Linux administrators cannot live by the GUI alone. That's why we've compiled the most essential Linux commands into this convenient guide. We designed this guide specifically for Linux managers and system administrators as a reference library of only the most useful utilities. By learning a few simple tools, command-line cowards can turn into scripting commandos, getting the most out of Linux by executing kernel and shell commands. Enjoy our guide and be sure to post your own Linux question or browse Linux answers on our IT Knowledge Exchange community.
alias
Alias allows you to substitute a small or more familiar name in place of a long string.
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apt-get
The apt-get program searches for and installs software packages on Debian-based systems.
How to manage software on Ubuntu Server with "aptitude" and "apt-get"
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aspell
GNU Aspell is a free, open source spell checker. It's known for its stellar list of replacements for misspelled words.
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awk, gawk
Awk searches for patterns in a file and processes them. It enables a programmer to write small programs in the form of statements to make changes in text files when certain patterns appear or extract data from those files. This command simplifies a process historically done in C or Pascal languages.
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Gawk stands for "GNU awk" and is commonly used in Linux.
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bzip2
Reduce the size of backup files by by compressing them with bzip2, which can also be used for decompressing files.
More info:
Try before you buy with Linux 2.6
Beating Trojan horses and backup blues
bzip.org
cat
Abbreviated from the word "concatenate," which means to link things together, cat is used in Linux to link file contents and output them for viewing or printing.
'Cat' is where it's at
cd
The cd command sets the working directory of a process.
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chmod
Chmod is a utility that changes the permission of a file.
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chown
Chown is a utility that is also used to change file ownership.
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cmp
Cmp compares files and lets you know if two or more files are identical.
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comm
Comm compares sorted files and selects or rejects lines common to two files.
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cp
The cp command is used to copy files.
Cp is used to copy the kernel to the boot area in "Try before you buy with Linux 2.6."
cpio
Back up empty directories with cpio, which restores files from an archive or creates an archive.
Moving files in a Unix-to-Linux port
Samba-E: Windows files and directory ACLs
cron
Cron is used for scheduling tasks.
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date
An essential command to set the date and time. Also a useful way to output current information when working in a script file.
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declare
Declare variables and/or give them attributes with this command.
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df
The df command reports filesystem disk space usage.
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echo
This command lets you echo a string variable to standard output.
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enable
Enables or disables a printer.
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env
To check environment variables on another account to troubleshoot dotfiles.
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eval
This POSIX special built-in command evaluates several arguments by reading them as one concatenated argument, then reports on that argument's status.
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exec
Short for "execute," exec replaces the parent process by whatever command is typed.
There is more than one use for exec. Learn some new ones in this excerpt
from Unix Power Tools, 2nd Edition.
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exit
Allows you to exit from a program, shell or UNIX network.
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expect
Use expect to automate repetitive tasks.
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export
Export sets the value of a variable so it is visible to all sub-processes that belong to the current shell.
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find
Find searches the directory tree rooted at each given file name by evaluating the given expression from left to right, according to the rules of precedence (see section OPERATORS), until the outcome is known (the left hand side is false for and operations, true for or), at which point find moves on to the next file name.
This manual page documents the GNU version of find.
for, while
The commands for and while are used to loop through a list of items and do something for each one.
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free
The free command enables admins to find statistics about memory usage, showing the total of free, used, physical, swap, shared and other memory used by the kernel.
Put these troubleshooting tools in your toolbox
gawk
See awk
grep
Grep is a command used for searching one or more files for a given character string or pattern. It can also be used to replace the character string with another one.
More info:
Put these troubleshooting tools in your toolbox
Ask the expert: Trouble with the scheduler in RHEL 2.6
gzip
gzip is a compression utility designed to be a replacement for compress. Its main advantages over compress are much better compression and freedom from patented algorithms. It has been adopted by the GNU project and is now relatively popular on the Internet. gzip was written by Jean-loup Gailly and Mark Adler for the decompression code.
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ifconfig
ifconfig checks a network interface configuration. It can be used, for example, to verify a user's configuration if the user's system has been recently configured or if the user's system cannot reach the remote host while other systems on the same network can.
More info:
Unix-to-Linux migration: Setting up a network
RHEL4 review, part two
RHEL4 administration tools: ifconfig, arp, tcpdump and iptraf
ifup
Starts up network interface.
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ifdown
Shuts down network interface.
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less, more
The less command lets an admin scroll through configuration and error log files, displaying text files one screen at a time. The command will enables a search for text within files.
For more about the command less, see Put these troubleshooting tools in your toolbox
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More goes hand-in-hand with the less command and displays text one screen at a time.
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locate, slocate
Locate lists files in a database that match a pattern.
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Slocate, or secure locate provides a secure way to index and quickly search for files on your system. It uses incremental encoding just like GNU locate to compress its database to make searching faster, but it will also store file permissions and ownership so that users will not see files they do not have access to.
This site gives variations on locate and slocate and how to use them. It also offers different methods for finding files in Linux.
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lft
lft is like traceroute but gives a lot more information for debugging connections or just finding where a box/system is.
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ln
The ln command makes new, alternate file names for a file by hard linking, letting multiple users share one file.
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ls
The ls command shows information about files. With it, admins can list the contents of a directory in order to determine when the configurations files were last edited. There are many subcommands under ls, such as ls-r, which can reverse the order in which files are displayed.
The ls command is also discussed in this tip: Put these troubleshooting tools in your toolbox
Debugging IPC with Shell commands from "The Linux Programmer's Toolbox Get advice about utilities for debugging communication between processes, find out how to handle open files and dumping data from files, discover commands and more from Chapter 8, Debugging IPC with Shell commands, in "The Linux Programmer's Toolbox."
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man
Short for "manual," man unveils information about commands and a keyword search mechanism for needed commands.
The man command is also discussed in this tip: Put these troubleshooting tools in your toolbox
Information about the Unix/Linux man command
mc
A visual shell file manager.
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more
See less
neat
Neat is a GNOME GUI admin tool. Among other things, net lets admins specify information needed to set up a network card.
More info:
Setting up an NTL Cable Modem on Linux using an Ethernet Card
Unix-to-Linux migration: Setting up a network
netconfig, netcfg
Netconfig, a command used in configuring a network, displays a series of screens that ask for configuration information.
More info:
Unix-to-Linux migration: Setting up a network
Linux Forum netconfig page
Network Configuration Using Red Hat netcfg
netstat
The netstat command shows the network status by symbolically displaying the contents of various network-related data structures. There are a number of output formats, depending on the options for the information presented.
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nslookup
Look up the name of a computer
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od
Used to dump binary files in octal (or hex or binary).
Debugging IPC with Shell commands from "The Linux Programmer's Toolbox
Get advice about utilities for debugging communication between processes, find out how to handle open files and dumping data from files, discover commands and more from Chapter 8, Debugging IPC with Shell commands, in "The Linux Programmer's Toolbox."
passwd
A quick and easy way to change passwords on a system.
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ping
The ping command (named after the sound of an active sonar system) sends echo requests to the host you specify on the command line, and lists the responses received their round trip time. When you terminate ping (probably by hitting control-C) it summarizes the results, giving the average round trip time and the percent packet loss. This command is used constantly to determine whether there is a problem with the network connection between two hosts.
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ps
Get details on a specific process.
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pwd
pwd is short for print working directory. The pwd command displays the name of the current working directory.
$ PATH vs. pwd
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read
Used to read lines of text from standard input into shell variables for further processing.
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rpm
The Red Hat Package Manager (RPM) is a command-line driven package-management system capable of installing, uninstalling, verifying, querying and updating computer software packages. Each software package consists of an archive of files along with information about the package like its version and a description.
RPM.org
Learn more in this white paper: Maximum RPM: Taking the Red Hat package manager to the limit
Expert response: Yum vs. RPM
rsync
Rsync is focused on synching data from one disk location to another. It was created by Andrew Tridgell, one of Samba's core team.
Learn more in this tip on Rsync and Amanda
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screen
The screen utility is a terminal multiplexor; in essence this means that you can use a single terminal window to run multiple terminal applications.
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Learn more in this tip: Screen: The terminal baby-sitter in the sysadmin's toolbox
sdiff
sdiff produces a human-friendly description of the differences between two text files. It shows the files side-by-side with the symbols to indicate lines only in the left file, lines that differ between the two files and lines only in the right file. Much easier to read than the output of diff.
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sed
Sed (streams editor) isn't really a true text editor or text processor. Instead, it is used to filter text, i.e., it takes text input and performs some operation (or set of operations) on it and outputs the modified text. Sed is typically used for extracting part of a file using pattern matching or substituting multiple occurrences of a string within a file.
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shutdown
Shutdown is a command that turns off the computer and can be combined with variables such as -h for halt or -r for reboot.
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slocate
See locate
snort
Snort is an open source network intrusion-prevention and detection system utilizing a rule-driven language, which combines the benefits of signature, protocol and anomaly based inspection methods. With millions of downloads to date, Snort is the most widely deployed intrusion detection and prevention technology worldwide and has become the de facto standard for the industry.
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sort
Used to sort lines of text alphabetically or numerically; supports multiple sort keys.
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sudo
Sudo (superuser do) allows a system administrator to give certain users (or groups of users) the ability to run some (or all) commands as root or another user while logging the commands and arguments.
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ssh
Ssh is used for secure network connections and tunneling of TCP services.
OpenSSH is one open source SSH version.
tar
The tar program provides the ability to create tar archives, as well as various other kinds of manipulation. For example, you can use tar on previously created archives to extract files, store additional files, or update or list files. Initially, tar archives were used to store files on magnetic tape. The name "tar" comes from this use; it stands for "tape archiver." Despite the utility's name, tar can direct its output to available devices, files, or other programs. Tar may even access remote devices or files.
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top
Like vmstat, get a view of how the system is performing, see which processes are hogging all the memory.
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tr
Used to translate or delete characters from a text stream.
A little devil called tr
traceroute
Traceroute determines a route to the host and is very useful for distinguishing network/router issues. If the domain does not work or is not available you can traceroute an IP.
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uname
This command is helpful when working on different computers which may not be in synch at the OS level. Also, you can print information about those systems.
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uniq
Used to remove duplicated lines from a list.
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vi
Vi is a screen-based editor preferred by many Unix users. The Vi editor has powerful features to aid programmers.
Learn more in this tutorial: Mastering the vi editor
vmstat
The vmstat command is used to get a snapshot of everything in a system, helping admins determine whether the bottleneck is CPU, memory or I/O. Run this command to get virtual memory statistics.
How to keep an eye on Linux performance.
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wc
Word count, used to count words (and characters, and lines) in files.
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wget
Wget is a network utility to retrieve files from the Web using http and ftp, the two most widely used Internet protocols. It works non-interactively, so it will work in the background, after having logged off.
Information on GNU Wget
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while
See for
whoami
Tells you what userid you are running under
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xargs
xargs create command line from data on standard input.
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