Command Line Interpreters: POSIX Shell, Cmd.exe, PowerShell

a side-by-side reference sheet

grammar and invocation | variables and expression | arithmetic and logic | strings | regexes | dates and time | resizable arrays | functions | execution control | exceptions | streams | files | directories | processes and environment | libraries and namespaces | reflection | debugging and profiling

posix shell cmd.exe powershell
version used
dash; POSIX 2008 6.3 4.0
show version
displayed at startup $host.version
grammar and invocation
posix shell cmd.exe powershell
$ dash when foo.bat is in the search path:
> foo
PS> .\foo.ps1

DOS> powershell -file foo.ps1
$ dash > cmd > powershell
execute command and exit $ dash -c 'echo hi' > cmd /c "echo hi" > powershell -command 'write-output "hi"'
statement separator pipelines separated by
; & && ||

lists of pipelines separated by newlines unless newline is preceded by a backslash or inside these characters:
"" '' `` ()
pipelines separated by & && ||

lists of pipelines separated by newline unless preceded by caret ^ which is not inside double quote "
; or newline; a newline can be escaped with a backtick: ` newlines are permitted in double quotes and after a pipe: |
word separators | & ; ( ) < > space tab | & < > space tab
escape special character # write "foo" to foo.txt:
echo foo > foo.txt

# write "foo > foo.txt" to stdout:
echo foo \> foo.txt
rem write "foo" to foo.txt
echo foo > foo.txt

rem write "foo > foo.txt" to stdout:
echo foo ^> foo.txt
block delimiters {}
do done
( ) { }
are expressions statements no no

relational expressions can only be used in the conditional of an
if statement

arithmetic expressions can only be assigned to a variable using
set /a
end-of-line comment
# comment rem comment

:: comment
# comment
comment out multiple lines <<EOF
another comment
goto comment
another comment
<# comment
another comment #>
variables and expressions
posix shell cmd.exe powershell
assignment a=1
whitespace next to = not permitted
set a=1 $a = 1
parallel assignment
none none $a, $b = 1, 2
swap tmp=$a
set tmp=%a%
set a=%b%
set b=%tmp%
$a, $b = $b, $a
compound assignment operators: arithmetic, string, bit none += -= *= /= %=
«= »= &= |= ^=
+= -= *= /= %=
+= *=
increment and decrement none none $x = 1
variable declaration
assignment, export, readonly assignment assignment
are identifiers case sensitive? yes no no
'' Variables must contain a non-empty string. This deletes the variable x:
set x=
null test
if [ -z $v ]
  echo not defined
if not defined v echo not defined $v -eq $null
arithmetic and logic
posix shell cmd.exe powershell
true and false
status codes:
true false

inside [ ]:
1 ''
no booleans; conditional of if statement must contain a relational expression $true $false
falsehoods status codes:
nonzero integers

inside [ ]:
relational expressions which evaluate to false 0 0.0 "" ''
logical operators
status codes:
&& || !

inside [ ]:
-a -o !
short-circuit operators:
&& ||

inside conditional of if:
-and -or -not
conditional expression
$(( x>0 ? x : -x )) none none
relational expression
[ $a -gt 3 ] %a% gtr 3 $a -gt 3
relational operators
-eq -ne -gt -lt -ge -le

= != > < none none
equ neq gtr lss geq leq -eq -ne -gt -lt -ge -le
arithmetic expression
$(( 1 + 3 )) arithmetic expression must be stored in a variable:
set /a "foo=1+3"
1 + 3
arithmetic operators
+ - * none / % **

operators are integer-only in most shells; use bc for float arithmetic
+ - * none / % none

operators only work on integers
+ - * / ?? % ??
integer division
$(( $a / $b )) set /a "result=7/3" $rem = $null
$quot = [Math]::DivRem($a, $b, [ref] $rem)
integer division by zero writes "division by 0" error message; statement terminates with a 1 status Writes "Divide by zero error." and sets %errorlevel% to nonzero value. error: Attempted to divide by zero
float division
`echo " scale=5; $a / $b " | bc` none $a / $b
float division by zero writes "division by 0" error message; statement terminates with a 1 status none evaluates to Infinity which is not a float literal
none [Math]::pow(2, 32)
none [Math]::sqrt(2)
sqrt -2
no sqrt none evaluates to NaN which is not a float literal
transcendental functions e l s c none none none a none
how to use:
`echo 'e(2)' | bc -l`
none [Math]::exp [Math]::log
[Math]::sin [Math]::cos [Math]::tan
[Math]::asin [Math]::acos [Math]::atan
float truncation
round towards zero, round to nearest integer, round down, round up
none and no floats none [Math]::truncate(3.14)
absolute value
and signum
none [Math]::abs(-7)
integer overflow
modular arithmetic sometimes modular arithmetic; sometimes writes error message and sets %errorlevel% to nonzero value. converts to float
float overflow
no floats none evaluates to Infinity which is not a float literal
random integer, uniform float echo $RANDOM 15 bit integer rem integer in range 0 to 32767:
echo %random%
random 100
random 1.0
seed random numbers RANDOM=17
none $r = random -setseed 17
bit operators
<< >> & | ^ ~ use with set /a:
<< >> & | ^ ~
none none -band -bor -bxor -bnot

# powershell 3.0:
-lsl -lsr
posix shell cmd.exe powershell
string literal 'don'\''t say "no"'
"don't say \"no\""
$'don\'t say "no"'
None; barewords are used for strings.

Double quotes are used for file names which contain spaces and other special characters. However, the double quotes are stored in the variables and passed to commands.
'don''t say "no"'
"don't say `"no`""
newline in literal yes No; a bareword string can be continued on the following line by ending a line with ^; the CRLF is not part of the value of the string; it is not possible to store a CRLF in a variable. yes
barewords yes yes yes
bareword escape character backslash: \; caret: ^<

Characters special to the interpreter can be stored in a variable by preceding them by a caret: ^; the interpreter will attempt to interpret them when the variable is dereferenced, however.
backquote:: `$
escapes gray|in double quotes##
\\ \"
in $' ' quotes:
\a \b \e \f \n \r \t \v \\ \' \cc \xhh \ooo
`' `" ``
`0 `a `b `f `n `r `t `v
in other backtick sequences the backtick is ignored
variable interpolation count=3
"$count ${item}s"
$count = 3
$item = "ball"
"$count $($item)s"
length s="hello"
see footnote $s = "hello"
string comparison [ $USER = foo ]
[ $USER != foo ]
# case insensitive:
-eq -ne -gt -lt -ge -le

# case sensitive:
-ceq -cne -cgt -clt -cge -cle
index of substring none returns -1 if not found:
"foo bar".indexof("bar")
extract substring s="foo bar"
set s=foo bar
echo %s:~4:3%
"foo bar".substring(4, 3)
string concatenation c="hello, ""world" rem trailing whitespace is stored:
set part1=hello, 
set part2=world
echo %part1%%part2%
$c = "hello, " + "world"
string replication $hbar = "-" * 80
split none "foo,bar,baz" -split ","
join none @("foo","bar","baz") -join ","
sprintf `printf "tie: %s %d %f" "Spain" 13 3.7` $a = "Spain", 13, 3.7
"tie: {0} {1} {2}" -f $a
case manipulation echo "hello" | tr [a-z] [A-Z]
echo "HELLO" | tr [A-Z] [a-z]
echo -n ${A:0:1} | tr [a-z] [A-Z]; echo -n ${A:1}
strip none " hello ".trim()
pad on right, pad on left `printf "%-10s" "hello"`
`printf "%10s" "hello"`
$s = "hello"
$s + " " * (10 - $s.length)
" " * (10 - $s.length) + $s
string to number A="12"
$(( 7 + $A ))

`echo 73.9 + $B | bc`
set x="12"
set /a "result=7+%x%"
7 + "12"

73.9 + ".037"
number to string
all values are strings all values are strings [convert]::tostring(7) + " items"

# or use variable interpolation
regular expressions
posix shell cmd.exe powershell
regex match s=hello
if expr $s : $rx > /dev/null
set s=hello
echo %s%|findstr /r "[a-z][a-z]*" && (echo match ) ^
  || (echo does not match)
if ("hello" -match "^[a-z][a-z]*$") {
single substitution s='do re mi mi mi'
s=$(echo $s | sed s/mi/ma/)
global substitution s='do re mi mi mi'
s=$(echo $s | sed s/mi/ma/g)
set "s=do re mi mi mi"
echo %s:mi=ma%
$s = "do re mi mi mi"
$s = $s -replace "mi", "ma"
dates and time
posix shell cmd.exe powershell
current datetime date +'%Y-%m-%d %H:%M:%S' date /t && time /t get-date -format 'yyyy-MM-dd HH:mm:ss'
get-date -uformat '%Y-%m-%d %H:%M:%S'
current unix epoch date +'%s' get-date -uformat %s
format datetime date -d '1970-01-01 00:00:00 UTC' +'%s' get-date -date '1970-01-01 00:00:00' -uformat %s
resizable arrays
posix shell cmd.exe powershell
literal nums=(1 2 3 4) set nums=(1 2 3 4) $nums = 1 ,2 ,3, 4
$nums = @(1 ,2, 3, 4)
size ${#nums[@]} set numsc=0
for %i in %nums% do set /a numsc+=1
echo %numsc%
${nums[0]} $nums[0]
nums[1]=5 $nums[0] = 5
${nums[@]:1:2} $nums[1..2]
concatenate a=(1 2 3)
b=(4 5 6)
c=(${a[@]} ${b[@]})
@(1, 2, 3) + @(4, 5, 6)
iterate over elements for i in ${nums[@]}
do echo $i
foreach ($i in $nums) {
  write-output $i
sort $a = 3, 2, 4, 1
$b = $a | Sort-Object
reverse $a = 1, 2, 3
posix shell cmd.exe powershell
define add() { echo $(( $1 + $2 )); }
function add { echo $(( $1 + $2 )); }
  set /a sum=%~1+%~2
  echo %sum%
function add {
  param ($a, $b)
  $a + $b
add 1 2 call:add 1 2 add 1 2
missing argument behavior '' parameter treated as undefined variable or as containing the empty string $null
extra argument behavior ignored none ignored
default argument none none function add {
  param ($a=0, $b=0)
  $a + $b
named parameters none none add -a 1 -b 2
return value return arg available in $? variable if a positive integer smaller than 256
anonymous function literal none none $f = { write-output "foo" }
call anonymous function none none & $f
default scope
global global local
nest Nested function visible outside containing function. none function add {
  param ($a, $b)
  function add2 {
    param ($a2, $b2)
    $a2 + $b2
  add2 $a $b

# Nested function not visible outside of containing
# function; nested function can see local variables
# of containing function.
execution control
posix shell cmd.exe powershell
if if [ $n -eq 0 ]
  echo "no hits"
elif [ $n -eq 1 ]
  echo "1 hit"
  echo $n " hits"
if %n% equ 0 (
  echo no hits
) else (
  if %n% equ 1 (
    echo one hit
  ) else (
    echo %n% hits
if ($n -eq 0) {
  write-output "no hits"
} elseif ($n -eq 1) {
  write-output "one hit"
} else {
  write-output "$n hits"
while i=0
while [ $i -lt 10 ]
do i=$(($i + 1))
  echo $i
set i=0
  set /a i+=1
  echo %i%
if %i% lss 10 goto :loop
$i = 0
while ($i -lt 10) {
  write-output (++$i)
for for i in 1 2 3
  echo $i
set nums=(1 2 3 4)
for %%n in %nums% do echo %%n
for ($i=1; $i -le 3; $i++) {
  write-output $i
break break
continue continue
posix shell cmd.exe powershell
raise Commands which fail return nonzero exit status.

Exit status of last command stored in $?
Commands which fail return nonzero exit status.

Exit status of last command stored in %errorlevel%
throw "bam!"
handle trap 'echo "risky failed"' ERR

if errorlevel 1 (
  echo risky failed
try {
  throw "bam!"
catch {
  write-output "caught!"
uncaught exception behavior stderr and continue stderr and continue script exits
posix shell cmd.exe powershell
sleep sleep 10 vista and later:
timeout 10
timeout 10
posix shell cmd.exe powershell
read line from stdin # stored in $ine:
read line

# with prompt:
read -p 'line: ' line
rem stored in %line%:
set /p line=

rem with prompt:
set /p line="line: "
$line = read-host

# with prompt:
$line = read-host 'line'
write line to stdout echo "hi world" echo hi world write-output "hi world"
write formatted string to stdout printf '%.2f\n' 3.1415
write file to stdout cat foo.txt
write to standard error echo "hi world" >&2 echo hi world >&2
iterate over file by line awk 'length($0) > 70 {print $0}' /etc/passwd
write to process echo "hi world" | wc echo hi world | find /c hi
write to file echo "hello" > /tmp/a echo hello > \tmp\a
append to file echo "hello" >> /tmp/a echo hello >> \tmp\a
posix shell cmd.exe powershell
create empty file # does not overwrite existing file, but updates
# last mod. time:

touch foo.txt
rem overwrites existing file:
type nul > foo.txt
# error if file exists:
new-item -type file foo.txt
file exists test, file regular test if [ -e foo.txt ]; then echo "exists"; fi
if [ -f foo.txt ]; then echo "regular"; fi
if exist foo.txt ( echo exists )

if (test-path foo.txt) { write-output "exists" }

if (test-path -pathtype leaf) {
  write-output "regular"
file size ls -l foo.txt dir foo.txt get-childitem foo.txt
is readable, is writeable, is executable if [ -r /etc/passwd ]; then echo readable; fi
if [ -w /tmp ]; then echo writeable; fi
if [ -x /bin/ls ]; then echo executable; fi
set file permisions chmod 0600 foo.txt
last modification time ls -l foo.txt dir foo.txt get-childitem foo.txt
copy file, remove file, rename file cp /tmp/foo.txt /tmp/bar.txt
rm /tmp/foo.txt
mv /tmp/bar.txt /tmp/foo.txt
copy foo.txt bar.txt
del foo.txt
move bar.txt foo.txt
copy-item foo.txt bar.txt
remove-item foo.txt
move-item bar.txt foo.txt
create symlink, symlink test, readlink ln -s /etc/passwd /tmp/passwd
if [ -h /tmp/passwd ]; then echo "symlink"; fi
readlink /tmp/passwd
generate unused file name mktemp /tmp/fooXXXXX [System.IO.Path]::GetTempFileName()
posix shell cmd.exe powershell
working directory
get and set
echo $(pwd)
echo $PWD

cd /tmp
echo %cd%

chdir \windows
$loc = get-location
echo $loc

set-location \windows
program directory bash only:
"$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
echo %~dp0
dirname and basename dirname /etc/hosts
basename /etc/hosts
iterate over directory by file for f in /etc/*
  echo $f
make directory mkdir /tmp/foo.d mkdir foo new-item -type directory foo
recursive copy cp -R /tmp/foo.d /tmp/bar.d
remove empty directory rmdir /tmp/foo.d rmdir foo
remove directory and contents rm -rf /tmp/foo.d rmdir /s /q foo
directory test if [ -d /tmp ]; then echo directory; fi
processes and environment
posix shell cmd.exe powershell
command line args $1

# number of args:

# pass args as $# words to twiddle
twiddle "$@"

# pass args as one word to twiddle
twiddle "$*"

rem number of args:
set #=0
for %%i in %* do set /a #+=1
echo %#%

rem no way to pass argument with space
twiddle %*
program name $0 %0
environment variable
get, set, clear
shell variables are environment variables
user id and name id

# user name only; most systems:
exit exit 0 rem batch script exits:
exit /b 0

rem also closes cmd.exe window:
exit 0
external command ls dir dir
command substitution s=$(ls) none $s=dir
speech mac only:
say "I'm a Mac"
$sp = new-object -comobject "SAPI.SpVoice"
$sp.speak("I'm a PC")
command path which ping get-command ping
set signal handler function int_handler {
  echo "exiting…";

trap int_handler INT
send signal kill -INT 1234
kill -KILL 1234
start job in background sleep 1000 &
suspend current job ^Z
terminate job kill %1
list jobs jobs
background suspended job bg %1
bring background job into foreground fg %1
disown job disown %1
libraries and namespaces
posix shell cmd.exe powershell
library $ cat
function add() {
  echo $(($1 + $2));
from directory in search path:

> type Modules\Foo\Foo.psm1
function add {
  param ($a, $b)
  $a + $b
import library source
add 3 7
import-module foo

add 1 2
library path environment variable none $env:psmodulepath
posix shell cmd.exe powershell
command line documentation man ls help dir get-help get-childitem
list defined variables printenv set get-variable
list available libraries get-module -listavailable
debugging and profiling
check syntax $ apt-get install shellcheck
$ shellcheck
flags for stronger errors bash only:
bash -eu -o pipefail

-e: exit with nonzero status if command fails
-u: exit with nonzero status if undefined variable accessed
-o pipefail: exit with nonzero status if command in pipeline fails
cpu usage time ls
posix shell cmd.exe powershell
__________________________________________________________ __________________________________________________________ __________________________________________________________


version used

The version of the language used for verifying the examples in the reference sheet.

show version

How to get the version.


The version number displayed at start up is the Windows kernel version. 6.3 is the Windows 8.1 kernel version number.

Grammar and Invocation


The customary name of the interpreter and how to invoke it.

posix shell:

On Unix, scripts are executing by passing the file containing the script to the interpreter as an argument:

$ dash ~/

If the executable bit is set, the file can be run directly:

$ ~/

To determine the name of the interpreter that will process the script, Unix will look for the presence of a shebang (#!) at the start of the file. If the pathname to a command follows the shebang, it will be used to interpret the script. If no shebang is present, the script will be interpreted with /bin/sh which is bash on Mac OS X and Cygwin and dash on Ubuntu.

Command line arguments which set the positional parameters $1, $2, and so on can be set upon invocation as follows:

$ dash ~/ arg1 arg2
$ ~/ arg1 arg2

Arguments can also be put on the shebang line, but this is only useful for passing options to the shell. If the file contains

#!/bin/sh -x

then invoking it as


is equivalent to

/bin/sh -x

Hardcoding a full path in a shebang is a common cause of portability problems because different systems may install the interpreter in different locations. The env command avoids the problem by searching the PATH directories for the command:

#!/usr/bin/env lua


On Windows, a file is a PowerShell script if it has a .ps1 suffix. There is no need to mark the script as executable. However, PowerShell is not configured to run scripts by default. To change the configuration, start a PowerShell as an Administrator and run the following command:

set-executionpolicy remotesigned

It is possible to invoke a PowerShell script by specifying the PowerShell interpreter as the command and the script as an argument, but the suffix of the file must still be ps1:

powershell -file .\test.ps1


How to invoke the REPL.

execute command and exit

How to pass a single command to be executed as a command line argument.

statement separator

How the parser determines the end of a statement.

posix shell:

A posix shell simple command consists of optional variable assignments, followed by a command and zero or more arguments and redirections. The command can be an external command, user defined function, or built-in.

A posix shell pipeline is a sequence of one or more simple commands joined by pipes |. The shell executes the commands in parallel and redirects the stdout of each command to the stdin of the following command. The exit status is the status of the last command.

The control operators ; & && || are pipeline separators. The semicolon ; enforces sequential execution. The ampersand & executes in parallel. The && executes to the first pipeline that returns a nonzero status. The || executes to the first pipeline that returns a zero status.

A list is one or more pipelines joined by control operators. A list can have a semicolon ; or ampersand & at the end. A list is terminated by a newline.

A newline does not terminate a list when:

  • inside single quotes '', double quotes "", backticks ``, or parens ()
  • inside a block started by the keywords: for, select, case, if, while, until

A newline that would normally terminate a statement can be escaped with a backslash.

Multiple lists can be grouped inside parens ( ) or curly brackets { }. When grouped inside parens, the lists are executed inside a subshell. The curly brackets, but not the parens, must be separated from their contents by white space. Also, within curly brackets, but not within parens, the last list must be terminated by a semicolon or newline.

word separators

escape special character

How to escape a special character.

block delimiters

How blocks are delimited.

posix shell:

Blocks can be delimited with {}, (), or the do,done keywords.

If a block is started with an open curly bracket {, then the block must be terminated with a line containing a close curly bracket by itself }.

If a block is delimited by (), then the commands in the block are executed in a subshell.

A block is delimited by do and done when using the execution control keywords for, select, while, and until.

The then and else keywords of an if statement start blocks which are terminated by a following elif, else, or fi.

The function and if keywords open blocks which are terminated by end keywords. The repeat keyword opens a block which is terminated by until.

to end-of-line comment

How to make the remainder of the line a comment.

multiline comment

How to comment out multiple lines.

posix shell:

The method described is the syntax for a here document, which is a multiline string literal.

Variables and Expressions


How to assign a value to a variable.

parallel assignment

How to assign values to variables in parallel.


How to exchange the values held by two variables.

compound assignment operators: arithmetic, string, bit

The compound assignment operators for arithmetic, string, and bit operations


Note that /= performs float division, even when both operands are integers.

When the left operand is a string, += concatenates the right operand to the left operand.

When the left operand is a string and the right operand an integer, *= concatenates the left operand with itself right operand times.

increment and decrement

The C-style increment and decrement operators which can be used in expressions.

variable declaration

How to declare a variable.

posix shell:

The following three lines have identical behavior:

A="hello, world"
declare A="hello, world"
typeset A="hello, world"

It is possible to make a read only variable. Again there are three ways:

readonly A="can't change"
declare -r A="can't change"
typeset -r A="can't change"

Variables are not exported to subprocesses unless declared to be exported:

export A="exported to subprocess"
declare -x A="exported to subprocess"
typeset -x A="exported to subprocess"

Variables assigned on the same line as a command are not created, only exported to the subprocess that instantiates the command:

EDITOR=emacs svn commit

By default variables defined inside functions are global. They can be declared to be local:

function foo () {
  local b=17
  # echoes 17:
  echo $b
# echoes nothing:
echo $b

are identifiers case sensitive?


PowerShell identifiers are case insensitive.


The null literal.

null test

How to test if a value is null.

Arithmetic and Logic

posix shell:

The POSIX shell provides at least three different environments for logical expressions, each with their own operators and values for true and false.

Logical expressions are usually encountered in the conditionals of if, elif, while, and until. A command is expected as the conditional expression. The command is executed, and a return value of zero is treated as true and nonzero as false.

status codes [ ] $(( ))
where used command command argument
true true no canonical true value 1
false false '' 0
falsehoods nonzero exit status '' 0
logical operators && || ! -a -o ! && || !
grouping { } \( \) ( )
string relational operators none = != \< \> none
arithmetic relational operators none -eq -ne -lt -gt -le -ge == != < > <= >=
arithmetic operators none none + - * / % **
bit operators none none << >> & | ^ ~

posix shell: status codes:

Logical expressions can be formed using status codes returned by commands. The commands can be external, built-in, or user defined functions. A status code of zero is used to indicate success, and for the purpose of logic zero is treated as true and all other status codes as false.

The && and || operators are short circuit operators. An exclamation point ! can be used to negate the status code of a command. It is not necessary to separate && and || from their operands with whitespace, but it is necessary to mark off a ! used as negation with whitespace.

posix shell: test command:

posix shell: arithmetic expansion:

true and false

The literals for true and false.


Values which are false in conditional expressions.

logical operators

Logical and, or, and not.

posix shell:

&& || and ! are available inside [[ ]], (( )), and $(( )) expressions. Inside [ ] expressions use -a, -o, and !.

conditional expression

The syntax for a conditional expression.

are expressions statements

Whether an expression can be used where a statement is expected.

relational expressions

posix shell:

Bash has three types of relational expressions: [[ ]], [ ], and (( )). For a description of [ ], read the man page for test.

(( )) evaluates its contents in the same manner as the arithmetic expansion $(( )). If the result is zero, it returns 1 (false). Otherwise it returns 0 (true).

[[ $a == pattern ]] and [[ $a != pattern ]] interpret * and ? on the right side as patterns. Thus "hello" == "hell*" is true. For numeric comparison, use [ $a -eq num ] or [ $a -ne num ].

Bash expressions discusses the different types of bash expressions in more detail.

relational operators

posix shell:

If == and =! have an unquoted string on the right, then * and ? within the string will be treated as wild cards for matching.

convert from string

posix shell:

All values are strings. The $(( )) operator will interpolate any variables and then evaluate the resulting string as an arithmetic expression composed of integers. The variables are not limited to containing integers. The following script outputs 10:

echo $(($A))

To perform floating point arithmetic, bash must shell out to a floating point utility such as bc.

convert to string

arithmetic expressions

How to evaluate an arithmetic expression.

posix shell:

Bash arithmetic is available within $(( )) and (( )). The latter form evaluates the arithmetic expression and returns status 1 if the result zero, and 0 otherwise.

Bash only has integer arithmetic. For floating point arithmetic, use the external commands bc or dc.

arithmetic operators

The operators for addition, subtraction, multiplication, float division, integer division, modulus, and exponentiation. Some languages provide a function pow instead of an operator for exponentiation.

posix shell:

arithmetic operators are available in $(( )) and (( )).

integer division

How to perform integer division.

float division

How to perform floating point division, even if the operands might be integers.

posix shell:

The bash shell lacks built-in floating point arithmetic. bc is an arbitrary precision calculator, and scale is the number of digits to the right of the decimal point. If scale is not specified, it defaults to zero, which results in integer division.

It is also possible to use dc, which is a reverse polish notation arbitrary precision calculator:

`echo " 5 k $a $b / p " | dc`

arithmetic functions

Functions for computing square root, natural exponent, natural logarithm, sine, cosine, tangent, arcsine, arccosine, arctangent, and atan2.

The trigonometric functions are all in radians. atan2 takes two arguments which are the x and y co-ordinates of a vector in the Cartesian plane. It returns
the angle to the positive x-axis made by the vector.

arithmetic truncation

division by zero

integer overflow

float overflow

sqrt -2

The result of taking the square root of -2.

random integer, uniform float

The examples show how to generate a uniform random integer in the range from 0 to 99, inclusive; how to generate a uniform float in the range 0.0 to 1.0; how to generate a float from a standard normal distribution

posix shell:

$RANDOM evaluates to a random integer between 0 and 32767 inclusive.

seed random numbers

posix shell:

Bash 3.2.48 seeds the random number at start up using the current time and the PID:

  /* Seed the random number generator. */
  sbrand (dollar_dollar_pid + shell_start_time);

Here is the random number generation code:

/* A linear congruential random number generator based on the example
   one in the ANSI C standard.  This one isn't very good, but a more
   complicated one is overkill. */

/* Returns a pseudo-random number between 0 and 32767. */
static int
brand ()
  rseed = rseed * 1103515245 + 12345;
  return ((unsigned int)((rseed >> 16) & 32767));    /* was % 32768 */


The initial seed is set to a value that varies each time PowerShell is started up.

If a repeatable sequence of random numbers is desired, the seed can be set to a specific value using the -setseed option on the first call to random.

bit operators

posix shell:

The bit operators are available in $(( )) and (( )).


string literal

The syntax for a string literal and how to escape the delimiter.

newline in literal

Whether a newline character sequence can be included in a string.

For all the languages described in this reference sheet a string literal is permitted to encompass multiple lines in the source code and the resulting string will contain the same number of lines.


Character escape sequences which can be used in string literals.

variable interpolation

How to interpolate variables in a string.

posix shell:

A dollar sign $ can be backslash escaped to prevent variable interpolation:

echo "the value of \$a is $a"


A dollar sign $ can be backtick escaped to prevent variable interpolation:

write-output "the value of @@`@@$a is $a"


How to get the length of a string.


:strLen string len -- returns the length of a string
::                 -- string [in]  - variable name containing the string being measured for length
::                 -- len    [out] - variable to be used to return the string length
    set "str=A!%~1!"&
    set "len=0"
    for /L %%A in (12,-1,0) do (
        set /a "len|=1<<%%A"
        for %%B in (!len!) do if "!str:~%%B,1!"=="" set /a "len&=~1<<%%A"
    IF "%~2" NEQ "" SET /a %~2=%len%

string comparison

How to determine if two strings are equal or unequal.

index substring

How to find the index of the start of a substring in a string.

extract substring

string concatenation

The string concatenation operator.


How to split a string into an array of strings.


When splitting a string into words, no delimiter need be specified and the string to be split can follow the -split operator:

-split "foo bar baz"


How to concatenate the elements of an array into a string with a separator.



How to create a string using a printf style format.

case manipulation


pad on right, pad on left

Regular Expressions

regex match

How to test whether a regular expression matches a string.

posix shell:

The double square bracket operator [[ ]] is not part of the POSIX standard but it is a feature of bash, ksh, and zsh. It supports a match test operator:

if [[ "hello" =~ ^[a-z][a-z]*$ ]]; then

single substitution

How to replace the first occurrence of a pattern in a string.

posix shell:

The following parameter expansion is not part of the POSIX standard but provided by bash, ksh, and zsh:

str='do re mi mi mi'
echo ${str/mi/ma}

global substitution

How to replace all occurrences of a pattern in a string.

posix shell:

The following parameter expansion is not part of the POSIX standard but provided by bash, ksh, and zsh:

str='do re mi mi mi'
echo ${str//mi/ma}

Dates and Time

Resizable Arrays

array literal

Array literal syntax.

array size

How to get the number of elements in an array.

array lookup

How to access a value in an array by index.

array slice

How to slice a subarray from an array.

array iteration


How to test for membership in an array.


How to compute an intersection.





universal predicate

How to test whether a condition holds for all members of an array. Always true for an empty array.

existential predicate

How to test whether an item in an array exists for which a condition holds. Always false for an empty array.

dictionary literal

dictionary size

dictionary lookup

dictionary iteration

out of bounds behavior


Python has both functions and methods. Ruby only has methods: functions defined at the top level are in fact methods on a special main object. Perl subroutines can be invoked with a function syntax or a method syntax.

function definition

posix shell:

A bash function definition can alternately be preceded by the keyword function, and when used, the parens following the function name are prohibited.

function invocation

posix shell:

The syntax for invoking a function is the same as the syntax for invoking a command. If a function is defined with the same name as a command in the search path, the function will be executed.

missing argument value

Value of an argument variable if a function is invoked with fewer arguments than are declared.

extra arguments

If a function is invoked with more arguments than are declared, how the function can access them.

default argument value

How to declare a default value for an argument.

variable number of arguments

How to write a function which accepts a variable number of argument.

named parameters

How to write a function which uses named parameters.

return value

posix shell:

Bash functions can only return small integers via return. However, a function can echo to stdout and the caller can invoke it with backticks to get a string value.

lambda declaration

How to define a lambda function.

lambda invocation

default scope

posix shell:

By default, bash and variables inside functions have global scope.

Execution Control


Some optional branching constructs:

posix shell:

case $a in (0) echo "no";; (1) echo "yes";; (2) echo "maybe";; (*) echo "error";; esac


posix shell:

Also has an until loop.


break exits a for or while loop immediately. continue goes to the next iteration of the loop. redo goes back to the beginning of the current iteration.


posix shell:

A C-style for loop:

for ((i=0; i<10; i++ )); do echo $i; done


raise exception

How to raise an exception.

catch exception

How to handle an exception.

uncaught exception behavior

System behavior if an exception goes uncaught. Most interpreters print the exception message to stderr and exit with a nonzero status.

posix shell:

The bash interpreter writes a message to stderr whenever a command returns a nonzero status. By default, the interpreter does not exit, but if this behavior is desired, then the following should be put at the top of the script:

trap exit ERR

wait on thread


write to standard out

posix shell:

To prevent echo from appending a newline to the output, use

echo -n "hello"

standard filehandles

read line

read file

write to file

append to file



Processes and Environment

external command

posix shell:

The syntax for calling an external command is the same as the syntax for invoking a function. If a function is defined with the same name as an external command in the search path, the function is invoked.


command line args


How to make the computer talk.

posix shell:

On Mac OSX the command say can also be executed from the bash or zsh prompt:

say "I'm a Mac"

On Ubuntu Linux this command can be used:

espeak "I'm Unix"

environment variable

command path

The directory containing a command. Also indicates if the command is a built-in, alias, or function. Shows the definition of aliases and functions.


posix shell:

The exit status of a bash script is the return status of the last command executed, or the argument of exit.

set signal handler

Libraries and Namespaces


What a library looks like.

import library

library path

namespace declaration

namespace separator



Debugging and Profiling


POSIX 2008: Shell & Utilities

Command Prompt

Command-line reference A-Z


Windows PowerShell User's Guide

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