Differences Between nohup, disown, and & in Linux

In the world of Linux, managing processes efficiently is crucial for system administrators. Three common tools that help in managing long-running processes are nohup, disown, and the & operator. Each serves a unique purpose but they are often used in conjunction to manage background processes and ensure they continue running even after a user has logged out. Let’s delve into the details of each, explore their differences and similarities, and share some tips and tricks for using them effectively.

What is nohup?

nohup, short for “no hang up,” is a command that is used to run another command that ignores the HUP (hangup) signal. This signal is sent to a process when its controlling terminal is closed (for example, when the user logs out). By ignoring this signal, the process continues running in the background even after the user logs out.

Syntax:

nohup command-to-run &

Example:

nohup python script.py &

Tips and Tricks:

• Output redirection: By default, nohup redirects the standard output to a file named nohup.out if no output file is specified. You can redirect the output to a file of your choice:

  nohup python script.py > output.log &

• Combine with & to run the process completely in the background.

What is disown?

disown is a shell builtin command that removes a shell job from the shell’s job table, making it no longer a child of the shell. This means the process will not receive a HUP signal from the shell when the shell session is terminated.

Usage:

1. Start a process in the background:

   python script.py &

2. Use jobs to see the job running in the background:

   jobs

3. Disown the job:

   disown %1

Tips and Tricks:

• Use disown with a specific job number to remove only that job from the shell’s job table.
• You can use disown -a to remove all jobs from the job table.

What is &?

The & operator in Linux is used to put a command in the background. It’s a way to start a process and return immediately to the shell prompt, without waiting for the process to complete.

Example:

python script.py &

Tips and Tricks:

• Combine & with nohup or disown to run processes in the background that will survive your logout.
• Use fg to bring a background process to the foreground if needed:

  fg %1

Differences and Similarities

• Similarities: All three methods (nohup, disown, and &) are used for managing background processes in Linux.
• Differences:
• nohup is used to run a command that ignores the hangup signal and is often used with & to run it in the background.
• disown is used to remove a job from the shell’s job table, ensuring it continues running without being attached to a specific terminal.
• & simply puts a command in the background but does not protect it from hangup signals.

Understanding the nuances of nohup, disown, and & can greatly enhance your ability to manage processes on a Linux system. Each tool has its specific use case, and knowing when and how to use them can make your system administration tasks more efficient. Whether you are running scripts that take a long time to complete or setting up services that should run continuously, these tools are indispensable in a Linux environment.