Linux has more than 650 commands and every command has its own set of options all performing different operations. Going through each and every one of these commands will be a very tedious task. However limiting yourself to only a few of them is never an option. The trick here to learning all these commands, is to categorise them according to their function. By doing this, you will know atleast the basic commands and have some measure of control over the Linux command line.
When you go through these commands, you will be able to perform different function as per your need in the Linux command line.
The ‘must’ know commands fall in this category. I have distributed the commands into two sub categories: Directory commands and File commands.
- pwd : Print working directory
- mkdir : Create directories
- cd : Change the current directory
- rmdir : Remove directories
- ls : List directory contents
- rm : Remove files
- cp : Copy files from a source to the same or different target(s).
- mv : Move file to different targets.
- cat : Read one or more files and print them to standard output. If you need to view contents of a short file, cat is recommended.
- cmp: Compare two files byte by byte.
- wc: Print the number of new lines, words, and bytes in files.
- du : Estimate disk usage of each file and recursively for directories.
- find: Search for files in directory hierarchy, e.g. find notes.txt
- grep: Print lines matching a pattern,e.g. grep –i topic notes.txt (topic is the pattern)
- sort: Sort lines of text files
Every Linux program is an executable file. For instance, the cp command is provided by the file in /bin/sh which holds the list of machine instructions. Similarly, if you are installing a package , let’s say vsftpd, your focus will be modifying its configuration file , vsftpd.conf present in /etc directory. This is where you will be using editors.
‘Vim’ is used quite frequently. It’s an advanced text editor that comes with a more complete feature than the ‘Vi’ text editor. The other text editors are: nano, vi, kate, (KDE Advanced Text Editor), gedit (graphical user interface).
e.g. vim rabi.c ( vim filename ).
In linux, you can say that using the ‘root’ account is like having the powers of God. You will have access to almost each and every file(configuration files, system , text files etc) with no interruption and restriction. You need to be very careful while doing work as the root (super user). Therefore, managing user accounts and groups is an essential part of a system administrator.
For example, an organization ABC has three departments: Marketing, Technical and Account, each department having 3-4 employees. The organization demands you to verify the users of marketing, technical and account departments so that the employees of each group will be able to view his/her own department file.
Given a scenario, if you know how to manage these accounts, you can easily set permissions for the users mentioned above. If not, these commands are essential for the task.
- useradd : for creating user account. This command can be executed by administrators only. On debian, you should use adduser. For other options like adding expiry date, home directory etc refer man useradd.
- passwd : for changing user password.
If the user has set password before, he/she will be prompted for the first password whereas superuser is permitted to bypass the step so that forgotten passwords may be changed.
- usermod : modify user accoun.
- userdel : delete a user account and related files This command can be executed by administrators only.
- groupadd : create a new group
- groupdel : delete the group and entries referring to the group
- groupmod : modify a group definition on the system
- chmod : modify properties for users
- chown : change file owner and group
- chgrp : change group ownership
Linux is predominantly known for its use in servers. In 2009 it held a server market share ranging between 20–40%(source : wikipedia). One should know the commands to check the ip address, download files from the net, get DNS, etc.
- wget : a non-interactive network downloader
- Even if a download fails due to a network problem, it will keep retrying until the whole file has been retrieved. The server will instruct to continue to download from where it it left off.
- $ wget url-for-file
- ping : send ICMP ECHO_REQUEST to network hosts, you will get back ICMP packet if the host responds. This command is useful when you are in a doubt whether your computer is connected or not.
- $ ping IP or host name
- hostname : show or set the system’s host name
- dnsdomainname : show the system’s DNS domain name
- netstat : displays the status of ports ie. which ports are open, closed, waiting for connections. It displays the contents of /proc/net file.
- ifconfig : configure a network interface, or to display their current configuration. It is also useful to get the information about IP address, Subnet Mask,set remote IP address , Netmask etc.
- ifup : bring a network interface up
- ifdown : take a network interface down
You want to install a package from its source code. You find out that the source code of the package is archived in a file xxx.tar. In this situation, the command-line utility ‘tar’ proves to be a vital resource for you. The ‘tar’ is probably the most popular Linux backup utility. If the ‘tar’ file is compressed with the compression utility like ‘bzip’ or ‘gzip’, the resulting file is the famous ‘tarballs’ which is a common method to deliver software installation archives.
tar : an archiving program designed to store and extract files from an archive known as a tarfile.
- -c : create a new archive
- -r : append files to the end of an archive
- -t : list the contents of an archive
- -u : only append files that are newer than copy in archive
- -x : extract files from an archive
- -C : change to directory Dir
- -j : filter archive through bzip2, use to decompress .bz2 files.
- -v : verbosely list files processed
- -f : use archive file
- -z : filter the archive through gzip
- tar -xvf test.tar ( extract foo.tar to the current location)
- tar -xvzf test.tar.gz ( extract gzipped test.tar.gz )
- tar -cvf test.tar foo/ ( compress the contents of foo folder to foo.tar )
There are manual pages for almost all the commands of Linux. You can access the manual pages using man command. The man command offers documentation of the command. If you type:
You will be seeing the manual page of ls with its name, synopsis, description, author, copyright etc. Remember, there is a manual page for the man command itself.
If you desire to have a brief reference of the command, use -help option with the command. ls -help
You can even use info command to have a quick overview of the command. info ls
Remember, that memorizing all the commands in Linux along with all its options is a very difficult job. So memorise the command and options which has frequent usage and leave the rest to the HELP commands.
Package Management Utilities
On RED HAT, SUSE, and many similar Linux distributions, the RPM Package Manager (RPM) format is used. Ubuntu and Debian, however, uses the Debian Pacakge (DEB) format. Therefore, I have categorised it into two, one for RPM and the other for Debian.
For RPM format, the rpm and yum is prefered.
rpm options rpm-package-name
(use man rpm for further more information) The -q option tells you if a package is already installed, and the -qa option displays a list of all installed packages.
-qa : List all installed RPM applications -qf : Lists applications that own filename -qR : Lists applications on which this application depends -qi : Displays all application information -qd : Lists only documentation files in the application -qc : Lists only configuration files in the application
If you add p qualifier to the above options, gives information about specific package. For e.g. -qpl : Lists files in the RPM package
Yum (Yellowdog Update modifier)
yum is an automatic updater and package installer/remover for rpm systems. It automatically computes dependencies and figures out what things should occur to install packages. You need to install yum in your Linux system.
yum command package-name
e.g. $yum install package-name
Its configuration file is /etc/yum.
For Debian packages, Advanced Package Tool (APT) and Debian Package Tool (dpkg) is preferred.
apt command package-name
use apt-get install package-name to install a package.
Similarly, if you want to upgrade a package use apt-get upgrade package-name. With no package specified, apt-get with the upgrade command will upgrade your entire system for FTP site, or CD. You can find configuration files in /etc/apt. There are sources.list, apt.conf files to look for.
dpkg(Debian package tool) is another method to install a binary file with the format .deb. To install , type $dpkg -i xxxx.deb
To remove, $dpkg -r xxxx.deb.
In order to execute a command in the background, place an ampersand(&) on the command line at the end of the command. A user job number(placed in brackets) and a system process number are displayed. A system process number is the number by which the system identifies the job whereas a user job number is the number by which the user identifies the job.
- sudo cp -rf * ~/ss &
-  9144
- jobs : lists the jobs being run at the background
- - Running sudo cp -rf * ~/ss &
- + Running sudo cp -rf * ~/yy &
- The ‘+‘ sign indicates the job currently being processed , ‘-’ sign indicates the upcoming jobs to be executed. The ‘% ‘ used with the job number refrences a job. e.g. Used in fg.
- fg : a process running in the background will be processed in the foreground
- $ fg % 2
- cat *.cpp > mytext
- kill : cancels a job running in the background, it takes argument either the user job number or the system process number.
-  + Running cp *.c > mytext
-  - Running cp *.dat >>mytext
- $kill %2
- bg: places a suspended job in the background
- $ cat *.cpp > mytext
- ( Ctrl + Z will suspend the process running at the moment )
- ps : reports a snapshot of the current processes
- top : displays Linux tasks
- at : executes commands at a specified time.
- $ at 8:00
- at > echo “HI” > /dev/tty1
- (Press ‘ctrl + d’ to return to the command line. This will display the message in tty1 at 8′o clock.)
- To view the schedule : $ atq
- To cancel a job : $atrm 5 [job ID]
- crontab :crontab is a file which contains the schedule of entries to run at specified times.
- shutdown : bring the system down
- -r Requests that the system be rebooted after it has been brought down.
- -c Cancels a running shutdown.
- whoami : displays the login name of the current effective user.
- logname : print user´s login name
- quota : display disk usage and limits, e.g $ quota -v
- su : switch to super user or change user ID
- which : returns the pathnames of the files which would be executed in the current environment.
Type $which ls, you will get /bin/ls.