Here is a list of most used / most discussed / most important *nix commands. Mastering this will surely get your confidence level high in the black background and white characters’ world !!

My experience says that for starters in UNiX / LiNuX , these commands are enough to get you in smoothly !

clear Clears the terminal window

usage: [root@test~]$ clear

ls List contents of a directory

usage: [root@test~]$ ls
usage: [root@test~]$ ls  -l

cat Displays the contents of a file in the terminal

usage: [root@test~]$ cat filename.extension

rm Removes a file

usage: [root@test~]$ rm filename.extension
usage: [root@test~]$ rm -i filename.extension  [prompts for confirmation before removing a file]
usage: [root@test~]$ rm -f filename.extension  [force removal of the file regardless of it bieng write-protected or open]
usage: [root@test~]$ rm -r directory		   [remove files recursively in 'directory']

cp Copies a file

usage: [root@test~]$ cp file1.ext file2.ext
usage: [root@test~]$ cp file1.ext /some/other/location/file2.ext
usage: [root@test~]$ cp file1.ext user@some.other.server:~/some/other/location/file2.ext

passwd Changes password

usage: [root@test~]$ passwd
usage: [root@test~]$ passwd SomeNewUser [root access only]

motd Message of the Day

usage: [root@test~]$ motd

finger User information lookup program

usage: [root@test~]$ finger   -p
Login     Name       Tty      Idle  Login Time   Office     Office Phone
xuser              pts/0          Oct 22 22:04 (ool-4578671d.dyn.optonline.net)

startx Starts an X Window System server

usage: [root@test~]$  startx -dpi 300 -depth 32

less Displays the contents of a file in the terminal one page at a time

usage: [root@test~]$ less filename.ext
usage: [root@test~]$ less +G filename.ext [start from end]

info Displays information and documentation on shells, utilities and programs

usage: [root@test~]$ info
usage: [root@test~]$ info less

lpr Sends file to printer

usage: [root@test~]$ lpr filename.ext
usage: [root@test~]$ lpr -# 3 filename.ext [3 copies]

grep looks through files for strings

usage: [root@test~]$ grep "needle" haystack.ext
usage: [root@test~]$ grep -i "neEdlE" haystack.ext
usage: [root@test~]$ grep -i -r "neEdlE" directory/

head Displays first 10 lines of file

usage: [root@test~]$ head filename.ext

tail Displays last 10 lines of file

usage: [root@test~]$ tail filename.log
usage: [root@test~]$ tail -f filename.log [don't exit // realtime]

mv Moves or renames file

usage: [root@test~]$ mv filename.ext /some/other/directory/
usage: [root@test~]$ mv filename.ext newname.ext [rename]

file Displays information about file contents

usage: [root@test~]$ file program.php
program.php: PHP script text

usage: [root@test~]$ file archive.tar.gz
archive.tar.gz: gzip compressed data, from Unix

echo Copies string to terminal

usage: [root@test~]$  echo Hello Think-Lamp
usage: [root@test~]$  echo * | wc

date Displays current date and time

usage: [root@test~]$ date
usage: [root@test~]$ date -R

cal Displays calendar

usage: [root@test~]$ cal
usage: [root@test~]$ cal -y 2009

gzip Compresses a file

usage: [root@test~]$ gzip filename.etx > archive.gz
usage: [root@test~]$ gzip -f -q filename.etx  > archive.gz[force and supress errors]
usage: [root@test~]$ gzip -f -q -r directory/ > archive.gz

gunzip Decompresses a compressed file

usage: [root@test~]$ gunzip archive.gz

which Displays path to command

usage: [root@test~]$ which echo
usage: [root@test~]$ which less

whereis Displays paths to locations of commands

usage: [root@test~]$ whereis echo
usage: [root@test~]$ whereis less

who Lists currently logged on users

usage: [root@test~]$ who
usage: [root@test~]$ who -b

w Lists currently logged on users with processing usage

usage: [root@test~]$ w

mesg Sets options for letting other users write you messages

usage: [root@test~]$ mesg

write Sends message to other users

usage: [root@test~]$ write xuser
hello! This is test message

usage: [xuser@test~]$
Message from [root@test~]$ on pts/1 at 13:25 ...
hello! This is test message

talk Allows two way chat to other users

usage: [root@test~]$ talk xuser@test

usage: [xuser@test~]$
Message from root@test
talk: connection requested by root@test.
talk: respond with: talk xuser@test

chmod Changes file access permissions

usage: [root@test~]$ chmod 775 file.ext
usage: [root@test~]$ chmod -r 777 directory/

CHMOD can also to attributed by using Numeric Permissions:

400 read by owner
040 read by group
004 read by anybody (other)
200 write by owner
020 write by group
002 write by anybody
100 execute by owner
010 execute by group
001 execute by anybody
____
777  read/write/execute by everybody !

mkdir Makes a directory

usage: [root@test~]$ mkdir newdirectory

rmdir Removes an empty directory

usage: [root@test~]$ rmdir newdirectory

ln Creates link to an existing file [hard, soft]

usage: (hard link) [root@test~]$ ln targetfile.ext linkname.ext
usage: (soft link) [root@test~]$ ln -s targetfile.ext linkname.ext

df Displays all mounted filesystems

usage: [root@test~]$ df
usage: [root@test~]$ df -h

top Displays updating list of currently running processes

usage: [root@test~] top
# Once in  TOP screen, use shift+F to select 'sort by' field.
# Once in Top screen, use Shift+R to 'sort' ascending OR descending

tty Displays the name of the terminal in which the command was issued

usage: [root@test~] tty

kill Aborts a process by PID (Process Identification Number) or job number

usage: [root@test~] kill -p 2345

jobs Displays a list of current jobs

usage: [root@test~] jobs
usage: [root@test~] jobs -l

netstat Displays network connections

usage: [root@test~] netstat -a -e