Linux Command Quick Reference

The following list describes some of the most useful and popular Linux commands. Consult the man page for each command to learn about additional arguments and details of operation.

adduser userid

Creates a new userid, prompting for necessary information (requires root privileges).

apropos keyword

Searches the manual pages for occurrences of the specified keyword and prints short descriptions from the beginning of matching manual pages.

at time

at -f file time

Executes commands entered via stdin (or, by using the alternative form, the specified file) at the specified time. The time can be specified in a variety of ways; for example, in hour and minute format hh: mm or in hour, minute, month, day, and year format hh: mm mm/ dd/ yy.

atq

Prints descriptions of jobs pending via the at command.

atrm job

Cancels execution of a job scheduled via the at command. Use the atq command to discover the identities of scheduled jobs.

bg

bg jobs

Places the current job (or, by using the alternative form, the specified jobs) in the background, suspending its execution so that a new user prompt appears immediately. Use the jobs command to discover the identities of background jobs.

cal month year

Prints a calendar for the specified month of the specified year.

cat files

Prints the contents of the specified files.

cd

cd directory

Changes the current working directory to the user's home directory or the specified directory.

chgrp group files

chgrp -R group files

Changes the group of the specified files to the specified group. The alternative form of the command operates recursively, changing the group of subdirectories and files beneath a specified directory. The group must be named in the /etc/groups file, maintained by the newgroup command.

chmod mode files

chmod -R mode files

Changes the access mode of the specified files to the specified mode. The alternative form of the command operates recursively, changing the mode of subdirectories and files beneath a specified directory.

chown userid files

chown -R userid files

Changes the owner of the specified files to the specified userid. The alternative form of the command operates recursively, changing the owner of subdirectories and files beneath a specified directory

clear

Clears the terminal screen.

cmp file1 file2

Compares two files, reporting all discrepancies. Similar to the diff command, though the output format differs.

cp file1 file2

cp files directory

cp -R files directory

Copies a file to another file or directory, or copies a subdirectory and all its files to another directory.

date

date date

Displays the current date and time or changes the system date and time to the specified value, of the form MMddhhmmyy or MMddhhmmyyyy.

df

Prints the amount of free disk space on each mounted filesystem.

diff file1 file2

Compares two files, reporting all discrepancies. Similar to the cmp command, though the output format differs.

dmesg

Prints the messages resulting from the most recent system boot.

du

du directories

Prints the amount of disk space used by the current directory (or the specified directories) and its (their) subdirectories.

echo string

echo -n string

Prints the specified text on the standard output stream. The -n option causes omission of the trailing newline character.

fdformat device

Formats the media inserted in the specified floppy disk drive. The command performs a low-level format only; it does not create a filesystem. To create a filesystem, issue the mkfs command after formatting the media.

fdisk device

Edits the partition table of the specified hard disk.

fg

fg jobs

Brings the current job (or the specified jobs) to the foreground.

file files

Determines and prints a description of the type of each specified file.

find path -name pattern -print

Searches the specified path for files with names matching the specified pattern (usually enclosed in single quotes) and prints their names. The find command has many other arguments and functions; see the online documentation.

finger users

Prints descriptions of the specified users.

free

Displays the amount of used and free system memory.

ftp hostname

Opens an FTP connection to the specified host, allowing files to be transferred. The FTP program provides subcommands for accomplishing file transfers; see the online documentation.

grep pattern files

grep -i pattern files

grep -n pattern files

grep -v pattern files

Search the specified files for text matching the specified pattern (usually enclosed in single quotes) and print matching lines. The -i option specifies that matching is performed without regard to case. The -n option specifies that each line of output is preceded by the file name and line number. The -v option reverses the matching, causing non-matched lines to be printed.

gzip files

gunzip files

Compress (or expand) the specified files. Generally, a compressed file has the same name as the original file, followed by . gz.

head files

Prints the first several lines of each specified file.

hostname

hostname name

Displays (or sets) the name of the host.

info

Launches the GNU Texinfo help system.

init run_level

Changes the system run level to the specified value (requires root privileges).

insmod module

Dynamically loads the specified module (requires root privileges).

jobs

Displays all background jobs.

ispell files

Checks the spelling of the contents of the specified files.

kill process_ids

kill - signal process_ids

kill -l

Kills the specified processes, sends the specified processes the specified signal (given as a number or name), or prints a list of available signals.

killall program

killall - signal program

Kills all processes that are instances of the specified program or sends the specified signal to all processes that are instances of the specified program.

ln old new

ln -s old new

Creates a hard (or soft) link associating a new name with an existing file or directory.

locate pattern

Locates files with names containing the specified pattern. Uses the database maintained by the updatedb command.

lpq

Prints the entries of the print queue.

lpr files

Prints the specified files.

lprm job

Cancels printing of the specified print queue entries. Use lpq to determine the contents of the print queue.

ls

ls files

ls -a files

ls -l files

ls -lR files

Lists (non-hidden) files in the current directory or the specified files or directories. The -a option lists hidden files as well has non-hidden files. The -l option causes the list to include descriptive information, such as file size and modification date. The -R option recursively lists the subdirectories of the specified directories.

mail

Launches a simple mail client that permits sending and receiving email messages.

man title

man section title

Prints the specified man page.

mkdir directories

mkdir -p directories

Creates the specified directories. The -p option causes creation of any parent directories needed to create a specified directory.

mkfs -t type device

Creates a file system of the specified type (such as ext2 or msdos) on the specified device (requires root privileges).

mkswap device

Creates a Linux swap space on the specified hard disk partition (requires root privileges).

more file

Lets the user peruse a file too large to be displayed as a single screen (page) of output. The more command provides many subcommands that let the user navigate the file. For example, the Space key moves forward one page, the b key moves back one page, and the q key exits the program.

mount

mount device directory

mount -o option -t type device directory

Prints the mounted devices or mounts the specified device at the specified mount point (generally a subdirectory of /mnt). The mount command consults /etc/fstab to determine standard options associated with a device. The command generally requires root privileges. The -o option allows specification of a variety of options; for example, ro for read-only access. The -t option allows specification of the filesystem type (for example, ext2, msdos, or iso9660, the filesystem type generally used for CD-ROMs).

mv paths target

Moves the specified files or directories to the specified target.

newgroup group

Creates the specified group.

passwd

passwd user

Changes the current user's password, or that of the specified user (requires root privileges). The command prompts for the new password.

ping host

Sends an echo request via TCP/IP to the specified host. A response confirms that the host is operational.

pr files

Formats the specified files for printing, by inserting page breaks and so on. The command provides many arguments and functions.

ps

ps -Aux

Displays the processes associated with the current userid or displays a description of each process.

pwd

Prints the absolute path corresponding to the current working directory.

reboot

Reboots the system (requires root privileges).

reset

Clears the terminal screen and resets the terminal status.

rm files

rm -i files

rm -f files

rm -if files

rm -rf files

Deletes the specified files or (when the -r option is specified) recursively deletes all subdirectories of the specified files and directories. The -i option causes the command to prompt for confirmation; the -f option suppresses confirmation. Because deleted files cannot generally be recovered, the -f option should be used only with extreme care, particularly when used by the root user.

rmdir directories

rmdir -p directories

Deletes the specified empty directories or (when the -p option is specified) the empty directories along the specified path.

shutdown minutes

shutdown -r minutes

Shuts down the system after the specified number of minutes elapses (requires root privileges). The -r option causes the system to be rebooted once it has shut down.

sleep time

Causes the command interpreter to pause for the specified number of seconds.

sort files

Sorts the specified files. The command has many useful arguments; see the online documentation.

split file

Splits a file into several smaller files. The command has many arguments; see the online documentation.

su

su user

su -

su - user

Changes the current userid to root or to the specified userid (the latter requires root privileges). The - option establishes a default environment for the new userid.

swapon device

Enables use of the specified device for swapping (requires root privileges).

swapoff device

Disables use of the specified device for swapping (requires root privileges).

sync

Completes all pending input/output operations (requires root privileges).

tail file

tail - n file

tail -f file

Prints the last several lines of the specified files. The -n option specifies the number of lines to be printed. The -f option causes the command to continuously print additional lines as they are written to the file.

talk user

Launches a program that allows a chat-like dialog with the specified user.

tar cvf tar_file files

tar zcvf tar_file files

Creates a tar file with the specified name, containing the specified files and their subdirectories. The z option specified that the tar file will be compressed.

tar xvf tar_file

tar zxvf tar_file

Extracts the contents of the specified tar file. The z option specified that the tar file has been compressed.

telnet host

Opens a login session on the specified host.

top

Prints a display of system processes that's continually updated until the user presses the q key.

traceroute host

Uses echo requests to determine and print a network path to the host.

umount device

Unmounts the specified filesystem (generally requires root privileges).

uptime

Prints the system uptime.

w

Prints the current system users.

wall

Prints a message to each user except those who've disabled message reception. Type Ctrl-D to end the message.

wc files

Prints the number of characters, words, and lines in the specified files.

Table E.1 identifies Linux commands that perform functions similar to MS-DOS commands. The operation of the Linux command is not generally identical to that of the corresponding MS-DOS command. See the index to this book or the Linux online documentation for further information about Linux commands.

Table E.1: MS-DOS Commands and Related Linux Commands

MS-DOS	Linux
ATTRIB	chmod
CD	cd
CHKDSK	df, du
DELTREE	rm -R
DIR	ls -l
DOSKEY	(built-in; no need to launch separately)
EDIT	ae, vi, and so on
EXTRACT	tar
FC	cmp, diff
FDISK	fdisk
FIND	grep
FORMAT	fdformat
MORE	more
MOVE	mv
SORT	sort
START	at, bg
XCOPY, XCOPY32	cp