With "Bashism" I mean shell syntax which is only understood by the bash shell and not other shells.

If you write a script which gets executed only in environments where /bin/bash exists, then I think avoiding Bashism is just useless and wasting time, but maybe I am missing something.

What is the benefit of scripting things in a more complicated way, when there is an easier solution if you are allowed to use a feature which is only available in the Bash?

There is a follow-up question: Are there concrete figures on the speed of bash vs dash?

I published my conclusion here: https://github.com/guettli/programming-guidelines/#portable-shell-scripts

  • 4
    Long story short: portability.
    – traducerad
    Jul 25, 2018 at 7:44
  • I vote to close this question as primarily opinion-based. Who says there’s anything wrong with bashisms in the first place? If you don’t need portability to systems without bash, why bother at all?
    – dessert
    Jul 25, 2018 at 7:55
  • @traducerad protability to what? Is there a system where bash does not exist? AFAIK the bash shell can be installed on all linux environments. I don't care for ms-windows or unix/aix/hp-ux systems. And even on these systems the bash can be installed.
    – guettli
    Jul 25, 2018 at 7:56
  • 1
    @dessert you understood me "why bother at all?". I see a lot of people wasting time because the want to avoid bashism. I don't get it. The bash is great, it is available. Why not use it?
    – guettli
    Jul 25, 2018 at 7:58
  • 6
    @guettli: My router doens't have enough memory to store a kernel, core userland utilities and Bash and it's cheaper and easier to replace Bashisms with whatever Busybox understands for what little I need that router to do. Jul 25, 2018 at 12:31

2 Answers 2


First, portability. Nothing wrong if you are sure bash ( and preferably same or newer ) version will be everywhere you use the script. If you're a developer or sysadmin that expects software to be used on Unix-like OS besides Ubuntu, and they may or may not have bash, then bashisms won't be understood by /bin/sh.

Second, POSIX compliance. If you write and submit scripts to be included as part of OS or a project, they often required to be in /bin/sh syntax, which is basically what POSIX standard is. /bin/sh is often preferred for performance reasons, so if you need speed in shell scripts, bashisms and therefore bash maybe something to avoid.

In short, bashisms aren't bad. It really depends on the context for which you write a script. If it's a certainty that bash will be available, and not very outdated, then by all meens use the features - they're there for a reason.

  • OP seems to premise bashisms should best be avoided at all – maybe write something about that too? ;P
    – dessert
    Jul 25, 2018 at 8:01
  • @dessert T_T I'm typing on phone. Jul 25, 2018 at 8:05
  • Honestly, how likely are you to encounter a Unix-like system anywhere today without Bash available? Jul 25, 2018 at 8:27
  • 5
    @leftaroundabout: Plenty of specialized embedded systems ship without bash. Jul 25, 2018 at 9:02
  • @leftaroundabout you talk about "specialized embedded systems". I care for them like I care for ms-windows or android (in the context of this question): Not at all. Please read the question carefully. "If you write a script which gets executed only in environments where /bin/bash exists ...".
    – guettli
    Jul 25, 2018 at 11:19

Nothing, as such, if you know you're using a Bash-specific feature, and remember to use the #!/bin/bash hashbang instead of assuming /bin/sh is Bash.

In Ubuntu (and Debian) "bashisms" in #!/bin/sh scripts are/were mostly an issue when the default /bin/sh was changed to Dash instead of Bash as it was earlier (see DashAsBinSh in Ubuntu wiki). All scripts running with /bin/sh had to be checked for bashisms and fixed to use standard features supported by Dash. The change wasn't about availability of Bash (it's still an Essential package, so always installed), but about speed: before systemd, the bootup process spawned numerous shell scripts, and changing the default shell to a faster one actually had an impact.

On other systems, /bin/sh might still be Bash, making it possible to accidentally use Bash-specific features in scripts marked with #!/bin/sh. They would not work directly in a system where sh is not Bash. Then there are systems that don't have Bash at all. Embedded systems often only have the Busybox shell. Non-Linux Unixen may not have Bash, though they often do have some version of ksh, which is where many of Bash's features come from. They're not 1:1 compatible, however.

Some of the non-standard features in Bash are very useful (e.g. arrays, substring slices (${var:n:m}), text replace (${var/foo/bar})), so if they make your script easier to write, by all means use Bash.

That said, there are some bashisms that have direct standard equivalents, meaning that there's little or no reason to use the non-standard variant. Some that come to mind:

  • the == operator in [ .. ] is non-standard, but equivalent to the standard = operator
  • function f { ... } and f() { ... } are equivalent in Bash, but the former is non-standard. (It's from ksh, where there's a difference.)
  • $((--n)) is non-standard, but can be replaced with $((n=n-1))
  • In the simple cases [[ ... ]] can be replaced with the standard [ .. ]

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