Running Commands in the Background (&) and $!

#!/bin/bash
sleep 0.2 &
echo "this prints immediately, before the sleep finishes"
wait
echo "the background sleep has now finished"

#!/bin/bash
sleep 0.2 &
pid=$!
echo "started background job with PID $pid"
wait "$pid"
echo "job $pid has completed"

#!/bin/bash
sleep 0.1 &
pid1=$!
sleep 0.1 &
pid2=$!
echo "started jobs $pid1 and $pid2"
wait "$pid1" "$pid2"
echo "both jobs finished"

1. Assuming a backgrounded command's output won't interleave with the rest of the script's output; both run concurrently and can print at overlapping times.
2. Forgetting $! must be captured immediately after the & command, since it always refers to the most recently backgrounded job's PID.
3. Not waiting for background jobs before the script ends, which in an interactive shell is fine but can mean a non-interactive script exits before the job's work (like a file write) is actually done.

• Appending & to a command starts it in the background and returns control immediately.
• $! holds the process ID (PID) of the most recently started background job.
• Multiple & jobs can run concurrently; use wait to block until they finish.
• Non-interactive scripts have job control disabled, so interactive builtins like fg/bg/jobs are unreliable there; $! and wait are the portable way to manage background work.