POSIX shell compatible and POSIX conforming dirname implementation

Question

The discussion in the comments of this answer made me wonder how hard it would be to write a built-in only version of dirname.

This is what I ended up with but I'm reasonably confident it should be able to be improved (I haven't spent much time thinking about it yet).

Any improvements/suggestions are welcome.

#!/bin/sh

p=$1
alt=$2
skip78=

stripslashes() {
    i=$1
    while [ "$i" != "$o" ]; do
        o=$i
        i=${i%/}
    done
    eval "$2=\$i"
}

[ "$p" = '//' ] || {
    case "$p" in
        *[!/]*|'')
            stripslashes "$p" p
            case "$p" in
                */*)
                    p=${p%/*}
                    ;;
                *)
                    p=.
                    skip78=skip78
                    ;;
            esac
            ;;
        *)
            p=/
            skip78=skip78
            ;;
    esac
}
[ -n "$skip78" ] || {
    { [ "$p" != '//' ] || [ -z "$alt" ]; } && {
        stripslashes "$p" p
        [ -z "$p" ] && p=/
    }
}

printf -- %s\\n "$p"

The POSIX spec for dirname is:

1. If string is //, skip steps 2 to 5.

2. If string consists entirely of <slash> characters, string shall be set to a single <slash> character. In this case, skip steps 3 to 8.

3. If there are any trailing <slash> characters in string, they shall be removed.

4. If there are no <slash> characters remaining in string, string shall be set to a single <period> character. In this case, skip steps 5 to 8.

5. If there are any trailing non- <slash> characters in string, they shall be removed.

6. If the remaining string is //, it is implementation-defined whether steps 7 and 8 are skipped or processed.

7. If there are any trailing <slash> characters in string, they shall be removed.

8. If the remaining string is empty, string shall be set to a single <slash> character.

Update: Made a gist.

Answer 1

I don't think there's a really compelling reason for this eval in stripslashes:

eval "$2=\$i"

It's a dirty hack to update the parameter variable in-place. But this is not a common practice, I don't know other commands or built-ins that work that way, and I would find such usage unexpected. I would find it more usable if stripslashes behaved more like other commands do:

• print the result on stdout
• set the result in a designated variable (like REPLY)

That would be more in-line with common UNIX practices, and unsurprising, familiar.

---

As a minor nitpick, I avoid quoting when it's unnecessary. For example instead of [ "$p" != '//' ], I would write [ "$p" != // ]

---

Instead of this:

        [ -z "$p" ] && p=/

This form is shorter:

        [ "$p" ] || p=/

---

This might be subjective, but I would rewrite this part using regular if-else:

[ -n "$skip78" ] || {
    { [ "$p" != '//' ] || [ -z "$alt" ]; } && {
        stripslashes "$p" p
        [ -z "$p" ] && p=/
    }
}