bg

Understanding Background Processes

Before exploring bg, understand background processes. These are processes that run independently of your current terminal session, allowing you to continue working on other tasks without interruption. You can initiate a background process using the ampersand (&) symbol at the end of a command. For instance:

sleep 60 &

This command starts a sleep process for 60 seconds in the background. Immediately after executing this, you can type other commands and the sleep process will continue running independently.

However, if a long-running command is interrupted (e.g., by pressing Ctrl+Z), it moves to a stopped state. This is where bg becomes invaluable.

Resuming Stopped Jobs with bg

The bg command takes a job ID as its argument. This job ID is a number assigned by the shell to each job it manages. You can view the list of currently running and stopped jobs using the jobs command:

sleep 1000 &  #Starts a long sleep process in background
sleep 10 &    #Starts a shorter sleep process in background
jobs

This might output something like:

[1]+  Running                 sleep 1000 &
[2]+  Running                 sleep 10 &

Now, let’s interrupt the first sleep process (job 1):

Ctrl+Z
jobs

This will stop the process, and jobs might show:

[1]+  Stopped                 sleep 1000
[2]+  Running                 sleep 10 &

Now, use bg to resume the stopped job:

bg %1
jobs

The %1 refers to job 1. You can also use % followed by the job name if it’s unique or use the % followed by a portion of the job name if it’s not unique. After running bg %1, jobs will likely display job 1 as running again.

Multiple Background Jobs

Let’s say you have many stopped jobs:

sleep 100 &
sleep 200 &
Ctrl+Z
Ctrl+Z
jobs

This might show:

[1]+  Stopped                 sleep 100
[2]+  Stopped                 sleep 200

You can resume them individually using bg %1 and bg %2, or you can resume both simultaneously by using bg %1 %2

bg Without Arguments

If you run bg without any arguments, it resumes the most recently stopped job. This is a convenient shortcut.

Advanced Usage and Considerations

While bg is straightforward, remember that background processes consume system resources. Excessive background processes can degrade performance. Always monitor your system resource usage and manage background processes appropriately using commands like top or htop. Furthermore, ensure your scripts and commands handle potential errors and unexpected terminations when running them in the background.