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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. In ksh both forms are present, but functions defined with "function" work slightly differently. || || case || `;;&` `;&` etc || none. duplicate the case (use a funnction to avoid code duplication) || `;;&` `;&` in bash4 is not defined by posix. ksh93 has `;&` but not ``;;&` || || 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. Present in ksh93. || || expand sequences || `echo $'hello\tworld'` || `printf "hello\tworld"` || `$' '` is not defined by posix and is bash-specific || || extended glob || `+( ) @( ) !( ) *( )` || not always possible, some times you can use several globs, sometimes you can use find(1) || not defined by posix. Present in ksh. || || select || select || some ideas: implement the menu yourself, use a command like dialog || not defined by posix. Present in ksh. || == Expansions == * Brace Expansion, eg {a,b,c} or {1..10} is not defined by posix. Both forms are present in ksh93, and the first form in older ksh. * <( ) >( ) process substitution is not defined by posix, but can be simulated with FIFOs: instead of `foo <(bar)`, write `mkfifo fifo; bar > fifo & foo fifo` (this is basically how process substitution is implemented on OSes that don't have a mechanism like `/dev/fd/` to refer to unnamed pipes with filenames). It is present in ksh93. == 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. This form is present in ksh93. * ${name/ } -- you can use `$(printf '%s\n' "$name" | sed 's/foo/bar/')`, after changing shell patterns to regular expressions. This form is prent in ksh93. * ${!name} -- bash-specific; it is possible, but dangerous, to use eval to achieve similar effects; see [[BashFAQ/006]]. Note that using `$( )` 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 == [[BashFAQ/005 | Arrays]] are not defined by posix (but are present in ksh); 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, but is present in ksh || || 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 || present in dash and ksh, 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; present in ksh || || check if 2 files are the same hardlink || `[[ file1 -ef file2 ]]` || ? || -et is not defined by posix, but is present in ksh || || `(( ))` || `(( ))` (without the $) acts like a command on its own || For simple comparison: `[ -lt ] (and -ne -gt -ge)` or `[ "$(( 3+1 < 5))" -eq 0 ]`. || present in ksh || || || || To assign a variable var=$((3+1)) || || == Arithmetic == || || '''Works in bash''' || '''Change to for dash''' || '''Comment''' || || pre/pos increment/decrement || `++ --` || `i=$((i+1))` || - || || - || `let` || `: $((i=i+1))` || The `:` command can be used to peform side effects with an expansion || == Redirections == || || '''Works in bash''' || '''Change to for dash''' || '''Comment''' || || redirect both stdout and stderr || `>&` and `&>`|| command > file 2>&1 or commnd 2>&1 | othercommand || - || || || `|&` (bash4)|| command 2>&1 | othercommand || - || || duplicate and close|| `m>&n- m<&n-` || `m>&n n>&-` ||not defined by posix || == Builtins == * `echo -n` or `-e` -- posix doesn't define any options, and furthermore allows `echo -e` to be the default behavior. Instead use `printf "%s\n"` (for normal echo) or `printf "%b\n"` (for `echo -e`); leave off the `\n` to simulate `echo -n`. * `printf -v` is not defined by posix. Also the `%q` format is not defined by posix. * `read` -- the only option defined by posix is `-r`; ksh has a different set of options that only partially overlaps with bash. * `shopt`, and therefore all the options it provides (extglob, nullglob, dotglob, etc.) are not defined by posix and are bash-specific * `local` -- there is no posix equivalent. You can use `$funcname_varname` to reduce the likelihood of conflicts, but even that is not enough for recursive functions. You can ensure that recursive calls occur in subshell environments (so there is a "local" copy of all variables), or pass all "local variables" as parameters (because the positional parameters $@, $1, $2, etc are truly local). dash explicitly supports `local` as a non-Posix extension; ksh uses `typeset` instead, which works like bash's `declare`. == Special Variables == || || '''Works in bash''' || '''Change to for dash''' || '''Comment''' || || keep track of the times|| SECONDS || before=$(date +%s) ....seconds=$(( $(date +%s) - $before)) || date +%s is not posix; see [[BashFAQ/070|this faq for more info]]. Present in ksh|| || Generate a random number|| RANDOM || {{{random=$(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. 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 [[ 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. * there is a handy perl script checkbashisms which is part of the debian devscripts package which can help point out bashisms in a particular script. ---- CategoryShell |
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