Shell – wait and recycling of PID

background-processprocessshell

I have a concern whereby in a script, I will be calling parallel versions of a function to run in the background and then issuing a wait.

a () {
  sleep $1
}

a 10 &
PIDs="$PIDs $!"
a 50000 &
PIDs="$PIDs $!"

wait $PIDs

The concern is the first function call takes 10 seconds (sleep 10), but the second takes nearly 14 hours (sleep 50000).

I am concerned the PID from the first invocation will be recylced and 14 hours later, when the second invocation is terminates, that PID is being used by another process and will prevent the script from continuing. Or does wait remove the first PID from the list as soon as that invocation completes and just simply waits for the second process to complete rather than waiting for both of them to finish at the end?

Best Answer

See for yourself that the wait builtin will not wait for a random process -- only children of the current shell.

#!/bin/bash
sleep 2 &
P=$!
echo sleeping 3 seconds...
sleep 3
echo waiting for $P ...
wait $P
R=$RANDOM
echo waiting for $R ...
wait $R
echo done

$ ./t.sh
sleeping 3 seconds...
waiting for 93208...     ## this returns immediately, since that PID is gone
waiting for 31941 ...
./t.sh: line 10: wait: pid 31941 is not a child of this shell
done

Related Solutions

Parallel Tasks – How to Run Four Tasks in Parallel

If you have a copy of xargs that supports parallel execution with -P, you can simply do

printf '%s\0' *.png | xargs -0 -I {} -P 4 ./pngout -s0 {} R{}

For other ideas, the Wooledge Bash wiki has a section in the Process Management article describing exactly what you want.

Bash – Script to Wait for Processes and Get Return Code

You can do this by using a temporary directory.

# Create a temporary directory to store the statuses
dir=$(mktemp -d)

# Execute the backgrouded code. Create a file that contains the exit status.
# The filename is the PID of this group's subshell.
for i in 1 2; do
    { ssh mysql "/root/test$i.sh" ; echo "$?" > "$dir/$BASHPID" ; } &
done

# Wait for all jobs to complete
wait

# Get return information for each pid
for file in "$dir"/*; do
    printf 'PID %d returned %d\n' "${file##*/}" "$(<"$file")"
done

# Remove the temporary directory
rm -r "$dir"

Best Answer

Related Solutions

Parallel Tasks – How to Run Four Tasks in Parallel

Bash – Script to Wait for Processes and Get Return Code

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