs.ext2  ;creates a ext2 partition, use mkfs.ext3 to create a ext3 partition, use -t to define the type mkfs -t ext4 /dev/sda1

mke2fs  ;creates a ext2 partition, use mke2fs -j to create a ext3 partition; the valid settings for file systems with the mke2fs are small, news (creates 1 x inode per 4-kb block) largefile (creates 1 x inode per megabyte) and largefile-4 (creates 1 x inode per 4MB block)

  • mkfs ;create most filesystems
  • mkfs, msdos, mkdosfs or mkfs.vfat ;creates a DOS FAT filesystem
  • mkfs.ext2 or mke2fs ;creates an ext2 filesystem
  • mkfs.ext3 or mke2fs -j creates an ext3 filesystem
  • mkisofs ;creates a CD-ROM filesystem
  • mkfs,reiserfs or mkreiserfs ;creates a reiserFS filesystem

e2fsck ;ensures that a  ext2 file system that was improperly shutdown is error-free

parted  ;shows you the partitions that you have on your system

sudo mkfs.ext4 /dev/sdb32  #creates a file system type ex4 in the partition 32

xfs_growfs ;this command is used to expand an existing XFS file system

  • xfs_growfs -n ;returns information of the file system geometry
  • xfs_info /dev/sda1 ;also returns information of the file system geometry

lsblock ;shows you the attached block devices of your system (hard drives), very handy

fdisk is for MBR while gdisk is for GPT partition tables

104.2 Maintain the integrity of filesystems (2)

Use the fsck (pronounced 'fisk') to check for errors on a file system (ensure the drive is unmounted), you can do fsck.ext2 (but you're screw if the filesystem is 'ext3), so thanks God fsck will auto-detect the file system. Note that fsck should always run in single-user mode.

  • Use the -N switch and it will show you the file format it has detected on the partition

Use the xfsprogs (on Ubuntu) to check for error of XFS file format, like xfs_repair, xfs_check (not recommended, run repair first)

For the xfs file system you have to use the xfs command to check it, a kind of beast on his own way

tune2fs  ;change ext2 to ext3, so you add journal, remember to modify the /etc/fstab to reflect the new file system, changes are effective after a reboot. Tune2fs also allows you to view block counts for a file system, allowing you to adjust the number of mounts or days (-i for interval) after which a volume will be checked by e2fsck

  • tune2fs -0 has_journal /dev/sda1  ;another way of converting an ext2 to an ext3 file system
  • tune2fs -0 ^has_journal /dev/sda1 ;this will do the opposite, converting ext3 to ext2

dumpe2fs  ;shows all superblock info

debugfs  ;interactive filesystem editor, if you use this you're definitely a professional 

Use du / -hd1 (h for human, to show detail level 1 only) to see the space of different folders

df ;shows disk free, use the -h for human, use the option -T to show the file type system

104.3 Control mounting and unmounting of filesystems (3)

Use mount or umount to mount hard drives into empty folders, if you want the mount to be permanent then add it to the /etc/fstab

Use smbmount to mount a samba filesystem

The first partition of the first SCSI drive is /dev/sda1

The first SCSI tape drives load with /dev/st0

The first Parallel port is /dev/lp0 while the 2nd parallel port would be /dev/lp1

The first RAID volume is referred as /dev/md0

mount -t iso9660 image.img /dev/mnt/image ;this mount the .img file as if it was a partition

104.4 Manage disk quotas (1)

If we want to have quotas on a drive, we need to add the options "usrquota,grpquota" on the entry in /etc/fstab for the drive, quite easy, add both entries if you want to have quotas for both users and groups

sudo apt-get install quota   #for Ubuntu systems
sudo mount -a #remount the drives after adding the quota setting on the /etc/fstab
sudo quotacheck -avmug #rebuild table and create *.quota files, u(user),g(group), v(verbose)
sudo quotaon /dev/mnt   #finally, enable the quotas
sudo quotaoff /dev/sda3   //**disables the quota on the drive
sudo quotacheck -cug /dev/mnt  #creates (c) quotas for users (u) and group (g)

sudo edquota -u mmunoz /dev/mnt  #edits the quota for user mmunoz
sudo repquota /dev/mnt  #reports the quota usage for the drive

Once the quota is enable on the drive, it will display the "" and "aquota.user" files on the root of the drive

104.5 Manage file permissions and ownership (3)

sudo chgrp root file1.txt  #change the group owner to the grooup "root"
sudo chmod g-rw file1.txt  #removes the rw rights for the group
sudo chmod u+rw, g+rw, o -rw file1.txt #executes as stated

In Linux the listing of permissions are the user-owner (u) of the file,  the group the owner belongs to (g) and the group others-everyone (o), the string (a) applies permission to ALL

  • chmod ugo+w file1.txt ;u(users) g(grouip) o(other), set every to write for the given file
  • chmod u+rw,g+rw,o-rwx ;set user and the group to be rw but remove the rw access from the other

Octal Notation, most common values:

  • 7 = rwx       777  = a+rwx                    =  rwxrwxrwx
  • 6 = rw         755  = u+rwx,g=rx,o=rx    =  rwxr-xr-x
  • 5 = rx          644 = u=rw,g=r,o=r          = rw-r--r--
  • 4 = r           700  = u=rwx,g-rwx,o-rwx  = rwx------

-(file) d(directory) | owner | group permissions | other, anybody who is not the owner or does not belong to the group

The umask is the default permission that are assigned when you create a file or directory (umask -S to see values), the umask is set in /etc/profile /etc/bashrc or /etc/login.defs and the way it works is by subtracting the value from the default all-permissions, for example if you execute the umask 077, the effective permissions created during the session would be:

  • 600 for files (rw only, as-)
  • 777 for directories (rwx)

The special permissions are set on the "executable" bit of the relevant user, group or everybody (other). The sticky bit is set ONLY in the other field. When you see the lower case "s" it means that the executable bit has also been set, while if you see only the capital "S" the executable bit is not set

SUID ;[set user id](value of 4) executes with permissions of the file owner, like for example ping, or the /usr/bin/passwd file, that runs with the root credentials even if you are a user (it will display with an "s" e.g. passwd = -rwsr-xr-x.

GUI ;(value of 2) the file is executed with the permission of the group ID, this option is not used very much, however any new files created on the folder will be executed with the permission of the group

Sticky bit  (value of 1);nowadays the sticky bit is set on a folder, and that means that the files inside that folder can only be deleted by the owner

chmod u+s file1.txt #sets the User ID (SUID) on the file
#If it is converting something that did not have the executable turn on, 
#the bit shows as capital "S", otherwise it will show as lower "s"
chmod o+t  #sets the sticky bit


0767 #the leading zero is the special bit, not set in this example, then users rwx, group rw and other rwx. 
The sticky bit (set on the other group) is set mostly on the tmp folder, so others user can't delete files in there. To set it you either of the follow:

chmod o+t
chmod o 1777

DANGER!# if you happens to set the SUID in the rm program, everybody will actually execute the program 
with the owner permissions, meaning that anybody will be able to delete anything! Disaster!

setfacl ;use this command to configure extended ACL permissions to a file (chmod cannot configure ACL), use getfacl filename to get current info of a file/directory, using getfacl -t filename gives you a different view. Linux permission have got some limitations, only 1 x user and only 1 x group can own the file/directory, while other could be anybody; this is where ACL comes into place

  • setfacl -m user:bob:rwx filename ;this will set tbob as a user in addition to whichever user is the user of the 'filename' file. if you type ls-l that will show you a '+' at the end, inficationg ACL permissions have been set
  • setfacl -m u:bob:rwx,g:accouting:rw filename ;another way of doing it, the -m switch is for modify
  • ACL are turn on on CentOS but only on the / partition!
  • To remove ACL use these switches
    • -x remove specific ACLs; setfacl -x bob filename or setfacl -x group:marketing filename
    • -k remove all default ACLs
    • -b remove all ACLs

chown ;change the ownership of files chown user:group -R /myfolder will assign the ownership to the user and the group, use it without the -R if you want to take ownership of a file only

chattr ;if you set the immutable feature on, those files cannot even be deleted by the root chattr -i

104.6 Create and change hard and symbolic links (2)

Hard links are files pointing to the inode of the hard drive, you cannot create hard links in between 2 x disks

Soft links are just shortcuts in between files, you update the master and the shortcut gets updated. If you move the original file (mv command) the soft link will be broken

ln File1..txt HardlinkFile1.txt  #createds a hard link
ln -s File1.txt Softlink.txt     #creates a soft link
ls -li   #displays the inode where you could see both files are in the same inode

104.7 Find system files and place files in the correct location

locate ;similar to find, a bit quicker but works by creating a database which is updated once a day (sudo updatedb updates de locate database)

  • sudo vi /etc/updated.conf  //**contain the setting for the locate database

which ;use to find binary files (#which pint)

whereis  ;similar to "which", also find the binary file together with the man pages



--- --- --- --- --- {[---  EXAM PART 2 ---]} --- --- --- --- ---



105.1 Customize and use the shell environment

Working on the command line, alt+b moves one character back, while alt-f moves it forward. The shell allow you to execute OS command by sending them to the kernel; when a command is entered in the terminal, the command execution order is the following:

  1. The Shell will check if it's a built-in command
  2. The Shell will check if it's an alias of a command configure by the OS or another user
  3. The Shell will check if the command is on the hard drive, and will display command not found if all of the above is false

There is a standard to write script, so they can be portable, called POSIX-compliant shell ;bash shell is currently version 4 but there are other popular shells like cash (for debian, using less memory and faster execution than bash), fish -friendly interactive shell- is designed for interactivity and only available in third-party EPEL software repository, Zsh is very similar to bash but with a wonderful command completion. Most of the shells can be installed using yum install -y zsh / ksh /csh To switch shell use chsh then type the path /bin/zsh for the desired shell, then logout and log back in again

  • Environment variables are defined for the current shell and inherited by any child shells or process, printenv to see those ones
  • Shell variables are contained exclusively within the shell in which they were define; use set to see shell variables as well as functions, to see them without the functions use set -o posix and then set | less ;to include a variable as part of the environment use the export command (use the export -n to unset it); if what you want is to unset shell variables use the unset command instead. You can see all the sell options by typing set -o (or set +o) and if you like you can for example set posix off by typing set -o posix ,however the changes of this bash variables values only last for your current logon, once you log out all is back to normal, to make these configuration persistent we need to add them to one of the bash files

Logon Process ;when a user logs onto into an interactive shell, the Linux OS process the logon as follows

  • /etc/profile ;this file loas, which in term load all the scripts under /etc/profile.d/*.sh ; both contain System environment and Shell variables
  • ~/.bash_profile ;this file loads, containing the user environment and shell variables; in CentOS this file also execute the~/bashrc that contain the user aliases and shell functions specific to the user
  • /etc/bashrc ;this file loads, containing the System wide aliases and Shell functions

When you need to add aliases to the ~/.bashrc file you must add them to the end of the file, after /etc/bashrc has been called so that you override systemwide aliases and functions

The commands in Linux normally are Command + Options + Arguments. When a login shell launches, it reads the files in this order:

  • /etc/profile first ;either it exist or not
  • ~/.bash_profile
  • ~/.bash_login
  • ~/.profile

Non-login shell will read only the /etc/bash.bashrc and then the ~/.bashrc files. To change shells in the middle of your session use the dot (.) command . bashrc will switch you to the bashrc shell.

Any content found in the /etc/skel directory will be copied to the home directories of newly created users; the file ~/.bash_logout runs when you exit from a session

Run echo $BASH_VERSION to find out what release of bash you have (4.2.46 r2 at the time of writing). Bash stands for Bourne Again BAsh, since 1989

#!/interpreter ;this is called the "Shebang", (the command interpreter) meaning that this is the interpreter (executable) that will run that particular script; if you don't specify a "Shebang" interpreter, the script will be execute on whichever bash you are

 Conditionals and loops

  • if [conditional statement] then {stuff} else {stuff} fi
  • for VAR in [some sequience] do {stuff} {stuff with VAR} done ;in this loop the sequence statement gets accumulative to the VAR, if 1 then 2, then 3, etc

for VAR IN $ ( ls );
cat $VAR

While loops ; while [conditional loop] do {stuff, stuff} done ;conditional loops can be dangerous and it may loop in the infinite if not properly coded


while [ $VAR -lt 10 ]
   echo "VAR is $VAR"
   let VAR=VAR+1

 declare ;when you declare variables you can use -i (specify it is an integer, therefore can do math) or -r (read-only)


105.2 customize or write simple scripts

 Testing values, notice the operator NOT equal to ( != ), you can use other ones like lt (less than) gt (greater than), etc

echo "Please enter some more text " 
read text2 
if test $text1 != $text2; then 
echo "they're not identical" 
echo "they are identical" 
exit 0 

The if statement works well if there are only 2 choices to choose from, but if you have 3 or more choices, you better use the "case" statement (notice that it ends with the clause esac, meaning case in reverse)

#A simple script to demonstrate the case structure. 
echo "What's your favorite car?" 
read CAR 
case $CAR in 
volvo | ford ) echo "Nice! I love" $CAR 
porsche | vw ) echo "Not bad…" $CAR"'s are ok, too" 
yugo | fiat ) echo "Yuk!" $CAR"'s are ugly" 
* ) echo "Sorry, I’m not familiar with that make!" 
exit 0 

 Loops are scripts that required a repeated action like while or until a certain condition is met

#example of while loop: 
declare -i COUNTER 
while [ $COUNTER -gt 2 ]; do 
echo The counter is $COUNTER 
exit 0 

 This next example is an "until" script; while continues until the condition is TRUE, and until continues until the condition IS NOT true, get it? Nice one

#example of until loop: 
until [ $COUNTER -lt 10 ]; do 
exit 0 

for loop ;a loop constructed with the for will take all the values returned by a command and act on them one at a time

#example of for loop:
    for i in $ (ls); do 
       echo item: $i
exit 0

seq ;the sequence command produces number in sequence, for example seq 5 14

exec ;the exec command is use to launch Linux shell commands written in other languages like C, PHP or Perl, for example exec("dir", $output, $return);



105.3 SQL data management

Run the following to install MySQL in Ubuntu (do not forget the 'sudo' in front of the commands):

sudo apt-get install mysql-server
sudo mysql_secure_installation #starts the wizard where you'd set the root passwd

#MySQL -u sa -p food ;this will use the username "sa" to connect to a database "food" and will prompt for a password "-p" to login to it

In MySQL interactive mode you can use the following commands:

  • mysqlL> SHOW TABLES;
  • mysql> SELECT * FROM fruit;
  • mysql> SELECT name FROM fruit;  [select the column name from table fruit]
  • mysql> SELECT * FROM fruit WHERE color = "yellow"; 
  • mysql> SELECT * FROM fruit JOIN vegetable ON fruit.color = vegetable.color;


106.1 Install and configure X11

xorg.conf file is a historically file that existed in /etc/X11 and by default is not in the new distributions, nowadays everything is automatically detected. Back on the day computers used to be so slow that there was a font server to load the fonts and renders them on the computer. If in the xorg.conf file the driver for the card is "visa", most likely means that the card is not supported.

  • xwiminfo ;tool that give you info about the monitor/graphics setting
  • xdpyinfo ;another tool with tons of information
  • xhost ;if you type "xhost +" that means that the X11 can listen to connections, however the nowadays X11 server do not longer listen to connections!
    • xhost + ;if the X Server was listening for external connections, the computer will be able to connect
    • export DISPLAY=  ;old stuff, how everything used to work, but this used to display your screen to the IP
  • ssh -X manuel@uranus ;this command is used nowadays! once you logon to URANUS you go and type "xeyes" and the graphical display in Uranus will be displayed on your computer

Display Managers; there are a few (lightdm ,KDM, XDM, GDM). with Ubuntu you get lightdm, the config files for it are in "/etc/lihgtdm" and "/etc/lightdm.conf.d". To make changes to the configuration edit "/etc/lightdm/"

  • apt-cache search lightdm | grep greeter ;shows you the greeter that you can use in Ubuntu
  • Edit "/etc/default/grup" and set to 'text' the GRUB_CMDLINE_LINUX_DEFAULT="text" , then issue "sudo update-grub" and the Ubuntu will default to text mode. For CentoOS you just edit the


106.2 Setup a display manager



106.3 Accessibility



107.1 Manage user and groups accounts and related system files

Execute su (substitute ) to change the user (su -l root). It is obviously NOT GOOD PRACISE to logon with the root account.

  • sudo -k ;give up the privileges after using sudo, logging out and leaving the user logon
  • sudo -s ;switch over to fully root account, notice how the prompt changes to the hash sign (#) so you know you are root, type 'exit' to go back the ($) user-sign

Type "passwd" to change your own password. Type "sudo passwd bob" to change bob password.

useradd ; m (create home directory), -d /home/user (specify home directory) -G group1,group2 (add user to groups) -c string (comment, puts the name of the user)

usermod ;almost as above, but modify the user like -L (lock account) -U (unlock account) -aG (add to additional group)

userdel ; with the -r you delete the files in home folder along with the account

id username //**shows info for the username

getent passwd //**shows the whole password file

usermod -s /bin/bash username //**assign the bash shell to the user

groupadd -g 10000 marketing  //**creates group marketing with id 10000
     groupmod -g 20000 marketing //**modifies the id of the group
     groupdel marketing //**deletes the group

usermod -aG marketing jsmith //**add user jmisth to group marketing

cat /etc/passwd ;all users need to be able to read this file; the password will appear as "x", meaning that you need to look at the shadow file to see the password. The /etc/shadow file is only readable by the root user, the password will show as encrypted, but this is where the system looks for the passwords. (1/1/1970 is the date the password are counted from)

Accounts with the shell /bin/false cannot login, this normally relates to deamon or service accounts

chage (for CHange AGe), tool use to change the password expiration of users

chage -l username  //**shows setting for password expiration

chage -E 2018-08-25 username //**set the password to expire on 25th August

cat /etc/group ;shows the group memebership. The /etc/gshadow is similar to /etc/shadow, but obviously there is no password at all for the groups

cat userpassword | chpasswd ;changes all the users password with whichever password is in the "userpassword" file, that file must be in the format username:password, and you can use the -e switch to tell chpasswd that the file is encrypted

groups manuel  //**shows the group the user is a member of

User Limits

ulimit ;type with -a to see all the limits that have been set, how many open files, etc

ulimit -f 0 //**set the size of the file to create to zero!!! not good

cp file1 file2 //**that will give an error saying your limit has been exceeded

To make limits changes that are global for everybody edit /etc/security/limits.conf so you can comment out and set limits per users or groups (nproc is number of processes that are allowed)

who ;it shows you who is login into the system

w ;it shows what the users are doing in the system

last ;shows you what users have been doing in the system, as well as the reboot times, etc, great command


107.2 Automate system administration tasks by scheduling jobs

Cron jobs could be specific to the user or run by the system. Both types of Cron jobs use the same format of the 5 digits-fields, which is as follows: [minute 0-59] [hour 0-23] [day of the month 1-31] [month 1-12] [day of the week, 0-7, with both 0 and 7 being Sunday], leave all asterisk [*] to apply for all digits.  The sixth field is the job to run while the seventh stipulates the cron.daily, hourly, monthly or weekly. If you add the @reboot the job will run every time the system reboots, if you type @daily it will do it daily

  •  0 15 3 2 3 = 15:00 3rd February Sunday
  • */ * * * * = execute the tasks every 5'
  • 6 3 * * 1-5 = weekedays at 03:06
crontab -e  #edit the cron file where the schedule jobs are store, for the 1st time you'll need to create the file
at now + 5 minutes #and you'll be prompted for a series of commands to run, press Crtl+D when you're done
     atq     #shows the queue of the at command with the ID number
     atrm 1  #removes the job with the number ID 1

The cron jobs users are stored in /var/spool/cron/username You can add users to the security files of cron to allow or denied access accordingly (follow the principle of most-restrictive always, deny everybody and allow only the users you want). Create these files accordingly to provide security

  • /etc/cron.allow
  • /etc/cron.deny
  • /etc/at.allow
  • /etc/at.deny

There is a online cron job generator at to help you out

System crontab; edit the system-wise crontab by editing the file (same name as the program) located in /etc/crontab, where you can specify the user under which credentials the task will run. Applications that have schedules task are automatically saved in the folder /etc/crontab/crontab.d under their own files. Note that inside the 'cron.daily', 'cron.weekly', etc there are just executable files that run at these regular intervals.

Anacron ;in case there is a power cut, etc, and the schedule tasks don't run, the anacron command takes charges. It works by taking a timestamp (located in /var/spool/anacron) of the tasks and checks to see if there is a miss entry, if there is it runs the schedule job. The anacron program is kept in /usr/sbin/anacron, and it needs to be monitoring the schedules on cron.daily, etc by adding an entry on the /etc/contab system-wide file, if the anacron program is not there monitoring those folders it will not keep track of the timestamps

at ;is a service use only for  jobs that run at certain times (for recurring jobs use crond) or in the case of a batch job when the CPU average drops below 0.8 ;at supports general syntax like tomorrow, midday or now, even teatime (specifically 4pm)

systemctl enable atd #ensures the service starts at boot
at now +5min #configures a job to start in 5 minutes, press Crtl+D to save your job
atq          #shows you the job in the queue, notice that you cannot edit it
at -c number #type the at number and you'll at the bottom what the job will do
atrm number  #removes the at job

batch ;batch jobs only run when the system is not busy, when the CPU average is below 0.8




107.3 Localisation and internationalization

The told "tzconfig" command is not longer use. The command tzselect is used to set the time for the user, appending the resulting entry into the profile file of the user. To make the time zone globally for the system, do as follows (this works for any system, Debian, Ubuntu, etc):

Inspect the file /usr/share/zoneinfo to see the timezone information of the system. To change it all globablly, please modify these 2 x files:

/etc/timezone  //**chagne this text file to read the output of tzselect

/etc/localtime //**this file needs to be ovewritten with your desired timezone, like this: cp "/usr/share/zoneinfo/Europe/London /etc/localtime"

 locale ;shows you the language setting of the system. To modify any of the entries type:

export LC_TIME="en_GB.UTF-8"

export LC=ALL"en _US.UTF-8"  //**if you set the LC=ALL variable it will override all other settings, including the user's profile ones

LAN=C //**if you issue this command the system will match all the profile languages with the "locale" setting

ASCII, 128 characters in English (all text), but it had lots of limitations, so it progressed to ISO-8859 (more characters and supported more languages), but also has lots of issues with "sets" of characters, so it got evolved to UTF-8 (Universal Transformation Format, 8 bits), which is the daddy of them old, basically universal!

 iconv ;convert back and forth from ASCII to UTF-8, ISO, etc



108.1 Maintain system time

Visit /etc/adjtime to check if the computer is using the hardware clock or time zone (UTC)

date //**shows the system clock time

date -s "03/19/2018 12:20:35" //**changes the system clock to that

hwclock //**shows you the battery internal time

hwclock --set --date="15/08/2018 07:00:41"  //**sets that hardware clock

hwclock -u -w //**sets the hardware clock to UTC time (-u) and sets the hardware clock to the system time (-)

The hwclock -w sync it from the system clock back to the hardware clock

The hwclock -sytohc will sync the BIOS hardware clock to the same value as the software clock

Do "apt-get install ntp" or "yum install ntp" depending on the distro. Edit /etc/ntp.conf and remove the Ubuntu/centos pools and replace them by your time zone ones found at Then execute "service ntp restart" to make the change effective

ntpq -p //**query your ntp servers



108.2 System logging

Any program that happens to log messgages will send its messges to the logger, and identify these messages with these categories (so that every message sent to the SysLog is tagged with these 2 categories):

  • Facilities ;auth,user,kerm,cron,daemon,mail,local1,local2
  • Priorities ;emer,panic,alert,crit,errr, warn, notice, info, debug

facility.priority > action ; for example a > /var/log/user.log but a kern.panic > @root (screen notification) OR *.* > /var/log/messages. If you execute "" all the priorities above info (emer,panic,etc) are going to be logged

In ubuntu edit the conf file under /etc/rsyslog.d to see the seeting regarding factory+priorities for services. To make your linux machine a syslog server, edit /etc/defaults/rsyslog and add -r to RSYSLOGD_OPTIONS_"-r" (remote) and it will listen for incoming messages

In CentOS systemd has its own binary logging program called journalctl, where the logs are kept in bibary code, run journalctl MESSAGE= to see if there is any message there

The file /etc/rc.d/init.d/syslog is used to start the System Log Daemon (syslog)

logger locatl4.crit This is a message  //**with the logger command you can inset a log to test the system or a note to yourself

tail /var/log/syslog  //**you will be able to see the incoming messages from the up

etc/logrotate //**the file that manages the logs rotations, so you don't have a single monster file like catalina

/etc/logrotate.d  //**this folder contains logs of apps that want theirs logs to be managed



108.3 Mail Transfer Agent (MTA) basics

sendmail ;the oldest program to send mail, there are new ones now (qmail, exim, postfix) but all of them are backwards compatible with sendmail

See the /etc/aliases to see the alias of the system, and if you make any changes to the aliases run newaliases so the system detects the changes

mail root //**to e-mail root, press Crtl+D when finished

mail //**to check your mail

su -l manuel //**logon as that user to check mail

If you create a file .forward in the home directory of the user and just type the name of the user, mail will be forwarded from user to user. The purpose of the ~./forward file in a user's home directory is to redirect mail for a single user


108.4 Manage printers and printing

The CUPS (developed by Apple) is done on the web browser, to visit it for the system go to http://localhost:631 and logon with the 1st user created on the system, or a user who is a member of the "LP Admin Group"

Edit the file /etc/cups/cupsd.conf if you like to make the changes on the cmd instead that on the browser; the printers.conf contain the configuration of the printers, to edit it first ensure the cups service is stopped

lpr   //**send file to the printer

lpq -a  //**show print jobs, type "lpq -P%printerName% to find the printer name

lprm 2  //**remove print job number 2, do "lprm -PHP_Inket -" to remove all the jobs in the queue

lpc   //**show printer statuses

You can use cupsenable printername, cupsdisable printername, and for the jobs that you want to deal in the queue you can use cupsreject printername, cupsaccept



109.1 Fundamentals of internet protocols



109.2 Basic network configuration

Cat the /etc/services to see the entirely list of ports the system recognize, some of these ports might include: port 53 (DNS), port 123 (NTP), port 139 (NetBIOS), port 143 (IMAP), port 161-162 (SNMP), por 636 (LDAPS), port 993 (IMAPS), port 995 (POP35) 

ifconfig eth0   //**changes the IP of eth0 to the given one

ifconfig eth0 netmask  //**changes the subject mask to the given one

ifdown eth0         //**shuts the interface down
ifconfig eth0 eth0  //**same as above

ifup eth0         //**enables the nic again
ifconfig eth0 up  //**set the nic up, with the original IP address, given that fact that ANY changes made with ifconfig are lost after reboot/restart

Historically, the DNS entries have been kept in /etc/resolv.conf

On a CentOS the configuration, visit the folder/etc/sysconfig/network-scripts/ and see the files in there, edit ifcfg-eth0 to change the IP on permanent basis. To modify the gateway use /etc/sysconfig/network

On an Ubuntu visit /etc/network/interfaces where all the stuff related to the nics (how many nics you have) and gateway are kept, lovely!

  • Edit /etc/hostname (and then reboot or logout) to change the hostname
  • Edit /etc/hosts to add additional DNS entries, like local intranet etc, that will be query before asking the legitimate DNS servers
  • The command route shows you gateway information, execute route add default gw to add default gateway; user traceroute and/or tracepath
  • User ping6 to execute ICMP packets by the ipv6 protocol
  • Use dig to query the DNS entries, similar to nslookup; execute dig @ to query a different server for the answer
  • Edit the file /etc/nsswitch.conf to arrange the order on which the Linux query the DNS servers, useful if you don't want the System to look at the host file first

To restart the network serive on different systems, do as follows:

  • For Debian: /etc/init.d/networking restart
  • For Red Hat: either service network restart OR /etc/init.d/network restart
  • For Ubuntu: sudo /etc/init.d/networking restart

 Routing ;to find your gateway type route -n and to delete use route del default gw

109.3 Basic network troubleshooting

  We know that "ls -l /bin/ping" will show that the ping command has the user bit set, so that program can run with root credentials to matter which user invokes it

find / -perm -u+s  //**find on the root directory all files that has the permission SUID set for the users (-u)

Running with -g to check for the Grooup User ID bit set

netstat -tuna  ;you definitely won't forget this command, it tells you the ports that are listening, not big problem is the local address show as the loopback

nmap ;use to scan a gateway to server to check the firewall, you probably need to install it on the 1st use

sudores ;to edit the file /etc/sudoers you actually need to type visudo ,in Ubuntu the 1st user created on the system is automatically added to the sudo group



109.4 Configure client side DNS



110.1 Perform security administration tasks



110.2 Setup host security



110.3 Securing data with encryption 

You create a Private Key and make it all secure (lock it away), while the Public Key can be shown to absolutely anybody; users can encrypt a file with your Public Key and only you can open it with your Private Key. To verify identity, users can decrypt files with your Public Key, so they know you have encrypt them (and only you) with your Private Key

/home/username/.ssh ;this folder contains the keys use by ssh, check the file "known_hosts"

/etc/ssh ;contain the different keys of the System, the file is the public key for ssh, send publically to the user when he/she tries to connect (the ssh_host_rsa_key file is actually the private key)

ssh-keygen  //**follow the wizard to create a private key

ssh-copy-id -i ~/.ssh/ //**ensure you ALWAYS specify the public key (pub)
The above command what is does is to copy the public key to the server, so you can login without a password

ssh  //**after the above, the system .130 will let you in without asking for passwd

ls .ssh  //**verify the "known_hosts" file has been created

cat authorized_keys  //**more checking for the keys

gpg ;tool to create encrypted keys that can be share among users. To create a key starts the wizard by executing gpg --gen-key to create a public key with your real details, it may take 5' minutes or so to generate the key

gpg --list-key  //**shows the keys that you have in the system

gpg --export username1 > username1.mykey.public  //**generate your public key so you can pass it around

gpg --import /tmp/username1.mykey.public  //**do this on username2 to import the key of username1

After the above, username2 can create a file (touch file3) and encrypt it using gpg --out file4 recipient username1 --encrypt file3 and that file4 can only be open by username1, if you then try to cat the file4 it will all garbish. To decrypt the file at username1 use gpg --out file5 --decrypt file4 and you'll be prompted for the passphrase to decrypt it



To install SSH on an Ubuntu server:

sudo apt-get install openssh-server

 If you get the error "E:  Unable to locate package openssh-server", run these two commands:

sudo apt-get upgrade

Applications that I always install on my Linux systems: tree, synaptic,


Install the development tools to compile a new kernel

sudo yum groupinstall "Development Tools"

London, 18 August 2018

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