wait

Additional Notes

wait is a shell builtin that pauses execution until one or more background processes complete. It is used in shell scripts to synchronize parallel tasks launched with &. Without arguments, it waits for all background processes to finish. With a process ID or job spec, it waits for a specific process.

The exit status of wait is the exit status of the waited-for process, which allows scripts to check whether background jobs succeeded. This is essential for robust parallelism in shell scripting.

Syntax

wait [process_id...] [job_spec...]

Parameters

• process_id: A numeric PID to wait for.
• job_spec: A job specification like %1 (first background job) or %+ (current job).

Examples

sleep 5 &
wait
echo "Background job finished"

Wait for all background processes.

gcc -o program program.c &
pid=$!
wait $pid
echo "Compilation finished with status $?"

Save the PID of a background process and wait for it specifically.

sleep 3 &
sleep 5 &
wait -n
echo "One of the sleep commands finished"

Wait for the next background job to finish (bash 4.3+).

job1 &
job2 &
wait %1 %2
echo "Both jobs done"

Wait for specific job specs.

Practical Notes

• Without arguments, wait waits for all background processes created in the current shell.
• The special variable $! holds the PID of the most recently started background process.
• wait -n (bash 4.3+) waits for the next job to complete, useful for managing parallel pools.
• If a process has already exited, wait returns immediately (the exit status is still available).
• wait is a shell builtin; the external /usr/bin/wait may exist but behaves differently.