interp: support all Bourne shell (POSIX) builtins

[mvdan]

As described in https://www.gnu.org/software/bash/manual/html_node/Bourne-Shell-Builtins.html. We have most of them already.

[mvdan]

In the past week:

• added more missing test operators
• added command
• added the dir stack ones - dirs, pushd and popd

[mvdan]

Only times, trap, and umask remain.

[masaeedu]

Not sure why it's not on the linked page, but there's also https://www.gnu.org/software/bash/manual/html_node/The-Shopt-Builtin.html

[mvdan]

@masaeedu not listed because shopt is bash, not sh (i.e. not POSIX shell or bourne shell).

The interpreter does support some bash builtins, but there is no ticket for supporting all of them. I might open one once this one is closed, but I don't expect I'll have the patience to implement all of those myself.

[sheldonhull]

I'm sorry if this is already answered, but I've spent a few minutes digging through issues, discussions, and the Go package and can't find a list of what utilities are matched.
Is there something in the Go package I can look at to match commands?

Specifically I had issues in the past with mkdir on windows, and not sure how to create a cross platform "create a directory" command with sh. Sorry if I missed the docs so let me know where to look if you don't mind! Thanks again! 👋

[mvdan]

Note that mkdir is a coreutil, not a shell builtin:

$ echo $0
/bin/bash
$ command -v mkdir
/usr/bin/mkdir

The issue tracking native support for coreutils is #93. It somewhat stalled as it's a huge amount of work, so instead I wanted to leverage an existing coreutils implementation. If someone wants to take a look at reusing u-root as explained in #93 (comment), I would be happy to review a PR :)