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When is it more appropriate to use #!/bin/bash rather than #!/bin/sh in a shell script?

In short:

• 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.