Regular Expression in Shell

Command	Description	Example
&	Run the previous command in the background	ls &
&&	Logical AND	if [ "$foo" -ge "0" ] && [ "$foo" -le "9"]
||	Logical OR	if [ "$foo" -lt "0" ] || [ "$foo" -gt "9" ] (not in Bourne shell)
^	Start of line	grep "^foo"
$	End of line	grep "foo$"
=	String equality (cf. -eq)	if [ "$foo" = "bar" ]
!	Logical NOT	if [ "$foo" != "bar" ]
$$	PID of current shell	echo "my PID = $$"
$!	PID of last background command	ls & echo "PID of ls = $!"
$?	exit status of last command	ls ; echo "ls returned code $?"
$0	Name of current command (as called)	echo "I am $0"
$1	Name of current command’s first parameter	echo "My first argument is $1"
$9	Name of current command’s ninth parameter	echo "My ninth argument is $9"
$@	All of current command’s parameters (preserving whitespace and quoting)	echo "My arguments are $@"
$*	All of current command’s parameters (not preserving whitespace and quoting)	echo "My arguments are $*"
-eq	Numeric Equality	if [ "$foo" -eq "9" ]
-ne	Numeric Inquality	if [ "$foo" -ne "9" ]
-lt	Less Than	if [ "$foo" -lt "9" ]
-le	Less Than or Equal	if [ "$foo" -le "9" ]
-gt	Greater Than	if [ "$foo" -gt "9" ]
-ge	Greater Than or Equal	if [ "$foo" -ge "9" ]
-z	String is zero length	if [ -z "$foo" ]
-n	String is not zero length	if [ -n "$foo" ]
-nt	Newer Than	if [ "$file1" -nt "$file2" ]
-d	Is a Directory	if [ -d /bin ]
-f	Is a File	if [ -f /bin/ls ]
-r	Is a readable file	if [ -r /bin/ls ]
-w	Is a writable file	if [ -w /bin/ls ]
-x	Is an executable file	if [ -x /bin/ls ]
parenthesis: ( … )	Function definition	function myfunc() { echo hello }