cmd: add 'help' subcommand to zpool and zfs

[robn]

Motivation and Context

program help subcommand is a reasonably common pattern for multifunction command-line programs.

At the very least, I use git help and cargo help fairly often, and I find myself typing zfs help or zpool help often enough, and being annoyed that I didn't land in the manpage. So something in me is expecting it to happen, and there's precedent, and I suggest (with no data to back it up at all) that it goes some way to make a command-line program more approachable and discoverable.

(I am willing to turn in my "old-timey-sysadmin" membership card if that's what it takes!)

Description

This commit adds support for that style to the zpool and zfs commands.

When run as zpool help [<topic>] or zfs help [<topic>], executes the man program on the PATH with the most likely manpage name for the requested topic: zpool-<topic> or zfs-<topic> for subcommands, or zpool<topic> or zfs<topic> for the concepts and props topics. If no topic is supplied, uses the top zpool or zfs pages.

There's loads more that could be done here, from being smarter about how the manpage names are constructed, to allowing alternate viewers or documentation formats (especially on non-Unix platforms). You might run git help help for ideas if you haven't seen this before.

How Has This Been Tested?

Tested by hand on Linux and FreeBSD:

FreeBSD:

$ ./zfs help | head -1
ZFS(8)                  FreeBSD System Manager's Manual                 ZFS(8)
$ ./zfs help list | head -1
ZFS-LIST(8)             FreeBSD System Manager's Manual            ZFS-LIST(8)
$ ./zfs help props | head -1
ZFSPROPS(7)        FreeBSD Miscellaneous Information Manual        ZFSPROPS(7)
$ ./zfs help badpage
No manual entry for zfs-badpage

Linux:

$ ./zpool help | head -1
ZPOOL(8)                  BSD System Manager's Manual                 ZPOOL(8)
$ ./zpool help import | head -1
ZPOOL-IMPORT(8)           BSD System Manager's Manual          ZPOOL-IMPORT(8)
$ ./zpool help concepts | head -1
ZPOOLCONCEPTS(7)     BSD Miscellaneous Information Manual     ZPOOLCONCEPTS(7)
$ ./zpool help dunno
No manual entry for zpool-dunno

(the difference in page title is because its just calling man, so showing the system manpages, which are slighly rebranded on FreeBSD's builtin OpenZFS).

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[behlendorf]

This is a nice addition. My only gripe is that it appears to cause cli_root/zfs_program/zfs_program_json to fail.

[robn]

Confirmed. Surprised me! That test includes the common usage trailer for all commands, which this PR changed. Feels like a bit of a brittle test (I'd at least prefer a "starts with" check) but I'm not interested in understanding and reworking it now, so I've just pushed an update to match the trailer. Lets see how this run goes.