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| = 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 == Parameter Expansions == List of expansions not defined by posix: * ${name:n:l} * ${name/ } * ${!name} == Arrays == [[BashFAQ/005 | Arrays]] are not defined by posix, there is no easy general workaround for arrays. Here are some hints: * The positional parameters are a kind of array (only one array): {{{ #build a command dynamically see [[BashFAQ/050]] set -- 'mycommand' 'needs some complex' 'args' "$@" #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 "$var" }}} * use IFS and set -f * eval, is powerful but dangerous so use it wisely. See [[BashFAQ/048 | Eval command and security issues]]. == Conditionals == || || '''Works in bash''' || '''Change to for dash''' || '''Comment''' || || simple test || `[[` || use [ and use double quotes around the expansions `[ "$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) - $before)) || date +%s is not posix see [[BashFAQ/070|this faq for more info]]|| || Generate a random number|| RANDOM || {{{random=$(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 |
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