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How to make bash scripts work in dash

This page is an attempt to list some of the most common bashisms, i.e. features not defined by POSIX (won't work in dash, or general /bin/sh). It probably won't be exhaustive. Note also we talk about "bashism" because this wiki is largely bash-centric but a number of these extensions work in other shells like ksh or zsh.

1. Syntax

Works in bash

Change to for dash


defining functions

function f { echo hello world; }

f() { echo hello world; }

"function" is not defined by POSIX, only "name ()" is. In ksh both forms are present, but functions defined with "function" work slightly differently.


;;& ;& etc

None. Duplicate the case (use a function to avoid code duplication)

;;& ;& in bash4 is not defined by POSIX. ksh93 has ;& but not ;;&`

numeric C-like for loop

for ((i=0; i<3; i++)); do
 echo $i

i=0 ; while test $i -lt 3 ; do
 echo $i ; i=$(($i+1))

this syntax is not defined by POSIX. Present in ksh93.

expand sequences

echo $'hello\tworld'

printf "hello\tworld\n"

$' ' is not defined by POSIX and is bash-specific

extended glob

+( ) @( ) !( ) *( )

not always possible, sometimes you can use several globs, sometimes you can use find(1)

not defined by POSIX. Present in ksh.



some ideas: implement the menu yourself, use a command like dialog

not defined by POSIX. Present in ksh.

2. Expansions

3. Parameter Expansions

List of expansions not defined by POSIX:

Note that using $( ) has the side-effect of removing trailing newlines from the results. Furthermore, since many standard Posix utilities, such as sed, require text files as input, you should ensure their input ends in a newline.

4. Arrays

Arrays are not defined by POSIX (but are present in ksh); there is no easy general workaround for arrays. Here are some hints:

#build a command dynamically see [[BashFAQ/050]]
set -- 'mycommand' 'needs some complex' 'args'
#access the i'th param
set -- one two three
eval var=\$$i #take care if i comes from some user input see below
echo "$var"

5. Conditionals

Works in bash

Change to for dash


simple test


use [ and use double quotes around the expansions [ "$var" = "" ]

[[ is not defined by POSIX, but is present in ksh

pattern matching

[[ foo = *glov ]]

use case or grep

see BashFAQ/041

equality with test


use = instead

only = is defined by POSIX, = works also in bash

compare lexicographically.

< >

no change

present in dash and ksh, but not defined by POSIX. See note below for possible workarounds.

compare modification times

[[ file1 -nt file2 ]] or -ot

[ "$(find 'file1' -prune -newer 'file2')" ] or [ "file1" -nt "file2" ]

-prune is required to avoid recursion; present in ksh. -nt and -ot aren't specified by POSIX.

check if 2 files are the same hardlink

[[ file1 -ef file2 ]]

[ "file1" -ef "file2" ]

-ef is not defined by POSIX, but is present in ksh and Dash.

(( ))

(( )) (without the $) acts like a command on its own

For simple comparison: [ -lt ] (and -ne -gt -ge) or [ "$(( 3+1 < 5))" -eq 0 ].

present in ksh

To assign a variable var=$((3+1))

Note: several standard POSIX utilities can be used for lexical comparisons. The examples below return a true (zero) exit status if the content of $a sorts before $b.

6. Arithmetic

Works in bash

Change to for dash


pre/pos increment/decrement

++ --





: $((i=i+1))

The : command can be used to peform side effects with an expansion

7. Redirections

Works in bash

Change to for dash


redirect both stdout and stderr

>& and &>

command > file 2>&1 or commnd 2>&1 | othercommand


|& (bash4)

command 2>&1 | othercommand


duplicate and close

m>&n- m<&n-

m>&n n>&-

not defined by POSIX



echo | command, or a here document to avoid a subshell (<<EOF)


8. Builtins

9. Special Variables

Works in bash

Change to for dash


keep track of the times


before=$(date +%s) ....seconds=$(( $(date +%s) - $before))

date +%s is not POSIX; see this faq for more info. Present in ksh

Generate a random number


random=$(awk 'BEGIN{srand(); printf "%d\n",(rand()*256)}') gives a number between 0 and 256
random=$(hexdump -n 1 -e '/1 "%u"' /dev/urandom) and random=$(od -A n -N 1 -t u1 /dev/urandom) give a timer-independent number between 0 and 256
random=$(hexdump -n 2 -e '/2 "%u"' /dev/urandom) and random=$(od -A n -N 2 -t u2 /dev/urandom) give a timer-independent number between 0 and 65535

Be sure to learn what srand() and rand() do, ie this method fails if you call awk several times rapidly. Instead generate all the numbers you need inside awk. Some systems also provide /dev/random and /dev/urandom , but this is not necessarily mandated by the POSIX standard. ksh has RANDOM

Get the status of all the commands in a pipeline


Simplest solution:
mkfifo fifo; command2 <fifo & command1 >fifo; echo $?
see NamedPipes

bash-specific; see this faq and this script pipe status for POSIX shell

10. More

Note that bash in POSIX mode is only guaranteed to run a shell written according to the POSIX specification. It doesn't mean that it will fail if you use bashisms in your scripts.


2012-07-01 04:05