How to make bash scripts work in dash

This page is an attempt to list some of the most common bashisms ie features not defined by POSIX (ie don't work in dash). 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.

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
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	the C-like for syntax (for (( ; ; ));do ..) is not defined by posix
case	;;&		;;& in bash4 is not defined by posix
case	;&		;& in bash4 is not defined by posix
select	select		select is not defined by posix
expand sequences	echo $'hello\tworld'	printf "hello\tworld"	$' ' is not defined by posix
extended glob	( +() @( ) etc...		not defined by posix

Expansions

Brace Expansion, eg {1..10} is not defined by posix
<( ) >( ) process substitution is not defined by posix

Parameter Expansions

List of expansions not defined by posix:

${name:n:l}
${name/ }
${!name}

Arrays

arrays are not defined by posix.

Conditionals

[[ is not defined by posix
== as an argument of test (aka [) is not defined by posix
< > to compare numbers as argument of test are not defined by posix, though dash implements them
-nt, -ot, -ef are not defined by posix
(( )) is not defined by posix

Arithmetic

++ -- are not defined by posix

Redirections

>& and &> are not defined by posix
m>n- m<n- ie duplicating and closing a descriptor at the same time is not defined by posix
|& (bash4) is not defined by posix

Builtins

echo. posix doesn't define any options, use printf
printf "-v" is not defined by posix. also the %b and %q format are not defined by posix
read, the only option defined by posix is "-r"
shopt, and therefore all the options it provide, extglob, nullglob, dot glob etc ..are not defined by posix

The bash manual has a list of the difference 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.

Bashism