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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.
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.
