bashisms

Bashisms are language features that are valid in the Bash shell but not in the POSIX standard shell. They arise when script authors assume Bash-specific behavior, making their scripts non-portable to systems where /bin/sh is linked to dash, ash, or other shells. The term is sometimes used pejoratively to encourage portability.

Common examples include: using arrays (for example, arr=(a b c)); the Bash conditional test [[ ... ]] instead of the POSIX [ ... ]; process substitution like <(command) or >(command); extended globbing patterns such as extglob; and advanced parameter expansions like ${var,,} to convert to lowercase or ${var//old/new} for substitution. Other frequently cited bashisms are the $'string' ANSI-C quoting and certain shell options that enable Bash-only behavior. The presence of a shebang such as #!/bin/bash rather than #!/bin/sh can also signal a non-portable script.

Detection and mitigation: to identify bashisms, developers use tools such as checkbashisms and ShellCheck, which warn about portability issues and Bash-specific constructs. The recommended practice for portable scripts is to write in POSIX sh syntax, avoid arrays, [[ ... ]] tests, and process substitutions, and explicitly invoke a POSIX shell (for example, #!/bin/sh). Testing scripts under multiple shells, such as dash, ash, or ksh, helps ensure compatibility.

Conclusion: Bashisms reflect the broader distinction between Bash's extensions and the POSIX shell, shaping debates about script portability and maintainability in Unix-like environments.