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Interactive job control and looping constructs


From: Ed Avis
Subject: Interactive job control and looping constructs
Date: Wed, 24 Dec 2014 11:51:23 +0000 (UTC)
User-agent: Loom/3.14 (http://gmane.org/)

At an interactive bash prompt, run a 'for' loop:

% for i in a b c; do echo $i; sleep 10; done

Then interrupt this with Ctrl-Z.  The process interrupted is just whichever
sleep process was running at the time.  You can then resume it with 'fg' but
the loop does not continue.

I understand that job control applies to processes and that bash does not
create a new process to run a 'for' loop.  You can force it to do so:

% (for i in a b c; do echo $i; sleep 10; done)

Now job control does something more sensible; the whole loop can be paused,
put into the background or the foreground.  But if you didn't have the
foresight to put () around your command, you are stuck if you later decide
you want to put it in the background.

Is there something bash could do to be more user-friendly here?
Could job control be made to apply to 'for' and 'while' loops, even if they
were run without forking a separate process?  After all, you can fork() at
any time, so perhaps a new child process could be created when needed for the
purpose of putting the loop in the background.

Failing that, it should at least be possible to interrupt a 'for' loop with
Ctrl-Z, then 'fg' and have it continue where it left off.  If putting it into
the background is not practical then 'bg' can give an error message.

Backgrounding or foregrounding just one individual command, and then
abandoning the rest of the loop somehow, is surely never the desired
behaviour.

-- 
Ed Avis <address@hidden>




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