bug#34488: Add sort --limit, or document workarounds for sort|head error messages

[Assaf Gordon]

Hello,

Thanks for all comments.

On 2019-02-24 11:33 a.m., Paul Eggert wrote:
> Thanks for doing all that. Although Pádraig is not enthusiastic about a 
> shortcut like -p, I'm a bit warmer to it, as it's an important special 
> case to fix a wart in POSIX. No big deal either way.

For now I kept "-p", can be removed later of course.
The first patch includes Pádraig's recent suggestions (slightly modified).

> The documentation should mention that SIGCHLD is special [...]

> The documentation should say what happens if mutually-contradictory 
> options are specified, [...]

> The documentation should echo this suggestion in 
> <http://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html>: 

I've added those, and I welcome all improvements suggestion to 
grammar/phrasing/etc.

> There should be options --block-signal[=SIG], --unblock-signal[=SIG],
> and --setmask-signal[=SIG] that affect the signal mask, which is also
> inherited by the child. These can be implemented via pthread_sigmask.

The second patch adds these new options (separated to ease review).
As for documentation - I'm not sure what to add beyond the basic
option description. When should these be used?

A third small patch adds "env ---list-signal-actions" and
"env --list-blocked-signals" - to ease diagnostics.
Might be worth adding for completeness (e.g., for users who
need to somehow know if SIGPIPE is being ignored by the shell
or not):

    $ ( trap '' PIPE && src/env --list-signal-actions )
    PIPE       (13): ignore

Comments very welcomed,
 - assaf

0001-env-new-options-p-default-signal-SIG-ignore-signal-S.patch
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0002-env-add-block-unblock-setmask-signal-SIG-options.patch
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0003-env-add-list-blocked-signals-list-signal-actions-opt.patch
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