Why is #!/usr/bin/env bash superior to #!/bin/bash?

There are a lot of systems that don't have Bash in /bin, FreeBSD and OpenBSD just to name a few. If your script is meant to be portable to many different Unices, you may want to use #!/usr/bin/env bash instead of #!/bin/bash.

Note that this does not hold true for sh; for Bourne-compliant scripts I exclusively use #!/bin/sh, since I think pretty much every Unix in existence has sh in /bin.

#!/usr/bin/env searches PATH for bash, and bash is not always in /bin, particularly on non-Linux systems. For example, on my OpenBSD system, it's in /usr/local/bin, since it was installed as an optional package.

If you are absolutely sure bash is in /bin and will always be, there's no harm in putting it directly in your shebang—but I'd recommend against it because scripts and programs all have lives beyond what we initially believe they will have.

For invoking bash it is a little bit of overkill. Unless you have multiple bash binaries like your own in ~/bin but that also means your code depends on $PATH having the right things in it.

It is handy for things like python though. There are wrapper scripts and environments which lead to alternative python binaries being used.

But nothing is lost by using the exact path to the binary as long as you are sure it is the binary you really want.

The standard location of bash is /bin, and I suspect that's true on all systems. However, what if you don't like that version of bash? For example, I want to use bash 4.2, but the bash on my Mac is at 3.2.5.

I could try reinstalling bash in /bin but that may be a bad idea. If I update my OS, it will be overwritten.

However, I could install bash in /usr/local/bin/bash, and setup my PATH to:

PATH="/usr/local/bin:/bin:/usr/bin:$HOME/bin"

Now, if I specify bash, I don't get the old cruddy one at /bin/bash, but the newer, shinier one at /usr/local/bin. Nice!

Except my shell scripts have that !# /bin/bash shebang. Thus, when I run my shell scripts, I get that old and lousy version of bash that doesn't even have associative arrays.

Using /usr/bin/env bash will use the version of bash found in my PATH. If I setup my PATH, so that /usr/local/bin/bash is executed, that's the bash that my scripts will use.

It's rare to see this with bash, but it is a lot more common with Perl and Python:

• Certain Unix/Linux releases which focus on stability are sometimes way behind with the release of these two scripting languages. Not long ago, RHEL's Perl was at 5.8.8 -- an eight year old version of Perl! If someone wanted to use more modern features, you had to install your own version.
• Programs like Perlbrew and Pythonbrew allow you to install multiple versions of these languages. They depend upon scripts that manipulate your PATH to get the version you want. Hard coding the path means I can't run my script under brew.
• It wasn't that long ago (okay, it was long ago) that Perl and Python were not standard packages included in most Unix systems. That meant you didn't know where these two programs were installed. Was it under /bin? /usr/bin? /opt/bin? Who knows? Using #! /usr/bin/env perl meant I didn't have to know.

And Now Why You Shouldn't Use #! /usr/bin/env bash

When the path is hardcoded in the shebang, I have to run with that interpreter. Thus, #! /bin/bash forces me to use the default installed version of bash. Since bash features are very stable (try running a 2.x version of a Python script under Python 3.x) it's very unlikely that my particular BASH script will not work, and since my bash script is probably used by this system and other systems, using a non-standard version of bash may have undesired effects. It is very likely I want to make sure that the stable standard version of bash is used with my shell script. Thus, I probably want to hard code the path in my shebang.