howto-enforce-bash-script-is-sourced-or-executed.md

Howto: enforce a bash script to be sourced or directly executed

When we write a bash script we have to take into consideration whether we intend the script to be sourced or executed.

In short, a "sourced" script ( source ./myscript.sh or . ./myscript.sh ) will execute the script in the current shell (versus forking a subshell). Therefore the environment of the current shell instance will be directly altered. Also, any exit commands will log you completely out of the shell.

Alternatively, an "executed" script ( ./myscript.sh ) will fork to a subshell and any changes to environment variables within that script will not impact the current shell you are working in. Similarly, any exit command within the script will only impact the subshell and not the working shell. An executable script needs to have its execution bit flipped on: chmod +x ./myscript.sh.

Make a decision! Will this script be a sourced script or an executed script?

Unless the script is a one-off, don't allow the end-user to pick one route or the other. The behavior is different for each path of execution and it is best practice to enforce one route or the other.

Enforcement can be achieved with a bit of inquiry at the top of a bash script. Here are some generic constructs that will give you a clue how things could work...

#!/usr/bin/bash
$(return 0 > /dev/null 2>&1) && echo "this is a sourced script" || echo "this is an executed script"
#$(return 0 > /dev/null 2>&1) && issourced=1 || issourced=0
(( $issourced )) && echo "this is a sourced script" || echo "this is an executed script"
(( $issourced == 1 )) && echo "this is a sourced script" || echo "this is an executed script"

...now let's enforce things..

Enforcing bash script to be sourced not executed

#!/usr/bin/bash
$(return 0 > /dev/null 2>&1) || ( echo "please do not run this directly. source it instead." && exit 1 )

That exit will end the script and $? will have a value of that exit code (in this case 1).

Enforcing bash script to be executed not sourced

#!/usr/bin/bash
$(return 0 > /dev/null 2>&1) && echo "please run directly, not as a sourced script" && return 1

That return will exit but only at the logic scope where it is called. In this case, the script. If you stick a return in a function, for example, it will only exit that function and the script will continue otherwise. This is a primary reason why "sourcing" instead of "executing" can have unexpected execution behaviors.

Leaving that scope, $? will be set to the return value.