Is there any mechanism in Shell script alike “include guard” in C++?

Question

let\'s see an example: in my main.sh, I\'d like to source a.sh and b.sh. a.sh, however, might have already sourced b.sh. Thus it will cause the codes in b.sh executed twice. Is

Answer 1

In bash, an early return does not affect the sourcing file, it returns to it as if the current file were a function. I prefer this method because it avoids wrapping the entire content in if...fi.

if [ -n "$_for_example" ]; then return; fi
_for_example=`date`

Answer 2

If you're sourcing scripts, you are usually using them to define functions and/or variables.

That means you can test whether the script has been sourced before by testing for (one of) the functions or variables it defines.

For example (in b.sh):

if [ -z "$B_SH_INCLUDED" ]
then
    B_SH_INCLUDED=yes
    ...rest of original contents of b.sh
fi

There is no other way to do it that I know of. In particular, you can't do early exits or returns because that will affect the shell sourcing the file. You don't have to use a name that is solely for the file; you could use a name that the file always has defined.

Answer 3

Personally I usually use

set +o nounset # same as set -u

on most of my scripts, therefore I always turn it off and back on.

#!/usr/bin/env bash

set +u
if [ -n "$PRINTF_SCRIPT_USAGE_SH" ] ; then
    set -u
    return
else
    set -u
    readonly PRINTF_SCRIPT_USAGE_SH=1
fi

If you do not prefer nounset, you can do this

[[ -n "$PRINTF_SCRIPT_USAGE_SH" ]] && return || readonly PRINTF_SCRIPT_USAGE_SH=1