Running Multiple Programs Simultaneously from a Bash Script

Introduction

When managing tasks that involve running multiple programs at once, it is often necessary to execute these operations concurrently rather than sequentially. This can improve efficiency and save time in various scripting scenarios. In this tutorial, we will explore methods for running multiple programs simultaneously within a bash script.

Running Programs in Parallel Using Background Execution

A straightforward way to run commands in parallel is by appending an & symbol at the end of each command line. This sends the command to the background, allowing subsequent commands to execute without waiting for the previous ones to complete. Here’s how you can achieve this:

prog1 &
prog2 &

In the example above, both prog1 and prog2 are executed simultaneously in the background.

Waiting for Background Processes

If your script needs to wait for all background processes to finish before proceeding, use the wait command. The wait command halts the execution of the script until all background jobs have completed. It can be particularly useful when subsequent steps depend on the completion of previous tasks:

prog1 &
prog2 &
# Perform other operations here...
wait  # Waits for both prog1 and prog2 to finish

Using Process IDs with Wait

To manage specific processes, you can capture their process IDs (PIDs) when starting them. The PID is a unique identifier used by the operating system to track each running process. By capturing PIDs, you can selectively wait for certain processes:

some_command &
PID1=$!  # Captures the PID of some_command

other_command &
PID2=$!  # Captures the PID of other_command

wait $PID1 $PID2  # Waits specifically for these two processes to finish

This method is beneficial if you need to control or terminate specific background tasks programmatically.

Handling Signals with a Subshell

Another powerful approach involves using subshells and signal trapping. A subshell can encapsulate multiple commands, allowing them to be managed collectively:

(trap 'kill 0' SIGINT; prog1 & prog2 &)

In this snippet:

• trap 'kill 0' SIGINT sets up a signal handler that sends the termination signal (SIGINT) to all processes in the current process group (using kill 0).
• Pressing Ctrl-C will terminate both prog1 and prog2, showcasing how complex command structures can be managed with ease.

Using GNU Parallel for Complex Task Management

For more advanced parallel processing, consider using GNU Parallel. This tool executes jobs in parallel on a single machine or across multiple machines:

echo prog1; echo prog2 | parallel

GNU Parallel manages job execution efficiently and is highly configurable to suit different scenarios.

Conclusion

Running programs simultaneously from a bash script can significantly enhance your workflow efficiency, especially when dealing with tasks that do not need to be executed sequentially. Depending on the complexity of your needs, you might choose simple background processes, specific process management using PIDs, subshell signal trapping for group operations, or GNU Parallel for advanced usage.

By understanding these methods, you’ll be well-equipped to handle parallel execution in various scripting environments effectively.