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	Comment
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.
case	`;;&` `;&` 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` `done`	`i=0 ; while test $i -lt 3 ; do` `echo $i ; i=$(($i+1))` `done`	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.
select	`select`	some ideas: implement the menu yourself, use a command like dialog	not defined by POSIX. Present in ksh.

2. Expansions

Brace Expansion, eg {a,b,c} or {1..10} is not defined by POSIX. Both forms are present in ksh93, and the first form in older ksh.
The <( ) and >( ) process substitutions are not defined by POSIX, but can be simulated with FIFOs: instead of foo <(bar), write mkfifo fifo; bar > fifo & foo fifo (this is basically how process substitution is implemented on OSes that don't have a mechanism like /dev/fd/ to refer to unnamed pipes with filenames). It is present in ksh93.

3. Parameter Expansions

List of expansions not defined by POSIX:

${name:n:l} -- if the variable contains no newlines, you can use $(printf '%s\n' "$name" | cut -c $n-$((n+l-1))). If there are newlines, you can use $(printf %s "$name" | dd bs=1 skip=$n count=$l 2>/dev/null), but printf may report a (harmless) broken pipe if the variable contents are large enough. This form is present in ksh93.
${name/ } -- you can use $(printf '%s\n' "$name" | sed 's/foo/bar/'), after changing shell patterns to regular expressions. This form is present in ksh93.
${!name} -- bash-specific; it is possible, but dangerous, to use eval to achieve similar effects; see BashFAQ/006.

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:

The positional parameters are a kind of array (only one array):

#build a command dynamically see [[BashFAQ/050]]
set -- 'mycommand' 'needs some complex' 'args'
"$@"
#access the i'th param
set -- one two three
i=2
eval var=\$$i #take care if i comes from some user input see below
echo "$var"

use IFS and set -f
eval is powerful but dangerous so use it wisely. See Eval command and security issues.

5. Conditionals

	Works in bash	Change to for dash	Comment
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.

awk -v v1="$a" -v v2="$b" 'BEGIN { exit !(v1 "" < "" v2) }'
expr "x$a" "<" "x$b" >/dev/null
printf "%s\n" "x$a" "x$b" | sort -c >/dev/null 2>&1 (also returns true if $a and $b are equal)

6. Arithmetic

	Works in bash	Change to for dash	Comment
pre/pos increment/decrement	`++ --`	`i=$((i+1))`	-
-	`let`	`: $((i=i+1))`	The `:` command can be used to peform side effects with an expansion

7. Redirections

	Works in bash	Change to for dash	Comment
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
herestring	`<<<"string"`	`echo \| command`, or a here document to avoid a subshell (`<<EOF`)	-

8. Builtins

echo -n or -e -- POSIX doesn't define any options, and furthermore allows echo -e to be the default behavior. Instead use printf "%s\n" (for normal echo) or printf "%b\n" (for echo -e); leave off the \n to simulate echo -n.
printf -v is not defined by POSIX. Also the %q format is not defined by POSIX.
read -- the only option defined by POSIX is -r; ksh has a different set of options that only partially overlaps with bash.
shopt, and therefore all the options it provides (extglob, nullglob, dotglob, etc.) are not defined by POSIX and are bash-specific
local -- there is no POSIX equivalent. You can use $funcname_varname to reduce the likelihood of conflicts, but even that is not enough for recursive functions. You can ensure that recursive calls occur in subshell environments (so there is a "local" copy of all variables), or pass all "local variables" as parameters (because the positional parameters $@, $1, $2, etc are truly local). dash explicitly supports local as a non-Posix extension; ksh uses typeset instead, which works like bash's declare.

9. Special Variables

	Works in bash	Change to for dash	Comment
keep track of the times	`SECONDS`	`before=$(date +%s) ....seconds=$(( $(date +%s) - $before))`	`date +%s` is not POSIX; see this faq for more info. Present in ksh
Generate a random number	`RANDOM`	`random=$(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	`PIPESTATUS`	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

The bash manual has a list of the differences between bash running in POSIX mode and a normal bash.

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.

There is a handy perl script checkbashisms which is part of the debian devscripts package which can help point out bashisms in a particular script.

CategoryShell