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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 || || 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 || the C-like for syntax (for (( ; ; ));do ..) is not defined by posix || || case || ;;& || || ;;& in bash4 is not defined by posix || || case || ;& || || ;& in bash4 is not defined by posix || || select || select || '''''' || select is not defined by posix || || expand sequences || echo $'hello\tworld' || printf "hello\tworld" || $' ' is not defined by posix || || extended glob || ( +() @( ) etc... || || 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/ } * ${!name} == Arrays == * arrays are not defined by posix. == Conditionals == * `[[` is not defined by posix * `==` as an argument of test (aka [) is not defined by posix * `< >` to compare numbers as argument of test are not defined by posix, though dash implements them * `-nt`, `-ot`, `-ef` are not defined by posix * `(( ))` is not defined by posix (but `$(( ))` substitution is OK) == Arithmetic == * `++ --` are not defined by posix * `let` is not defined by posix == Redirections == * `>&` and `&>` are not defined by posix * `m>&n- m<&n-` i.e. duplicating and closing a descriptor at the same time is not defined by posix * `|&` (bash4) is 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 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. |
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