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 (almost all) of these extensions work in at least some other shells like ksh or zsh with perhaps some differences in the details, as most of Bash's scripting features are derived from ksh. POSIX has simply required a much smaller number of them.

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. The function f {...} syntax originated in ksh (and predates the Bourne syntax). In ksh both forms are present, but in the AT&T implementations, functions defined with "function" work slightly differently. zsh also supports both syntax without distinction.

case

;;& ;& etc

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

;;& ;& in bash4 is not defined by POSIX. AT&T ksh (since ksh88e, where it originated), MirBSD ksh (since R40) and zsh (since 3.1.2) have ;& but not ;;&

numeric C-like for loop

for ((i=0; i<3; i++)); do
 echo "$i"
done

i=0 ; while [ "$i" -lt 3 ]; do
 echo "$i" ; i=$(($i+1))
done

this syntax is not defined by POSIX. Present in ksh93 where it originated and zsh.

expand sequences

echo $'hello\tworld'

printf "hello\tworld\n"

Historically $' ' was not defined by POSIX through 2008, but has been accepted for the next version. http://austingroupbugs.net/view.php?id=249. Originated in ksh93, also supported by zsh

extended glob

+( ) @( ) !( ) *( )

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

not defined by POSIX. Originated in ksh. Supported by zsh with an option like bash.

select

select

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

not defined by POSIX. Originated in ksh, present in zsh.

Expansions

Parameter Expansions

List of expansions not defined by POSIX:

Note that using $( ) has the side-effect of removing trailing newlines from the results.

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'
"$@"
#access the i'th param
set -- one two three
i=2
eval "var=\${$i}" # i should be controlled by the script at all times. If influenced by side-effects like user input, robust validation is required.
printf '%s\n' "$var"

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, originated in ksh and is also present in zsh

pattern matching

[[ foo = *glov ]]

use case or expr or grep

see BashFAQ/041

equality with test

==

use = instead

only = is defined by POSIX

compare lexicographically.

< >

no change

present in dash, ksh, yash and zsh, 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 dash, ksh, yash and zsh. -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, yash, zsh and Dash.

(( ))

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

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

present in ksh (where it originated) and zsh

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.

See http://austingroupbugs.net/view.php?id=375 for current work on extending the standard test builtin operators.

Arithmetic

Works in bash

Change to for dash

Comment

pre/pos increment/decrement

++ --

i=$((i+1)) or : $((i+=1))

-

comma operator

,

: "$((...))"; cmd "$((...))"

The comma operator is widely supported by almost everything except dash and yash -- even posh and Busybox.

-

let or ((...))

[ "$((...))" -ne 0 ]

Because of the above comma restriction, let can't be simulated exactly without a loop.

Redirections

Works in bash

Change to for dash

Comment

redirect both stdout and stderr

>& and &>

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

-

|& (bash4)

command 2>&1 | othercommand

Conflicts with ksh. Not recommended, even in Bash. Just use 2>&1.

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)

-

Builtins

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

Get the name of all / the current function name(s)

FUNCNAME

??

bash-specific see stackoverflow question

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.


CategoryShell

Bashism (last edited 2014-05-02 01:39:08 by ormaaj)