I don't like the `require_curl` example.

Here's what is done there:

    OK=0
    FAIL=1

    function require_curl() {
        which curl 2>&1 > /dev/null
        if [ $? -eq 0 ]
        then
          return $OK
        fi

        return $FAIL
    }

(`2>&1 > /dev/null` drops stdout and writes stderr to stdout—not what was meant. `which` doesn't write to stderr, so I drop that part.)

That can be shortened significantly by using the return code directly in the if branch:

    function require_curl() {
        if which curl > /dev/null
        then
          return $OK
        fi

        return $FAIL
    }

Or by just using the return code directly:

    function require_curl() {
        which curl > /dev/null
        return $?
    }

And as it will return the return code of the last statement executed:

    function require_curl() {
        which curl > /dev/null
    }