UNIX Commands 
(Remember: RTFMP)

Command		Purpose				

alias	Creates an alias for a command					
awk	Start the awk program to select and format output		
cal	Show the system calendar							
cat	Create or display or concatenate files						
cd	Change directories							
chmod	Set file or directory permissions					
clear	Clear the screen (terminal window)						
comm	Compare two files, output common lines				
cp	Copy a file								
cut	Extract specified columns or fields from a file			
date	Display the system date and time					
df	Report information about file systems				
diff	Compare two files, output lines that differ			
du	Report disk usage of current/subdirectories			
echo	Copy to stdout							
exit	Exit UNIX (also CTRL-D)						
export	Makes a local variable an environment variable			
find	Search in a directory and all its subdirectories			
gawk	Gnu awk—see awk
grep	Search for a string of characters in a text file			
head	Display the first few lines of a file					
kill	terminate one or more process ids				
less	Display a long file one screen at a time, scroll up & down	
logout	Exit UNIX (also CTRL-D)						
lpr	Print files								
ls	Display directory contents						
mail 	Send email from the command line or read mail
man	Display the online manual for a command			
mkdir	Make a new directory						
more	Display a long file one screen at a time, scroll down		
mount	Connect filesystems to a directory tree mount point		
mv	Move/rename files							
nawk	New awk—see awk
passwd	Change password							
paste	Combine fields from two or more files				
pr	Format a file for printing to stdout					
printenv Prints a list of environment variables to stdout (see set)	
pwd	Print working (current) directory					
rm	Remove (delete) file							
rmdir	Remove an empty directory					
sed	Apply editor commands to a (usually large) file			
set	Set environment variables or print to stdout (see printenv)	
set -o noclobber	Prevents files from being overwritten by ">"	
sh	Execute a shell script						
sleep	Wait for a specified number of seconds
sort	Sort a file								
tail	Display the last few lines of a file					
tee	Accept stdin and send it to both stdout and a file
touch	Update an existing file’s date/time stamp or create empty file	
tr	Translate characters						
trap	Executes a command on receipt of a signal from UNIX	 
umount	Disconnect filesystems from a directory tree mount point	
unique	Remove adjacent duplicate lines					
vi	Start the vi editor							
wc	Count the number of lines, words, or bytes in a file		
whatis	Display a brief description of a command			
who	See who is logged in						
xargs	Execute a command on multiple strings from stdin


The Channels of a UNIX Command:

stdin (0)  ->  command  ->  stdout (1) and stderr (2)

The standard out of one command can be piped ( | ) into standard in of 
another command. For example:

ls -al | grep joe	lists all the files in the current directory 
and searches for the string "joe" in the filenames and only prints 
those that match..

ls | xargs -i grep joe {}	
searches for the string "joe" within the body of all files in the 
current directory, illustrating the use of pipe and the xargs command.

find . –size +100000c –print > big1s 2>&1	
Finds all files in this and subordinate directories that are larger 
than 100,000 bytes, sending both stdout and stderr to file "big1s".