Add wait command

[s417-lama]

Description

We (@tasq0 and I) implemented wait command.

The implementation is almost following POSIX. As discussed in issue #1422, we can't retain the exit status of background jobs, but we thought it's not a critical problem. If we want to know the failure of background jobs, there are another ways to do it. Shell needs wait command.

The functions we implemented

Wait for all background jobs

With no options specified, wait for all background jobs finishing.

For example:

$ sleep 5 &; sleep 10 &; sleep 7 &
$ wait
Job 1, 'sleep 5 &' has ended
Job 3, 'sleep 7 &' has ended
Job 2, 'sleep 10 &' has ended
$

Wait for specified jobs

With job ids or process ids specified, wait for only specified jobs finishing.

For example:

$ sleep 5 &; sleep 10 &; sleep 7 &
$ wait %1 %3
Job 1, 'sleep 5 &' has ended
Job 3, 'sleep 7 &' has ended
$
Job 2, 'sleep 10 &' has ended

The wait command returns before job 2 (sleep 10 &) finishes.

Wait for any background jobs

With -n option given, wait for any background jobs finishing.

For example:

$ sleep 5 &; sleep 10 &; sleep 7 &
$ wait -n
Job 1, 'sleep 5 &' has ended
$
Job 3, 'sleep 7 &' has ended
Job 2, 'sleep 10 &' has ended

The wait command returns immediately when job 1 (sleep 5 &) finishes.

Acknowledgment

I would like to express my gratitude to @tasq0 who also greatly contributed to this PR.

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added
• User-visible changes noted in CHANGELOG.md

[ridiculousfish]

Well this looks awesome! I'll review!

[ridiculousfish]

Ugh, I'm sorry this sat so long! Merged as c7a682e, thanks!

[ridiculousfish]

Added to changelog as 51d34e1

[ridiculousfish]

Docs added as 1014995