Operators & Other Characters

Arithmetic operators

Note

The arithmetic operators are listed grouped in order of precedence.

Operator	Description	b/d/k/z
id++ id--	post-increment post-decrement	bdkz
++id --id	pre-increment pre-decrement
+ -	unary plus (positive number) unary minus (negative number)
! ~	logical negation bit-wise negation
**	exponentiation
* / %	multiplication division remainder (modulus)
+ -	addition subtraction
<< >>	bit-wise shift left bit-wise shift right
<= >= < >	comparison less than or equal to comparison greater than or equal to comparison less than comparison greater than
== !=	equality inequality
&	bit-wise AND
^	bit-wise XOR (exclusive or)
|	bit-wise OR
&&	logical AND
||	logical OR
expr?expr:expr	conditional operator
assignment	(see following table)
expr1 , expr2	comma assignment

Arithmetic assignment

Operator	Description
=	Assign expression to variable: variable=expression
*=	Assign product of expression multiplied by variable
/=	Assign dividend
%=	Assign remainer
+=	Assign sum
-=	Assign difference
<<=	assign bit-wise left shift
>>=	assign bit-wise right shift
&=	assign bitwise AND
^=	assign bit-wise XOR exclusive or
|=	assign bit-wise OR

See also

Online documentation at man bash.

Logic Operators

Note

Legend
Test	Column Test indicates which type of logic test this operator applies to. Values might be [, [[ or all.
U/B	The U/B column refers to the type of test: U for unary, or B for binary.

Logical reversal operator

Operator	Test	U/B	Description
!	all	U	“not” – reverses the sense of a logical operation

Compound comparison operators

Operator	Test	U/B	Description
-a	[	B	logical AND within test: [ expr1 -a expr2 ]
-o	[	B	logical OR within test: [ expr1 -o expr2 ]
&&	[[	B	logical AND double-bracket: [[ expr1 && expr2 ]]
||	[[	B	logical OR double-bracket: [[ expr1 || expr2 ]]

Comparison Tests

Generally, comparison tests are of three types: string comparisons, integer comparisons, and file status tests.

String comparisons

Operator	Test	U/B	Description
=	all	B	equal to, literal matching, whitespace req’d
==	[	B	equal to, within test, synonym for =
==	[[	B	equal to, double-bracket, pattern matching, see (2)
!=	[	B	not equal to, within test, whitespace req’d
!=	[[	B	not equal to, double-bracket, pattern matching, (2)
=~	[[	B	matches, double-bracket, pattern matching, see (2)
<	all	B	less than, escape req’d in [, see (1)
>	all	B	greater than, escape req’d in [, see (1)
-z	all	U	null (zero length)
-n	all	U	not null (length > zero), quotes req’d in [

Note

1. Lexicographical sort order for test or [ construct is ASCII; for [[ construct the ordering is determined using the current locale.
2. Construct [[ uses ==, !=, and =~ to match by pattern, with the pattern on the right. See Glob Pattern Matching.

Integer comparisons

Operator	Test	U/B	Description
-eq	[	B	equal to: [ number1 -eq number2 ]
-ne	[	B	not equal to: [ number1 -ne number2 ]
-gt	[	B	greater than: [ number1 -gt number2 ]
-ge	[	B	greater than/equal to: [ number1 -ge number2 ]
-lt	[	B	less than: [ number1 -lt number2 ]
-le	[	B	less than/equal to: [ number1 -le number2 ]
=	[[	B	equal to: (( number1 = expr2 ))
!=	[[	B	not equal to: (( number1 <> expr2 ))
<	[[	B	less than: (( number1 < expr2 ))
<=	[[	B	less than or equal to: (( number1 ,= expr2 ))
>	[[	B	greater than: (( number1 > expr2 ))
>=	[[	B	greater than or equal to: (( number1 >= expr2 ))

File test operators

Operator	Test	U/B	Description
-e -a	all	U U	file exists file exists (deprecated usage)
-f	all	U	a regular file (not a directory or device file)
-s	all	U	file is not zero size
-d	all	U	file is a directory
-b	all	U	file is a block device
-c	all	U	file is a character device
-p	all	U	file is a pipe
-h -L	all all	U U	file is a symbolic link file is a symbolic link
-S	all	U	file is a socket
-t	all	U	file (descriptor) is associated with a terminal. In a script, this test option can check whether the stdin [ -t 0 ] or stdout [ -t 1 ] is a terminal.
-r	all	U	file has read permission (for user running test)
-w	all	U	file has write permission (for user running test)
-x	all	U	file has execute permission (for user running test)
-g	all	U	set-group-id (sgid) flag set on file or directory. When sgid flag is set on a directory, files created in that directory belong to the group that owns the directory, not to the creating user’s group.
-u	all	U	set-user-id (suid) flag set on file
-k	all	U	sticky bit set
-O	all	U	you are owner of file
-G	all	U	group-id of file same as yours
-N	all	U	file modified since it was last read
f1 -nt f2	all	B	file f1 is newer than f2
f1 -ot f2	all	B	file f1 is older than f2
f1 -ef f2	all	B	files f1 and f2 are hard links to the same file

See also

Advanced Bash Scripting Guide

Expression constructs

Construct	Description
(a b ... n) (expr...)	assign an array of n elements: array=(1 2 3 4) group commands (expr1; expr2; ...) to execute in subshell
((math))	double-parentheses math construct: var = var1 + var2
${var}	evaluate a string variable
$(expr)	evaluate command expression (creates subshell)
$((math))	evaluate an integer arithmetic expression (C-style)
`expr`	evaluate a command, original Bourne notation (deprecated)
$[3+4+5]	evaluate an integer math expression (deprecated)
{var} {02..8..2} { expr..; }	brace expansion: echo {1,2,3} shows 1 2 3 expand series starting 02 step 2 with 0 padding code block: inline group commands into local anonymous function
${var:l:n}	at l from left extract n characters negatives start at right and extract by position, not count
${!var}	indirect reference (eval) to value of variable
[ logical ]	builtin shell logical test ([ is the “test” operator)
[[ logic ]]	shell keyword construct for logical test
array[index]	(de)reference an element of an array
[c d e]	delineates a range of characters to match in regular expression
> &> >&2 >> < <>	redirect output (stdout): ls > filename redirect errors, output (stdout, stderr): ls &> filename redirect output to error (stdout > stderr) append output (stdout) to filename: ls >> filename redirect input from expression: filename < ls open file and assign file descriptor: [i]<>filename
|	pipe output (stdout) of one process to input (stdin) of next
<(commands) >(commands)	substitutes output of one process into another process
& && |	at end of command, detach and run command in background linking two commands, run second iff for first, exit -eq 0 linking two commands, run second iff for first, exit -ne 0
- -- --	command option prefix: ls -al verbose option prefix: ls --all --list (builtin) end of options rm -- -badname*

Glob Pattern Matching

Some entries, such as file system paths, may be written as expressions to be expanded by substitution and pattern matching.

Expression	Interpretation
*	any string of 0 or more characters
?	any string of 0 or 1 character
X or \X	where X represents any (special) character
[XYZ]	where XYZ is a set of permitted characters
[x..z]	where x..z is a range of permitted characters

Special characters

Character
“X” ‘X’ \X `X`	partial quoting interprets embedded expressions (quotes) full quoting preserves literal contents of text escape single character to display literal value (like 'X') back-tick quote command to assign output to a variable
!	negates a conditional test result or exit status
:	synonym for true conditional result
;	semicolons separate commands appearing on one line: if [ -x "$filename" ] ; then cp $filename $dir/ ; fi
#	# after a newline or a command separator denotes a comment
;; ;;& ;&	double-semicolons terminate an option in case statements case "$variable" in abc)  echo "\$variable = abc" ;; xyz)  echo "\$variable = xyz" ;; esac
. .filename . or ./ .. /	period by itself indicates a source period as a filename prefix indicates a hidden file directory name . represents the working directory PWD directory .. represents the parent directory of PWD file path directory separator (forward slash)
* *text ** ,	return all visible files in the current directory return all files ending in “text” in current directory recursively return files in directory tree (shopt -s globstar) concatenate string results: for file in /{,usr/}bin/*calc
~ ~+ ~-	user home directory path: corresponds to $HOME present working directory: corresponds to $PWD previous working directory: corresponds to $OLDPWD
^ ^^ , ,,	parameter substitution: echo ${var^} uppercase first char parameter substitution: echo ${var^^} uppercase string parameter substitution: echo ${var,} lowercase first char parameter substitution: echo ${var,,} lowercase string
?	in globbing and regex, represents a single wild-card character operator in double-parentheses construct, parameter substitution
$ ${} $’ ..’ $n “$*” “$@” $? $$	in a regular expression, $ represents end of line of text in parameter substitution, evaluates a variable or expression in quoted string, expands octal/hex values into ASCII/Unicode the n-th positional parameter input to a command or script all positional parameters, seen together as single word list of positional parameters, each one quoted exit status of a command, function, or script process id of running script

See also

Shell scripting special characters.