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Every so often I'll bash out a bash script and it strikes me there are a few ways of setting a variable:

key=value
env key=value
export key=value

When you're inside a script or a single command (for instance, I'll often chain a variable with a Wine launcher to set the right Wine prefix) these seem to be completely interchangeable but surely that can't be the case.

What's the difference between these three methods and can you give me an example of when I would specifically want to use each one?

Definitely related to What is the difference between `VAR=...` and `export VAR=...`? but I want to know how env fits into this too, and some examples showing the benefits of each would be nice too :)

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3  
Note that export key=value is extended syntax and should not be used in portable scripts (i.e. #! /bin/sh). –  Simon Richter Oct 24 '12 at 11:49

1 Answer 1

up vote 28 down vote accepted

Let us consider a specific example. The grep command uses an environment variable called GREP_OPTIONS to set default options.

Now. Given that the file test.txt contains the following lines:

line one
line two

running the command grep one test.txt will return

line one

If you run grep with the -v option, it will return the non-matching lines, so the output will be

line two

We will now try to set the option with an environmental variable.

  1. Environment variables set without export will not be inherited in the environment of the commands you are calling.

    GREP_OPTIONS='-v'
    grep one test.txt
    

    The result:

    line one
    

    Obviously, the option -v did not get passed to grep.

    You want to use this form when you are setting a variable only for the shell to use, for example in for i in * ; do you do not want to export $i.

  2. However, the variable is passed on to the environment of that particular command line, so you can do

    GREP_OPTIONS='-v' grep one test.txt
    

    which will return the expected

    line two
    

    You use this form for temporarily change the environment of this particular instance of the program launched.

  3. Exporting a variable causes the variable to be inherited:

    export GREP_OPTIONS='-v'
    grep one test.txt
    

    returns now

    line two
    

    This is the most common way of setting variables for use of subsequently started processes in a shell

  4. This was all done in bash. export is a bash builtin; VAR=whatever is bash syntax. env, on another hand, is a program in itself. When env is called, following things happen:

    1. The command env gets executed as a new process
    2. env modifies the environment, and
    3. calls the command that was provided as an argument. The env process is replaced by the command process.

    Example:

    env GREP_OPTIONS grep one test.txt
    

    ...will launch two new processes: (i) env and (ii) grep (actually, the second process will replace the first one). From the point of view of the grep process, the result is exactly the same as running

    GREP_OPTIONS='-v' grep one test.txt
    

    However, you can use this idiom if you are outside of bash or don't want to launch another shell (for example, when you are using the exec() family of functions rather than the system() call).

Additional note on #!/usr/bin/env

This is also why the idiom #!/usr/bin/env interpreter is used rather than #!/usr/bin/interpreter. env does not require a full path to a program, because it uses the execvp() function which searches through the PATH variable just like a shell does, and then replaces itself by the command run. Thus, it can be used to find out where an interpreter (like perl or python) "sits" on the path.

It also means that by modifying the current path you can influence which python variant will be called. This makes the following possible:

echo -e '#!/usr/bin/bash\n\necho I am an evil interpreter!' > python
chmod a+x ./python
export PATH=.
calibre

instead of launching Calibre, will result in

I am an evil interpreter!
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Why does GREP_OPTIONS='-v' grep one test.txt work? I thought it needed a semicolon after '-v' (but I tried it and it does in fact work.) –  Joe Oct 26 '12 at 6:20
1  
Because with a semicolon, it is interpreted as two separate bash commands; the first one sets the variable (without exporting it), and the second starts with an environment that doesn't have the variable exported. Without the semicolon, however, this is one command (grep), preceded by setting a local environment. –  January Oct 26 '12 at 7:05
    
Slight correction: in step 4 where you have env GREP_OPTIONS grep one test.txt, I think that should actually be env GREP_OPTIONS='-v' grep one test.txt –  yshavit Sep 24 at 19:00
    
Where do all the variables from env come from though? I mean when you open a new shell you always have a few variables. So some program must have exported those, right? –  Pithikos Oct 30 at 14:05

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