Process Management in Scripts

Scripts often need to run long tasks, manage background jobs, or gracefully handle sudden termination (like when a user presses Ctrl+C).

Background Processes

To run a command in the background, append an ampersand & to the end of the command.

#!/bin/bash

echo "Starting a long process..."
sleep 10 &

Retrieving Process IDs (PIDs)

When you run a background job, its PID is stored in the special variable $!.

sleep 20 &
BG_PID=$!
echo "Background job started with PID: $BG_PID"

Waiting for Processes

If your script relies on a background task completing before it can continue, use the wait command.

echo "Starting job 1..."
sleep 3 &
PID1=$!

echo "Starting job 2..."
sleep 5 &
PID2=$!

echo "Waiting for jobs to finish..."
wait $PID1
wait $PID2

echo "All background jobs completed."

Trapping Signals

Signals are messages sent to processes. For example, SIGINT (Signal 2) is sent when you press Ctrl+C. You can "trap" these signals to run cleanup code instead of immediately terminating.

#!/bin/bash

# Define a cleanup function
cleanup() {
    echo -e "\nCaught Ctrl+C! Cleaning up temporary files..."
    rm -f /tmp/myscript_temp.txt
    exit 1
}

# Trap SIGINT (Ctrl+C)
trap cleanup SIGINT

echo "Creating temp file..."
touch /tmp/myscript_temp.txt

echo "Running... (Press Ctrl+C to stop)"
while true; do
    sleep 1
done

Terminating Processes

You can kill processes from within your script using the kill command.

sleep 60 &
PID=$!
echo "Process $PID running..."
sleep 2
echo "Killing process $PID"
kill $PID

Best Practices

• Always use wait if downstream commands depend on background tasks.
• Always use trap to clean up lock files, temporary directories, or sensitive data if your script is interrupted.
• Avoid using kill -9 inside scripts unless absolutely necessary, as it prevents the target process from performing its own cleanup.