Differences between revisions 4 and 16 (spanning 12 versions)

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.

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
case	`;;&` `;&` etc	none. duplicate the case (use a funnction to avoid code duplication)	`;;&` `;&` in bash4 is not defined by posix
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
expand sequences	`echo $'hello\tworld'`	`printf "hello\tworld"`	`$' '` is not defined by posix
extended glob	`+( ) @( ) !( ) *( )`	not always possible, some times you can use several globs, sometimes you can use find(1)	not defined by posix
select	select	some ideas: implement the menu yourself, use a command like dialog	select is not defined by posix

Expansions

Brace Expansion, eg {1..10} is not defined by posix
<( ) >( ) process substitution is not defined by posix, but can be simulated with FIFOs: instead of foo <(bar), write mkfifo fifo; bar > fifo & foo fifo

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.
${name/ } -- you can use $(printf '%s\n' "$name" | sed 's/foo/bar/'), after changing shell patterns to regular expressions.
${!name} -- 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.

Arrays

Arrays are not defined by posix, 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.

Conditionals

	Works in bash	Change to for dash	Comment
simple test	`[[`	use [ and use double quotes around the expansions `[ "$var" = "" ]`	[[ is not defined by posix
pattern matching	`[[ foo = *glov ]]`	use case or grep	see BashFAQ/041
equality with test	`==`	use = instead	only = is defined by posix, = works also in bash
compare lexicographically.	`< >`	no change	works with dash but not defined by posix a possible workaround with awk: `awk -v v1="1" -v v2="fcd" 'BEGIN{exit !(v1 "" < "" v2)}'`
compare modification times	`[[ file1 -nt file2 ]]` `or` -ot`	[ "$(find 'file1' -prune -newer 'file2')" ]	-prune is required to avoid recursion
check if 2 files are the same hardlink	`[[ file1 -ef file2 ]]`	?	-et is not defined by posix
`(( ))`	`(( ))` (without the $) acts like a command on its own	For simple comparison: `[ -lt ] (and -ne -gt -ge)` or `[ "$(( 3+1 < 5))" -eq 0 ]`.
		To assign a variable var=$((3+1))

Arithmetic

	Works in bash	Change to for dash	Comment
pre/pos increment/decrement	`++ --`	`i=$(($i+1))`	-
-	`let`	`: $((i=4+1))`	The `:` command can be used to peform side effects with an expansion

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>&m n>&-`	not defined by posix

Builtins

echo -n or -e -- 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 provides (extglob, nullglob, dotglob, etc.) are not defined by posix

Special Variables

	Works in bash	Change to for dash	Comment
keep track of the times	SECONDS	before=$(date +%s) ....seconds=$(( $(date +%s) - $before))	date +%s is not posix see this faq for more info
Generate a random number	RANDOM	`random=$(awk 'BEGIN{srand(); printf "%d\n",(rand*256)}'`) gives a number between 0 and 256	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 provides /dev/random and /dev/urandom
Get the status of all the commands in a pipeline	PIPESTATUS	Simplest solution `mkfifo fifo; command2 <fifo & command1 >fifo; echo $?` see NamedPipes	see this faq and this script pipe status for posix shell

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.

CategoryShell

-  ⇤ ← Revision 4 as of 2009-09-28 20:06:19 → 
  Size: 1932
  Editor: ppp089210033244
  Comment: a link to the manual
+   ← Revision 16 as of 2009-10-11 16:29:00 → ⇥
  Size: 6563
  Editor: NeilMoore
  Comment: process sub, parameter expansions
-Deletions are marked like this.
+Additions are marked like this.
 Line 1:
-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
+= 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
-Line 5:
+Line 7:
+== 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 ||
|| case || `;;&` `;&` etc || none. duplicate the case (use a funnction to avoid code duplication) || `;;&` `;&` in bash4 is not defined by posix ||
|| 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 ||
|| expand sequences || `echo $'hello\tworld'` || `printf "hello\tworld"` || `$' '` is not defined by posix ||
|| extended glob || `+( ) @( ) !( ) *( )`  || not always possible, some times you can use several globs, sometimes you can use find(1) || not defined by posix ||
|| select || select || some ideas: implement the menu yourself, use a command like dialog  || select is not defined by posix ||
-Line 9:
+Line 21:
- * <( ) >( ) process substitution is not defined by posix
+ * <( ) >( ) process substitution is not defined by posix, but can be simulated with FIFOs: instead of `foo <(bar)`, write `mkfifo fifo; bar > fifo & foo fifo`
-Line 15:
+Line 27:
- * ${name:n:l}
 * ${name/ }
 * ${!name}
+ * ${name:n:l} -- if the variable contains no newlines, you can use `$(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.
 * ${name/ } -- you can use `$(printf '%s\n' "$name" | sed 's/foo/bar/')`, after changing shell patterns to regular expressions.
 * ${!name} -- it is possible, but dangerous, to use eval to achieve similar effects; see [[BashFAQ/006]].
-Line 19:
+Line 31:
+Note that using `$( )` 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.
-Line 21:
+Line 34:
- * arrays are not defined by posix.
+[[BashFAQ/005 | Arrays]] are not defined by posix, 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 [[BashFAQ/048 | Eval command and security issues]].
-Line 25:
+Line 56:
- * [[ 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
+|| || '''Works in bash''' || '''Change to for dash''' || '''Comment''' ||
|| simple test ||  `[[`    || use [ and use double quotes around the expansions `[ "$var" = "" ]`   || [[ is not defined by posix ||
|| pattern matching || `[[ foo = *glov ]]` || use case or grep || see [[BashFAQ/041]] ||
|| equality with test || `==` || use = instead || only = is defined by posix, = works also in bash ||
|| compare  lexicographically. || `< >` || no change || works with dash but not defined by posix a possible workaround with awk: `awk -v v1="1" -v v2="fcd" 'BEGIN{exit !(v1 "" < "" v2)}'` ||
|| compare modification times ||  `[[ file1  -nt file2 ]]`` or `-ot` || [ "$(find 'file1' -prune -newer 'file2')" ] || -prune is required to avoid recursion ||
|| check if 2 files are the same hardlink || `[[  file1 -ef file2 ]]` || ? || -et  is not defined by posix ||
|| `(( ))` ||  `(( ))` (without the $) acts like a command on its own ||  For simple comparison:  `[ -lt ] (and -ne -gt -ge)` or `[ "$(( 3+1 < 5))" -eq 0 ]`. ||  ||
|| || ||  To assign a variable var=$((3+1))  || ||
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+Line 69:
- * ++ -- are not defined by posix
+|| || '''Works in bash''' || '''Change to for dash''' || '''Comment''' ||
||  pre/pos increment/decrement ||  `++ --` || `i=$(($i+1))` ||  - ||
||  - ||  `let` || `: $((i=4+1))` || The `:` command can be used to peform side effects with an expansion  ||

== 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>&m n>&-`  ||not defined by posix ||
-Line 35:
+Line 79:
-== 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
-Line 43:
+Line 81:
- * 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"
 * select is not defined by posix
 * shopt, and therefore all the options it provide, extglob, nullglob, dot glob etc ..are not defined by posix
+ * `echo -n` or `-e` -- 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 provides (extglob, nullglob, dotglob, etc.) are not defined by posix
-Line 49:
+Line 86:
-== more ==
+== Special Variables ==
-Line 51:
+Line 88:
- * [[ http://www.gnu.org/software/bash/manual/html_node/Bash-POSIX-Mode.html#Bash-POSIX-Mode | 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.
+|| || '''Works in bash''' || '''Change to for dash''' || '''Comment''' ||
|| keep track of the times|| SECONDS || before=$(date +%s) ....seconds=$(( $(date +%s) - $before))  || date +%s is not posix see [[BashFAQ/070|this faq for more info]]||
|| Generate a random number|| RANDOM  || {{{random=$(awk 'BEGIN{srand(); printf "%d\n",(rand*256)}'}}})  gives a number between 0 and 256 || 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 provides /dev/random and /dev/urandom||
|| Get the status of all the commands in a pipeline|| PIPESTATUS || Simplest solution {{{mkfifo fifo; command2 <fifo & command1 >fifo; echo $?}}} see NamedPipes || see [[ http://shell.cfajohnson.com/cus-faq-2.html#Q11 | this faq ]] and this script [[http://pipestatus.sourceforge.net/ | pipe status for posix shell]]  ||

 
== More ==

 * [[ http://www.gnu.org/software/bash/manual/html_node/Bash-POSIX-Mode.html#Bash-POSIX-Mode | 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. 

----
CategoryShell

Diff for "Bashism"