How to use the command 'disown' (with examples)
The disown command is a utility used in Unix-like operating systems to manage job control in the shell. It allows users to detach processes or jobs from the shell, enabling them to continue running even after the user closes the terminal session. This is particularly useful for long-running background processes that don’t need continuous user interaction. By using disown, system administrators and developers can ensure that critical tasks are not interrupted when the shell session ends. This command is closely related to job control commands like jobs.
Use case 1: Disown the current job
Code:
disown
Motivation:
Imagine you’re running a long-running background script in your terminal, like a data processing script or a web server. You initiated the script using an ampersand (&) to run it in the background. At some point, you realize you need to close your terminal session or log out of the remote server without terminating the script. By disowning the current job, you ensure that the background process will continue running even after your terminal session is closed.
Explanation:
Simply using disown without any arguments will disown the current foreground job that has been pushed to the background. This default behavior assumes that the user wants to disown the most recent background process.
Example Output:
There will be no output for this command, as the action it performs is internal. However, upon checking background processes with jobs, you will no longer find the previously running job in the list.
Use case 2: Disown a specific job
Code:
disown %1
Motivation:
Suppose you are running multiple background processes and you want to disown only one specific job. Identifying the job by its job number allows you to selectively keep certain jobs attached to the shell while detaching others. This targeted approach provides precise control over job management.
Explanation:
The % followed by a job number is used to reference specific jobs. In this example, %1 references the first job in the list of jobs. The disown %1 command will detach this specified job from the shell.
Example Output:
Again, no direct output is produced. Using the jobs command afterward will show that the specified job is no longer listed.
Use case 3: Disown all jobs
Code:
disown -a
Motivation:
If you have initiated several background processes and decide to close down your terminal session completely without affecting any of these jobs, you can disown all jobs at once. This is especially useful when working on scripts that start multiple services or processes which should continue independently of the user’s session.
Explanation:
The -a flag signifies “all,” instructing the disown command to detach all background jobs associated with the current shell. This ensures that every running background process will persist beyond the shell session.
Example Output:
There is no output from this command, but subsequent checks with jobs will confirm no jobs are currently tied to the session.
Use case 4: Keep a job, but prevent receiving SIGHUP
Code:
disown -h %2
Motivation:
Sometimes, users may want a process to remain associated with the shell for easier management but still wish to prevent it from being terminated by a hang-up signal (SIGHUP) when the shell exits. This use of disown is ideal in scenarios where processes should continue unaffected by shell termination without being entirely disowned.
Explanation:
The -h flag prevents the specified job from receiving a SIGHUP on shell exit. By using %2, we are specifying the second job in the job list. This form of the command keeps the job listed under jobs but marks it to ignore the hang-up signal.
Example Output:
Just like the other uses of disown, this command provides no visible output. However, after running, the specified job will not be affected by shell termination due to a SIGHUP.
Conclusion
The disown command in Unix-like operating systems is a powerful tool for managing background processes within the shell environment. By using it, users can control how jobs interact with the terminal session, ensuring continuity and stability for important tasks. Whether you need to keep certain processes alive past the shell session, manage multiple jobs, or selectively choose which processes receive SIGHUP signals, disown provides a flexible solution for efficient process management.
