This cheatsheet deals with commonly used Linux commands executed from the command line.  Most or all of these commands should work using any common unix shell interface, but this is written from the point of view of a bash user, so if there's anything here that doesn't work in another shell you will at least know why.  Because this is not intended to give comprehensive instructions on all commonly available commands, but only on those deemed most commonly important by the author, this will never be a comprehensive list.  Some commands herein may require that a given package needs to be installed on your system if it is not already present.  Commands that open captive interfaces requiring their own instructions are not included here, and may be (or become) the subject of other cheatsheets.

apt-cache search [string]
	search for packages matching string

apt-cache show [package]
	display summary detail of specified package

apt-get autoclean
	clean out cruft in local cache of packages

apt-get install [package]
	download and install package and dependencies

apt-get update
	download updated package list for local cache

apt-get upgrade
	upgrade installed packages to newest versions

aptitude [search|show|autoclean|install|update|upgrade]
	same purpose as apt-cache and apt-get, unified

cal [month] [yyyy]
	output formatted calendar: optional month and year arguments

cat [filename]
	output the contents of (a) file(s)

cd [destination]
	change directory

cdrecord dev=[path] [filename]
	commonly used to burn CD from image

chgrp [group] [filename]
	change group ownership of a file

chmod [nnn] [filename]
	change access permissions

chown [user] [filename]
	change ownership characteristics of a file

clear
	clears the screen

date
	display the current date and time

dd if=[inputfile] of=[outputfile]
	commonly used to make bootable floppy from image

df -h
	display disk usage by partition

du -h
	display disk usage by directory

echo [string]
	print the string to STDOUT

exit
	log out

grep [string]
	input to grep is filtered for a search string

gunzip [filename]
	unzip a gzip file

gzip [filename]
	zip a gzip file

head [filename]
	display the first ten lines of a file

id
	output user and group names and IDs

less [filename]
	display filename in a manner that allows scrolling and paging
	q=exit, ctrl-d=pagedown, ctrl-u=pageup, arrows=scroll

locate [string]
	list all files that match a search string

ls
	list directory contents

ls -a
	list all directory contents, including hidden

ls -al
	list all directory contents in long format

ls -l
	list directory contents in long format

man [command|file]
	display manpage for specified command or file

man -f [command|file]
	display short description from manpage if it exists

man [n] [command|file]
	display manpage number n for specified command or file
	"man 7 man" displays man(7)

mkdir [directoryname]
	create a new directory

mount
	displays currently mounted partitions

mount [device] [directory]
	mounts specified device in indicated directory

mv [filename] [path|filename]
	move and/or rename a file or directory

passwd
	change current user's password

ping [host|ip]
	ping at regular intervals

ping -c [n] [host|ip]
	ping n times then exit

ps aux
	list running processes

pwd
	path to working directory

rm [filename]
	remove/delete a file

rm -r [directoryname]
	remove/delete a directory and all its contents

rmdir [directoryname]
	remove/delete a directory

sftp [host|ip]
	ftp over ssh

sftp -oPort=[port] [host|ip]
	ftp over ssh on nonstandard port

ssh [host|ip]
	connect to an ssh server

ssh -p [port] [host|ip]
	connect to an ssh server on nonstandard port

tail [filename]
	display last ten lines of a file

tar -czf [archivename].tar.gz [sourcefilename]
	create a gzipped tarball; more than one filename may be specified

tar -zxf [filename]
	expand the contents of a gzipped tarball (.tar.gz file)

wc [filename]
	get line, word, and character count of a file

yafc [host|ip]
	yet another ftp client