6

I notice that with bash scripts, some people use a different shebang to the one that I'm used to putting at the top of my own.

Can someone simplify the difference between these two? I use the #!/bin/bash one all the time.

#!/bin/bash
#!/usr/bin/env bash
Gilles 'SO- stop being evil'
  • 829,060
Danijel-James W
  • 1,735
  • 2
    see http://unix.stackexchange.com/questions/29608/why-is-it-better-to-use-usr-bin-env-name-instead-of-path-to-name-as-my and take note of what it says about passing arguments to the interpreter. IMO using /usr/bin/env like this is an abuse of the env program perpetrated mostly by the python community because they can't manage interpreter and library versioning properly. – cas Oct 05 '13 at 05:58

1 Answers1

12

The #!/usr/bin/env bash results in the script using whatever bash is found first in $PATH.

While it is common for bash to be located at /bin/bash. There are cases where it is not (different operating systems). Another potential use is when there are multiple bash shells installed (newer version at an alternate location like /usr/local/bin/bash).

Doing #!/usr/bin/env bash just takes advantage of a behavior of the env utility.
The env utility is normally used for manipulating the environment when calling a program (for example; env -i someprog to wipe the environment clean). However by providing no arguments other than the program to execute, it results in executing the specified program as found in $PATH.

Note that there are both advantages and disadvantages to doing this.

The advantages are as mentioned earlier, in that it makes the script portable if bash is installed in a different location, or if /bin/bash is too old to support things the script is trying to do.

The disadvantage is that you can get unpredictable behavior. Since you're at the mercy of the user's $PATH, it can result in the script being run with a version of bash that has different behavior than what the script expects.

phemmer
  • 71,831