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; }
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, or an alias to let the shell expand the whole segment as it parses the script)

;;& ;& 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
 printf '%s\n' "$i"
done

i=0
while [ "$i" -lt 3 ]; do
 printf '%s\n' "$i"
 i=$((i+1))
done

This syntax is not defined by POSIX. Originated in ksh93; present in zsh.

expand sequences

echo $'hello\tworld'

printf "hello\tworld\n"

Will be defined by the next version of POSIX (albeit with fewer escape sequences than Bash supports). Originated in ksh93, also supported by zsh, mksh, FreeBSD and Busybox sh though with variations

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.

file slurp

$(< file)

$(cat file)

Or read the file line by line.

Expansions

Parameter Expansions

List of expansions not defined by POSIX:

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

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 but are considered for addition in a future release.

check if 2 files are the same hardlink

[[ file1 -ef file2 ]]

[ "file1" -ef "file2" ]

-ef is not defined by POSIX (yet, same as above), 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). To assign a variable var=$((3+1)). For full functionality, use [ "$(( (i+=2) < 5 && a > 3))" -ne 0 ].

present in ksh (where it originated) and zsh

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

See Arithmetic Precision and Operators and Arithmetic expansion for supported and required math expression features.

Works in bash

Change to for dash

Comment

pre/post 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. In ksh93 however, it conflicts with the decimal radix in locales where it's used in floating points instead of period

exponentiation

**

** is the only bash arithmetic operator that is not a standard C or C++ operator. ksh93 can use pow(x, y). The ** operator is supported by at least bash, zsh, ksh93, and busybox, but not by dash or mksh.

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

>& or &>

command > file 2>&1

-

|& (bash4)

command 2>&1 | othercommand

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

duplicate and close

m>&n- or 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 and zsh

Generate a random number

RANDOM

random=$(awk 'BEGIN{srand(); printf "%d\n",(rand()*256)}') gives a number between 0 and 256
random=$(LC_ALL=C od -vAn -N1 -t u1 /dev/urandom | tr -d '[:space:]') gives a timer-independent number between 0 and 256
random=$(LC_ALL=C od -vAn -N2 -t u2 /dev/urandom | tr -d '[:space:]') gives 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 specified by the POSIX standard. ksh and zsh have 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

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

FUNCNAME

??

bash-specific see stackoverflow question

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Bashism (last edited 2022-10-20 23:13:29 by larryv)