Howto Make Script More Portable With #!/usr/bin/env As a Shebang

Posted on in Categories Debian Linux, Linux, OpenBSD, OS X, Perl, RedHat/Fedora Linux, Shell scripting, Solaris, Suse Linux, Tips, Ubuntu Linux, UNIX last updated March 6, 2007

You may notice that most shell and Perl script starts with the following line:

It is called a shebang. It consists of a number sign and an exclamation point character (#!), followed by the full path to the interpreter such as /bin/bash. All scripts under UNIX-like os and Linux execute using the interpreter specified on a first line. However there is a small problem. BASH or Perl is not always in the same location (PATH) such as /bin/bash or /usr/bin/perl. If you want to make sure that script is portable across different UNIX like operating system you need to use /usr/bin/env command.

Fig.01: My Perl script start with /usr/bin/env
Fig.01: My Perl script start with /usr/bin/env

More on env

The env command allows to run a program in a modified environment. First, find line
Replace with
#!/usr/bin/env bash
For example here is a small script:

#!/usr/bin/env bash
echo "$x and $y"


#!/usr/bin/env perl
use warnings;
print "Hello " x 5;
print "\\n";

Now you do not have to search for a program via the PATH environment variable. It makes the script more portable. Also, note that it is not a foolproof method. Always make sure you have /usr/bin/env exists or use a softlink/symbolic link to point it to correct path. Moreover, your work or script looks more professional with this simple hack.

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

25 comment

  1. I am confused.
    Why is /usr/bin/env more portable than /bin/bash.
    Besides on most linux distros bash is found in /bin, but env is not guaranteed to be found under /usr/bin.
    e.g. in my case (FC5) env is under /bin.
    My point is , which ever way you need to know the absolute path to either env, or bash, so why bother ?

  2. If you move from Linux distro to BSD you will see bash is located at /usr/local/bin/bash OR to Solaris you will see bash at /opt or some other location. Instead of adjusting all the location admin can create a /usr/bin/env softlink and problem solved. Just imagine you have 100s of shell and perl scripts…

    This is not just about Linux. It is about running a script under different UNIX like oses.

  3. I do not feel very comfortable with the security aspects of this hack.
    It may be more portable but it reduces control.
    Bash and other shells are hardened so that they can be used for admin jobs withstanding tampering efforts by non root users.
    Using the described “env” solution may introduce vulnerabilities which are difficult to oversee or analyze.
    Instead of linking env and using it to deal with compatibity I rather add compatibility links to my systems which link bash or perl to a uniform location e.g. ln -s /usr/local/bin/bash /bin/bash

  4. @Terrible
    You could be correct, but I’d like to hear your opinion of why exactly is it more harmful?

    Well, it runs the command, perl for example, like you would run perl by hand – so I assume it searches via PATH – I can see that individual user may then create executable ‘perl’ of his own under his home directory and change PATH so that env will call that… However that already means that user in question is doing this on purpose, in which case he could just call the harmful program himself – no need to even include the script, perl nor env on that and if done that way it would not affect other users which would still get to run the script as intended.

    However security and holes in it can be complex issues and I’m not a professional at all… If indeed using env is more of a possible security issue than creating shebang pointing right to correct interpreter in install routine I would love to learn why exactly is it safer, about possible security issues in using env, etc.
    I would think that when installed system wide both ways would be as safe – and one trying to do harm could install the whole software locally under his home directory anyway but could not alter what is ran when other users call the script in question – not understanding makes me even more curious, but most importantly I want to ensure security…

    1. Imagine a trojan ‘bash’ placed in ‘/tmp’.. An sa with sudo all nopasswd.. Their path has ‘.’ at the beginning.. CWD as ‘/tmp’.. Now imagine them executing any of their scripts with this hack?

      That’s a lot of stars to align, but remember.. you only need this to happen once! The patient will be rewarded!

      1. >Imagine a trojan ‘bash’ placed in ‘/tmp’.. An sa with sudo all nopasswd.. Their path has ‘.’ at the beginning.. CWD as ‘/tmp’.. Now imagine them executing any of their scripts with this hack?

        How did a trojan arrive in /tmp ?
        Their path has ‘.’ at the beginning.. -> Call you trojan ls and you have way more chance for this to work… Don’t put ‘.’ in PATH.

  5. Here’s an idea, instead of wasting cpu and peoples time, suggest a link to /bin/bash always. This stupid idea of using env command is a waste of the unix operating system.

    Symbolic links were created to solve problems exactly like this. Some idiot came up with the great idea to make it more complicated by suggesting a link to bin env… I swear this is absolutely retarded.

    Quit being lazy and put a proper link in /bin. What is the point in spawning processes you don’t need? More professional? According to who? The idiots who never wrote a single program of assembler code in their life? The idiots who couldn’t write a bash script to remove duplicate words from a list without resorting to external commands?

    Oh yeah, you must mean those idiots…. The one’s who couldn’t code their way out of wet paper sack.

  6. And another thing, if its really that important to be portable, and you can’t create a link in /bin then your scripts need to be auto-generated on-site with the proper header line.

    Quit wasting resources and practice efficiency. Wastefulness is a bad habit that not only adds up, but is certain to be a perpetual mistake.

  7. @Triston J. Taylor
    I came here to solve a problem with the python interpreter changing based on which python virtual environment was activated at a given time. Remapping the symlink doesn’t work at all as I may have one shell in one environment and another in a completely different one – it invokes different python symlinks. Using env fixes this perfectly.

  8. I just want to add to this now seven-year-old conversation that modern developers often change their development environment depending on which project they are working on.

    For example using rvm you might often change which version of the ruby interpreter you are using to ensure compatibility across different versions or to work on legacy code. Switching rubies switches your PATH to point to whichever ruby you are using.

    Additionally a developer may want to develop in a different version of say, python, than the version that ships with OS X, but doesn’t want to overwrite the system’s version or uninstall it because they know they do not know what the consequences of doing so. Using a package manager like Homebrew makes it easy to install scripting languages for the local user rather than forcing you to install it as root. Is that not more secure?

    In each of these scenarios you want to be using /usr/bin/env in your scripts because hard-coding the binary location is not practical and will be irritating to anyone else on your team if you try and pass off a script with something like #!/Users/joejimbob/.rvm/rubies/ruby-1.9.3-p286/bin/ruby at the top. Saying MY script doesn’t work because YOU don’t know how to symlink might earn me 3 or 4 old school points, but I would still be a jerk.

    As for being wasteful, trading end-user CPU cycles for a shorter development time and broader compatibility is the world we live in. Rejecting this and calling people lazy for it, is an obsolete way of thinking and shows a distrust of modern methods and software which is the sign of a bad coder (see #6 here: Isn’t abstraction the way by which we accomplish the most amazing things with technology? Isn’t it more wasteful to waste development time by re-inventing the wheel and forcing your peers to understand YOUR style rather than what’s most practical?

    I inherited a system from such old-fashioned types of people and I can tell you I still haven’t unearthed all of their “gotchas” of absolute paths and i-dont-trust-this-new-language-so-im-going-to-be-clever types of garbage. The way they did things was to approach the problem as if they were the first person to ever tackle it or that at the very least, they knew better because they’ve been around longer. I swear these guys thought they were going to be buried with the server.

  9. @Ovidiu
    Well you have to start somewhere! For the most part *nix distributions have standardized the locations so that both /usr/bin/env and /bin/env exist.

    Totally agree. I came here because I am faced with Perl code that starts “#! /usr/bin/perl”, but I have a requirement that means using a later release of Perl than that installed in the system area and in the environment I work in we are not allowed to change the “system” perl or add to its modules. So it should have been coded “#! /usr/bin/env perl”.
    Thanks for the page you linked to.

  10. I would also like to add that this technique has allowed my scripts to be portable to OSX where bash is still stuck at version 3. For example, I’ve installed mac ports and set my path for ‘bash’ to take precedence. But if I were to hard-code #!/bin/bash then that would do no good.

    ‘which env’ on OSX 10.9 /usr/bin/env as the only result and appears to be the most portable between Cygwin, Linux, and OSX.

  11. Funny thing is that according to `man env` “env – run a program in a modified environment”. So running `#!/usr/bin/env bash` instead of `#!/bin/bash` should have no effect as there are no environment variables being set or unset. I guess when `env` runs a command passed to it the PATH environment variable is used to locate the command but why there’s no information about this in the man page of `env` is a mystery for me.

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