March 13, 2013

A bashism a week: assigning to variables and special built-ins

Assigning a value to a variable when executing a command is a way to populate the command's environment, without the variable assignment persisting after the command completes. This is not true, however, when a special built-in is the command being executed.

POSIX:2001 states that "Variable assignments specified with special built-in utilities remain in effect after the built-in completes".

Not only this is tricky because it depends on whether a utility is a special built-in or not, but the bash interpreter does not respect that behaviour of the POSIX standard. That is, special built-ins are not so "special" to the bash interpreter.

This leaves two things to take into account when assigning to a variable when executing a command: whether the command is a special built-in, and whether bash is interpreting the script.

Now, the list of special built-ins is rather short and it would be a bit unusual to perform variable assignments when calling them, except for some cases: "exec", "eval", "." (dot), and ":" (colon).

It is important to note that ":" and "true" differ in this regard; the former is a special built-in, the latter is just a utility. Watch out for this kind of differences when using ":" or "true" to nullify a command. E.g.

$ dash -c '
# some condition or user setting ends up making:
# later:
foo=bar $method
echo foo: $foo'
To (redacted for brevity):
$ dash -c '
foo=bar $method
echo foo: $foo'
foo: bar

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