How to Use the Command 'bg' (with Examples)

The bg command is a built-in utility in Unix-like operating systems that allows users to resume suspended jobs. When working in a command-line environment, you might occasionally want to pause a command or process temporarily. By pressing Ctrl + Z, you can suspend a running task and send it to the background. The bg command enables these suspended jobs to continue execution without tying up your terminal. This is especially useful when you need to perform additional tasks while having other jobs running concurrently.

Use Case 1: Resume the Most Recently Suspended Job and Run It in the Background

Code:

bg

Motivation:

Imagine you are compiling a large application in the terminal, and it starts taking longer than expected. You realize you need to perform another task, such as reading an email or checking system status, without terminating the current compile job. By using Ctrl + Z, you suspend the compiling process, which allows you to regain control of the terminal. Now, you can use the bg command to resume the most recently suspended job, allowing it to continue running in the background. This approach maximizes your multitasking abilities, ensuring that prolonged tasks don’t impede your workflow.

Explanation:

• bg: The command bg alone operates on the most recently suspended job. It changes the state of this job from suspended to running in the background. When a job is continued in the background, it means that the job will continue to execute and use system resources, but you won’t see its output or be able to interact with it directly in the terminal window.

Example Output:

When you execute the bg command, you may see an output indicating the job (often represented by a job ID) and the command that has been sent to the background:

[1]+ make myproject &

Use Case 2: Resume a Specific Job and Run It in the Background

Code:

bg %job_id

Motivation:

Consider a scenario where you have multiple jobs suspended, perhaps a database backup and a large file conversion. You need to prioritize which process to run in the background based on urgency or significance. Here, the bg %job_id command becomes invaluable. By specifying the job ID, you can selectively resume a particular job to continue its execution while addressing other tasks. This control is crucial in environments where different jobs have varying importance or resource requirements.

Explanation:

• bg: Just like in the previous use case, this command is used to send jobs into the background.
• %job_id: The % character is a job specifier prefix that helps the shell identify which suspended job you want to manage. You obtain job IDs by inspecting the list of jobs with the jobs command. Specifying job_id directly informs the shell of which particular process to resume running in the background.

Example Output:

Suppose you have run the command jobs to find out the job IDs, and you issue:

bg %2

The terminal could respond with:

[2]+ mysqldump -u user -p database_name > backup.sql &

Conclusion

The bg command is an essential tool for multitasking within Unix-like operating systems. It allows users to resume suspended jobs in the background, thereby facilitating optimal use of time and system resources. By mastering the use of bg in conjunction with job control commands like jobs and fg, users can efficiently manage running tasks without disrupting their workflow. Whether you are dealing with a single suspended process or managing several, understanding the use cases of bg can significantly enhance your command-line productivity.