Appendix A: Linux Commands

This appendix presents alphabetically arranged reference entries for the most important Linux commands. The goal here is to provide you with an overview of all commands needed to manage files and directories, start and stop processes, find files, work with text files, and access online help.

If you are looking for a command for a specific task, but don’t know which command to use, you may find it helpful to browse through the commands by category. Table A-1 shows the Linux commands organized by categories.

Table A-1: Linux Commands
Command Name	Action
Getting Online Help
apropos	Finds man pages for a specified keyword
info	Displays online help information about a specified command
man	Displays online help information
whatis	Similar to apropos, but searches for complete words only
Making Commands Easier
alias	Defines an abbreviation for a long command
type	Shows the type and location of a command
unalias	Deletes an abbreviation defined using alias
Managing Files and Directories
cd	Changes the current directory
chattr	Changes file attributes in Linux ext2 and ext3 file systems
chgrp	Changes group ownership of file
chmod	Changes file permissions
chown	Changes file owner and group
cp	Copies files
ln	Creates symbolic links to files and directories
ls	Displays the contents of a directory
mkdir	Creates a directory
mv	Renames a file as well as moves a file from one directory to another
rm	Deletes files
rmdir	Deletes directories
pwd	Displays the current directory
touch	Updates a file’s time stamp
Finding Files
find	Finds files based on specified criteria such as name, size, and so on
lsof	List open files
locate	Finds files using a periodically updated database
whereis	Finds files based in the typical directories where executable (also known as binary) files are located
which	Finds files in the directories listed in the PATH environment variable
Processing Files
cat	Displays a file on standard output (can be used to concatenate several files into one big file)
cut	Extracts specified sections from each line of text in a file
dd	Copies blocks of data from one file to another (used to copy data from devices)
diff	Compares two text files and finds any differences
expand	Converts all tabs into spaces
file	Displays the type of data in a file
fold	Wraps each line of text to fit a specified width
grep	Searches for regular expressions within a text file
less	Displays a text file, one page at a time (can go backwards also)
lpr	Prints files
more	Displays a text file, one page at a time (goes forward only)
nl	Numbers all nonblank lines in a text file and prints the lines to standard output
paste	Concatenates corresponding lines from several files
patch	Updates a text file using the differences between the original and revised copy of the file
sed	Copies a file to standard output while applying specified editing commands
sort	Sorts lines in a text file
split	Breaks up a file into several smaller files with specified size
tac	Reverses a file (last line first and so on)
tail	Displays the last few lines of a file
tr	Substitutes one group of characters for another throughout a file
uniq	Eliminates duplicate lines from a text file
wc	Counts the number of lines, words, and characters in a text file
zcat	Displays a compressed file (after decompressing)
zless	Displays a compressed file one page at a time (can go backwards also)
zmore	Displays a compressed file one page at a time
Archiving and Compressing Files
compress	Compresses files
cpio	Copies files to and from an archive
gunzip	Decompresses files compressed with GNU ZIP (gzip) or compress
gzip	Compresses files (more powerful than compress)
tar	Creates an archive of files in one or more directories (originally meant for archiving on tape)
uncompress	Decompresses files compressed with compress
Managing Processes
bg	Runs an interrupted process in the background
fg	Runs a process in the foreground
free	Displays the amount of free and used memory in the system
halt	Shuts down Linux and halts the computer
kill	Sends a signal to a process (usually used to terminate a process)
ldd	Displays the shared libraries needed to run a program
nice	Runs a process with lower priority (referred to as nice mode)
ps	Displays a list of currently running processes
printenv	Displays the current environment variables
pstree	Similar to ps, but shows parent-child relationships clearly
reboot	Stops Linux and then restarts the computer
shutdown	Shuts down Linux
top	Displays a list of most processor- and memory-intensive processes
uname	Displays information about the system and the Linux kernel
Managing Users
chsh	Changes the shell (command interpreter)
groups	Prints the list of groups that includes a specified user
id	Displays the user and group ID for a specified user name
passwd	Changes the password
su	Starts a new shell as another user or root (when invoked without any argument)
Managing the File System
df	Summarizes free and available space in all mounted storage devices
du	Displays disk usage information
fdformat	Formats a diskette
fdisk	Partitions a hard disk
fsck	Checks and repairs a file system
mkfs	Creates a new file system
mknod	Creates a device file
mkswap	Creates a swap space for Linux in a file or a disk partition
mount	Mounts a device (for example, the CD-ROM) on a directory in the file system
swapoff	Deactivates a swap space
swapon	Activates a swap space
sync	Writes buffered data to files
tty	Displays the device name for the current terminal
umount	Unmounts a device from the file system
Working with Date and Time
cal	Displays a calendar for a specified month or year
date	Shows the current date and time or sets a new date and time

The rest of this appendix covers individual reference entries for each command shown in Table A-1. Each reference entry is organized as follows:

Purpose tells you when to use the command.
Syntax shows the syntax of the command with a few common options. Also, typical option values are shown. All optional items are shown in square brackets.
Options lists most options, along with a brief description of each option. For many commands, you will find all options listed in this section. However, some commands have too many options to cover here. For those commands, I describe the most commonly used options.
Description describes the command and provides more details about how to use it.

alias

Purpose

Define an abbreviation for a long command or view the current list of abbreviations.

Syntax

alias [abbrev=command]

Options

None

Description

If you type alias alone, you get a listing of all currently defined abbreviations. Typically, you use alias to define easy-to-remember abbreviations for longer commands. For example, if you type ls -l often, you might add a line with alias ll=’ls -l’ in the .bashrc file in your home directory. Then, you can type ll instead of ls -l to see a detailed listing of a directory. alias is a built-in command of the bash shell and is described in Chapter 7.