If the shell scripts start with #!/bin/bash, they will always

Question

If the shell scripts start with #!/bin/bash, they will always

run with bash from /bin. If they however start with #!/usr/bin/env bash, they will search for bash in $PATH and then start with the first one they can find.

Why would this be useful? Assume you want to run bash scripts, that require bash 4.x or newer, yet your system only has bash 3.x installed and currently your distribution doesn't offer a newer version or you are no administrator and cannot change what is installed on that system.

Of course, you can download bash source code and build your own bash from scratch, placing it to ~/bin for example. And you can also modify your $PATH variable in your .bash_profile file to include ~/bin as the first entry (PATH=$HOME/bin:$PATH as ~ will not expand in $PATH). If you now call bash, the shell will first look for it in $PATH in order, so it starts with ~/bin, where it will find your bash. Same thing happens if scripts search for bash using #!/usr/bin/env bash, so these scripts would now be working on your system using your custom bash build.

One downside is, that this can lead to unexpected behavior, e.g. same script on the same machine may run with different interpreters for different environments or users with different search paths, causing all kind of headaches.