When you are creating a new shell script, you want to make sure it is as problem free as possible, but sometimes it can be a bit confusing to know which shebang is the best one for you to use. On that note, today’s SuperUser Q&A post has the answer to a confused reader’s question.
Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.
The Question
SuperUser reader Hendre wants to know when it is better to use #!/bin/bash instead of #!/bin/sh in shell scripts:
When is it better to use #!/bin/bash instead of #!/bin/sh in a shell script?
The Answer
SuperUser contributor grawity has the answer for us:
Have something to add to the explanation? Sound off in the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.
There are several shells which implement a superset of the POSIX sh specification. On different systems, /bin/sh might be a link to ash, bash, dash, ksh, zsh, etc. It will always be sh-compatible though, never csh or fish. As long as you stick to sh features only, you can (and probably even should) use #!/bin/sh and the script should work fine, no matter which shell it is. If you start using bash-specific features (i. e. arrays), you should specifically request bash because, even if /bin/sh already invokes bash on your system, it might not on everyone else’s system, and your script will not run there. The same applies to zsh and ksh, of course. Even if the script is for personal use only, you might notice that some operating systems change /bin/sh during upgrades. For example, on Debian it used to be bash, but was later replaced with the very minimal dash. Scripts which used bashisms but had #!/bin/sh suddenly broke.
However:
Even #!/bin/bash is not that correct. On different systems, bash might live in /usr/bin, /usr/pkg/bin, or /usr/local/bin. A more reliable option is #!/usr/bin/env bash, which uses $PATH. Although the env tool itself is not strictly guaranteed either, /usr/bin/env still works on more systems than /bin/bash does.
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