Importing functions from a shell script

According to the “Shell Builtin Commands” section of the bash manpage, . aka source takes an optional list of arguments which are passed to the script being sourced. You could use that to introduce a do-nothing option. For example, script.sh could be:

#!/bin/sh

foo() {
    echo foo $1
}

main() {
    foo 1
    foo 2
}

if [ "${1}" != "--source-only" ]; then
    main "${@}"
fi

and unit.sh could be:

#!/bin/bash

. ./script.sh --source-only

foo 3

Then script.sh will behave normally, and unit.sh will have access to all the functions from script.sh but will not invoke the main() code.

Note that the extra arguments to source are not in POSIX, so /bin/sh might not handle it—hence the #!/bin/bash at the start of unit.sh.