Re: [dev] New utility "when"

From: Truls Becken <truls.becken_AT_gmail.com>
Date: Thu, 12 Dec 2013 16:32:27 +0100

On 2013-12-12, at 13:28, Andrew Gwozdziewycz wrote:

> Are you suggesting that the shell handle the command after the &&?
> Or you let the subshell I spawn do it?

The shell would handle the command after &&. The command after recognized
arguments is handled using fork + execvp, without a subshell.

> For the -z mode (default) that could totally work. So, for example:
>
> when cat /file/that/will/eventually/exist && xmessage 'exists'
>
> Instead of running "/bin/sh -c 'cat /file/that/will/eventually/exist'"
> until the exit status is 0 and then running "/bin/sh xmessage
> 'exists'" you just repeatedly run "/bin/sh -c 'cat
> /file/that/will/eventually/exist && xmessage 'exists''"

Yes, repeat cat until it succeeds. Then return 0 to the shell.

> I agree that this works, but it's also incorrect because the second
> command must exit with 0, or it'll keep running. I've made the call
> that I don't care about what happens with the second command, only the
> first, and for that reason, that method produces the incorrect
> behavior.

Huh? The when command exits once, letting the shell call xmessage once.

> This doesn't work with -t for the simple reason that if you run
> everything after the when token via '/bin/sh -c ...' && waits for the
> first command to finish, which as stated previously is incorrect.

No. The shell will only let "when" see the stuff before &&.

> But, I'm pretty sure you had other intentions, and I'm not following
> your thinking. Could you clarify your point?


In my mind, when would simply:

1) Read all leading elements of argv that start with "-".

2) Repeatedly fork and exec argv (which now points to a command and its
   arguments).

3) Return 0 when the child process returns 0, or in the case of -t when the
   child process runs long enough.

The first part is known as chain loading or Bernstein loading, and is done by
programs such as nice, env, sudo, etc. The only issue I see in the context of
"when" is that the repeated child process will be orphaned in -t mode.

Perhaps my version should be called "retry" rather than "when".

-Truls
Received on Thu Dec 12 2013 - 16:32:27 CET