bash Cookbook

Finding bash Portably for #!

Problem

You need to run a bash script on several machines, but bash is not always in the same place. See Getting bash for xBSD.

Solution

Use the /usr/bin/env command in the shebang line, as in #!/usr/bin/env bash. If your system doesn’t have env in /usr/bin, ask your system administrator to install it, move it, or create a symbolic link because this is the required location. For example, Red Hat inexplicably uses /bin/env, but they at least create a symlink to the correct location.

You could also create symbolic links for bash itself, but using env is the canonical and correct solution.

Discussion

env’s purpose is to “run a program in a modified environment,” but since it will search the path for the command it is given to run, it works very well for this use.

You may be tempted to use #!/bin/sh instead. Don’t. If you are using bash-specific features in your script, they will not work on machines that do not use bash in Bourne shell mode for /bin/sh (e.g., BSD, Solaris, Ubuntu 6.10+). And even if you aren’t using bash-specific features now, you may forget about that in the future. If you are committed to using only POSIX features, by all means use #!/bin/sh (and don’t develop on Linux, see Developing Portable Shell Scripts), but otherwise be specific.

You may sometimes see a space between #! and /bin/whatever. Historically there were some systems that required the space, though in practice we haven’t seen one in a long time. It’s very unlikely any system running ...