Bashism

This page is an attempt to list some of the most common bashisms ie features not defined by POSIX (ie don't work in dash). 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

• "function" is not defined by posix, only "name ()" is
• the C-like for syntax (for (( ; ; ));do ..) is not defined by posix

• ;;& ;& etc in case (bash4) are not defined by posix

• the special quotes $' ' and $" " are not defined by posix
• select is not defined by posix
• the extended glob ( +() @( ) etc.. ) are not defined by posix
• [^a-b] to negate a range is not defined by posix, the standard way is [!a-b]

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

Arithmetic

• ++ -- are not defined by posix

Redirections

• >& and &> are not defined by posix

• m>n- m<n- ie duplicating and closing a descriptor at the same time is not defined by posix

• |& (bash4) is not defined by posix

Builtins

• echo. 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 provide, extglob, nullglob, dot glob etc ..are not defined by posix

more

• 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.