Linux Useful Commands
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A way to run a command or a series of Unix commands using a shorter name than those that are usually associated with such commands.
Apt-get is a tool to automatically update a Debian machine and to get and install Debian packages/programs.
Aspell is a free and open source spell checker designed to replace Ispell. It can either be used as a library or as an independent spell checker.
A programming language tool used to manipulate text. The language of the AWK utility resembles the shell programming language in many areas, although AWK's syntax is very much its own.
A portable, fast, open source program used to compress and decompress files at a high rate.
A Unix, GNU/Linux command that can read, modify or concatenate text files. Cat commands are most commonly used for displaying the contents of a file.
The cd command changes the current directory in GNU/Linux and can toggle between directories conveniently. Cd is similar to the CD and CHDIR commands in MS-DOS.
Chmod changes the access mode (permissions) of one or more files. Only the owner of a file or a privileged user may change the access mode.
Chown changes file or group ownership and has the option to change ownership of all objects within a directory tree, as well as having the ability to view information on objects processed.
The cmp utility compares two files of any type and writes the results to the standard output. By default, cmp is silent if the files are the same; if they differ, the byte and line number at which the first difference occurred is reported.
Comm compares lines common to file1 and file2.The output is in three columns; from left to right: lines unique to file1, lines unique to file2 and lines common to both files.
The cp command copies files and directories; copies can be made simultaneous to another directory if the copy is under a different name.
Cpio copies files into or out of a cpio or tar archive. A tar archive is a file that contains other files, plus information about them, such as their file name, owner, timestamps and access permissions. The archive can be another file on the disk, a magnetic tape or a pipe. Cpio has three operating modes and is a more efficient alternative to tar.
CRON is a GNU/Linux system process that will execute a program at a preset time. To use CRON, a user must prepare a text file that describes the program to be executed and the times at which CRON should execute them. Then the crontab program can be used to load the text file that describes the CRON jobs into CRON.
Date sets a system's date and time. This is also a useful way to output/print current information when working in a script file.
Declare declares variables, gives them attributes or modifies properties of variables.
Df displays the amount of disk space available on the file system containing each file name argument. With no file name, available space on all currently mounted file systems is shown.
Echo allows a user to repeat, or "echo," a string variable to standard output.
Enable will stop or start printers or classes.
Env runs a program in a modified environment or displays the current environment and its variables.
Eval evaluates several arguments, concatenates them into a single command and then reports on that argument's status.
Exec replaces the parent process with whatever command is typed. This command treats its arguments as the specification of one or more subprocesses to execute.
The exit command terminates a script and can return a value to the parent script.
Expect talks to other interactive programs according to a script and waits for a response, often from any string that matches a given pattern.
Export converts a file into a different format than the one in which it is currently. Once a file is exported, it can be accessed by any application that uses its format.
Find searches the directory tree to find particular groups of files that meet specified conditions, including --name and --type, -exec and --size and --mtime and --user.
For and while are used to execute or loop items repeatedly as long as certain conditions are met.
Free displays the total amount of free and used physical memory and swap space in the system, as well as the buffers and cache used by the kernel.
Grep searches files for a given character string or pattern and can replace the string with another. This is one method of searching for files within GNU/Linux.
Gzip is the GNU project's open source program used for file compression, compressing web pages on the server end for decompression in the browser. This is popular for streaming media compression and can concatenate and compress several streams simultaneously.
Ifconfig is used to configure the kernel-resident network interfaces. It is used at boot time to set up interfaces. After that, it is usually only needed when debugging or when system tuning is needed.
Ifup configures a network interface/enables a network connection.
Ifdown shuts down a network interface/disables a network connection.
The less command lets an admin scroll through configuration and error log files, displaying text files one screen at a time, with backward or forward moving available in files. There is more mobility within files.
Similar to less, more pages through text one screen at a time, but is more limited in moving in files.
Locate reads one or more databases and writes file names matching certain patterns to output.
Like locate, slocate, or secure locate, provides a way to index and quickly search for files, but also securely stores file permissions and ownership so unauthorized users will be unable to view such information.
Lft is similar to traceroute in determining connection routes, but provides a lot more information for debugging connections or finding where a box/system is. Lft also displays route packets and file types.
The ln command creates a new name for a file through hard linking, allowing multiple users to share one file.
The ls command lists files and directories within the current working directory, allowing admins to see when configuration files were last edited.
Short for "manual," man allows a user to format and display the user manual built into GNU/Linux distributions which documents commands and other aspects of the system.
A visual shell, text-based file manager for Unix systems.
Neat is a GNOME GUI admin tool which allows admins to specify the information needed to set up a network card, among other features.
Netconfig configures a network, enables network products and displays a series of screens that ask for configuration information.
Netstat provides information and statistics about protocols in use and current TCP/IP network connections. It is a helpful forensic tool in figuring out which processes and programs are active on a computer and are involved in network communications.
Nslookup allows a user to enter a host name and find the corresponding IP address. A reverse of that process to find the host name is also possible.
Od is used to dump binary files in octal (or hex/binary) format to standard output.
Passwd updates a user's authentication tokens (changes their current password).
Ping allows a user to verify that a particular IP address exists and can accept requests. Ping can be used to test connectivity and determine response time, as well as to ensure that a host computer the user is trying to reach is actually operating.
Ps reports the statuses of current processes in a system.
The pwd (print working directory) command displays the name of the current working directory. This is a shell built-in command.
Read is used to read lines of text from standard input and to assign values of each field in the input line to shell variables for further processing.
Red Hat Package Manager (RPM) is a command-line-driven program capable of installing, uninstalling and managing software packages in GNU/Linux.
Rsync syncs data from one disk or file to another across a network connection. Rsync is similar to rcp, but has more options.
The GNU screen utility is a terminal multiplexer in which a user can use a single terminal window to run multiple terminal applications or windows.
Sdiff finds differences between two files by producing a side-by-side listing indicating lines that are dissimilar. Sdiff then merges the files and outputs results to the outfile.
Sed is a stream editor that is used to filter text in a pipeline, distinguishing it from other editors. Sed takes text input, performs operations on it and outputs the modified text. Sed is typically used to extract part of a file using pattern matching or to substitute multiple occurrences of a string within a file.
Shutdown is a command that turns off the computer and that can be combined with variables such as -h, for halt after shutdown, or -r, for reboot after shutdown.
Snort is an open source network intrusion detection system and packet sniffer that monitors network traffic, looking at each packet to detect dangerous payloads or suspicious anomalies. Snort is based on libpcap.
Used to sort lines of text alphabetically or numerically according to fields; multiple sort keys can also be used.
Sudo allows a system admin to give certain users the ability to run some (or all) commands at the root level and logs all commands and arguments.
SSH is a command interface used for securely gaining access to a remote computer and is used by network admins to control servers remotely.
The tar program provides the ability to create archives from a number of specified files or to extract files from such an archive.
TOP is a set of protocols for networks that performs distributed information processing in offices and displays the tasks on the system that take up the most memory. TOP can sort tasks by CPU usage, memory usage and runtime.
Tr is used to translate or delete characters from a text stream. Tr writes to standard output, but does not accept file names as arguments -- it only accepts inputs from standard input.
Traceroute determines and records a route through the internet between two computers and is useful for troubleshooting network/router issues. If the domain does not work or is not available, an IP can be tracerouted.
Uname displays the name of the current operating system and can print information about the system.
Uniq compares adjacent lines in a file and removes/reports any duplicate lines.
Vi is a text editor that allows a user to control the system by solely using the keyboard instead of a combination of mouse selections and keystrokes.
Vmstat is used to get a snapshot of everything in a system and to report information on such items as processes, memory, paging and CPU activity. This is a good method for admins to use to determine where issues/slowdown in a system may be occurring.
Wc counts the number of words, lines and characters in text files and produces a count for multiple files if several files are selected.
Wget is a network utility that retrieves files from the web that support http, https and ftp protocols. Wget works non-interactively in the background while a user is logged off. This can create local versions of remote websites, re-creating directories of original sites.
Whoami prints or writes the user/login name associated with the current user ID to the standard output.
Xargs reads, builds and executes arguments from standard input; blank lines in the input are ignored.