Wait for one of two processes to complete in a shell script

nonoitall · 06-01-2008, 12:22 AM

Title says it all pretty much. Let's say I start process A, then start process B. How can I wait until either A or B completes (without waiting for both to complete), and then continue execution of my script?

ntubski · 06-01-2008, 12:57 AM

Code:

processA &
pidA=$!

processB &
pidB=$!

if <condition here> ; then
  wait $pidA
else
  wait $pidB
fi

EDIT: looking at it now, I have no I have no idea how I interpreted your question this way...

nonoitall · 06-01-2008, 01:50 AM

Maybe I didn't explain my problem clearly in my first post. I launch two processes, A and B. If A finishes before B, I want my script to resume as soon as A finishes. If B finishes before A, I want my script to resume as soon as B finishes.

matthewg42 · 06-01-2008, 05:29 AM

Code:

#!/bin/bash

t1=$(($RANDOM % 10))
t2=$(($RANDOM % 10))
echo "$(date +%T)|times are: $t1 and $t2"
sleep $t1 &
sleep $t2 &

keep_waiting=1
while true; do
        running=$(jobs |wc -l)
        if [ $running -gt 1 ]; then
                echo "$(date +%T)|still $running running... sleeping"
                sleep 1
        else
                echo "$(date +%T)|only $running running now... ending loop"
                break
        fi
done

echo "$(date +%T)|at least on of them finished..."
wait
echo "$(date +%T)|now both ar done"

nonoitall · 06-01-2008, 02:49 PM

Cool, polling never even occurred to me. I'll give that a shot - thanks!

osor · 06-01-2008, 04:08 PM

You could always exploit SIGCHLD. Here is one way to do so (there are many others in a similar fashion).

Code:

#!/bin/sh

echo "Starting both processes…"

sh -c 'trap "exit" CHLD
       process1 & process2 & wait'

echo "One process has exited."

Edit:
It seems that the above will not work if sh is symlinked to bash. It will work on ksh, zsh, dash, etc., but for some reason, trapping SIGCHLD doesn’t work in bash in a script.

As a workaround (if sh is a symlink to bash on your system), try this:

Code:

#!/bin/sh

echo "Starting both processes…"

sh -c 'set -m
       trap "exit" CHLD
       process1 & process2 & wait'

echo "One process has exited."

nonoitall · 06-02-2008, 03:00 AM

Thanks!

archtoad6 · 06-05-2008, 08:02 PM

Is this an OR situation or an AND? -- you do want to resume when EITHER completes, not when BOTH are done, don't you?

I ask because I assume that "either ... or ..." in your OP means what it says, whereas to me the logic in both post #4 & #6 seems to be AND logic.

osor · 06-05-2008, 09:34 PM

Quote:

Originally Posted by archtoad6

I ask because I assume that "either ... or ..." in your OP means what it says, whereas to me the logic in both post #4 & #6 seems to be AND logic.

I interpreted the question as “either … or …” and my post logic reflects this. I am not sure how you read otherwise. The line “One process has exited.” will be printed when either process exits.

Btw, AND logic is much easier to do (at least in shell), since the wait builtin may take multiple pid or job-id arguments, and will wait for all of them.

nonoitall · 06-05-2008, 10:24 PM

It was either or. I'm automating a series of tasks, keeping two running at a time (to make better use of a dual-core processor). So, as soon as one task finishes, another should start. Since the tasks vary in length, I'm arranging it like a queue where each 'slot' takes a new task of a list when its current process has finished.

osor · 06-06-2008, 03:10 PM

Quote:

Originally Posted by nonoitall

I'm automating a series of tasks, keeping two running at a time (to make better use of a dual-core processor). So, as soon as one task finishes, another should start. Since the tasks vary in length, I'm arranging it like a queue where each 'slot' takes a new task of a list when its current process has finished.

You can do this using SIGCHLD as well (although it would be much easier in C).

Try out the following (it should do what you want, barring race conditions):

Code:

#!/bin/bash

SLOTS=2

(	set -m

	trap 	"if read next ; then
			eval \$next &
		fi" CHLD

	for ((i = 0; i < $SLOTS; i++)); do
		read proc
		eval "$proc" &
	done

	wait
) << __EOF__
zenity --info --text=1st
zenity --info --text=2nd
zenity --info --text=3rd
zenity --info --text=4th
zenity --info --text=5th
zenity --info --text=6th
__EOF__

echo "Rest of script goes here"

I am not sure, however, of the portability to other shells.

When you say queue, generally you mean a First-In, First-Out (FIFO), so you could very well use pipes (named or anonymous) instead of hardcoding as the method of sending commands.

osor · 06-10-2008, 04:10 PM

I haven’t been here for awhile, but if anyone is still interested, I fixed the race condition in the above script. Basically, it involves setting up the SIGCHLD handler first, and then ensuring there are no calls to external programs (except those you have on queue). So that means instead of using seq(…), you have to use a counter.

I have edited the above accordingly, and it works (for me) on bash and zsh.