jobs

Understanding Background Processes

Before diving into jobs, let’s quickly review background processes. In Linux, you can run commands in the background using the ampersand (&) symbol. This allows you to continue using your terminal while the command executes. For instance:

sleep 60 &

This command starts a sleep process that will pause execution for 60 seconds, but it runs in the background, freeing your terminal. However, you might need a way to manage these background processes, and that’s where jobs comes in.

The jobs Command: Listing Your Background Processes

The simplest use of jobs is to list currently running background jobs:

jobs

This will output a list like this (the output may vary depending on your jobs):

[1]+  Running                 sleep 60 &
[2]-  Running                 long_running_script.sh &

Each line represents a background job. [1]+ and [2]- are job numbers. The + indicates the current job, while - indicates the previous job. Running shows the status, and the rest is the command itself.

Controlling Background Processes with jobs

jobs isn’t just for listing; it also lets you control your background processes.

Bringing a Job to the Foreground

To bring a background job to the foreground, use fg along with the job number or job specifier (e.g., %1 or %sleep).

fg %1  # Brings job 1 to the foreground
fg %sleep # Brings the job with "sleep" in its command to the foreground

Once a job is in the foreground, you can interact with it directly. Pressing Ctrl+Z will suspend it, returning it to the background.

Stopping a Job

You can stop a background job using kill with the job number. This sends a SIGTERM signal, which allows the process to gracefully shut down.

kill %1 # Sends SIGTERM to job 1
kill %2 # Sends SIGTERM to job 2

If the process doesn’t respond to SIGTERM, you can use kill -9 %1 (or kill -9 <job_number>), which sends a SIGKILL signal, forcefully terminating the process. Use this option cautiously as it doesn’t allow for a clean shutdown.

Listing Stopped Jobs

Sometimes, jobs might be stopped (e.g., by pressing Ctrl+Z). The jobs command will list these as well:

jobs -l # Show detailed job information including PID

This command displays detailed information, including the process ID (PID), for advanced process management.

Combining jobs with Other Commands

jobs is often used in conjunction with other commands like wait. wait pauses the script until a specified background process finishes.

long_running_command &
pid=$! # get PID of the last background process
wait $pid # wait until process with $pid completes
echo "Long running command finished!"

This example demonstrates how you can efficiently manage a background process and ensure other parts of the script only continue after it’s completed.

Advanced jobs Options

The jobs command has many other options you can look at to tailor its output:

• jobs -p: Displays only the process IDs.
• jobs -n: Shows only the new jobs since the last jobs command.

By mastering the jobs command, you gain significant control over background processes in your Linux environment, increasing your efficiency and allowing for more complex scripting and task management.