E. 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`