How to use the command 'nohup' (with examples)

The nohup command in Unix and Linux systems is a useful utility that allows a process or command to continue execution even after the user has logged out or the terminal session has been closed. This command is particularly valuable in maintaining persistent processes without the need to remain logged in, ensuring operations continue uninterrupted in the background. It essentially detaches the process from the terminal, so it remains unaffected by hang-ups.

Use case 1: Running a process that can live beyond the terminal

Code:

nohup command argument1 argument2 ...

Motivation:

Imagine you’re running a long-running scientific calculation or a server setup command that may take several hours, or even days, to complete. You don’t want a mere terminal closure or accidental logout to interrupt the process, potentially wasting hours of computation. Using nohup, you can launch such commands, ensuring they persist regardless of session interruptions.

Explanation:

• nohup: This is the command that tells the system to ignore the HUP (hangup) signal, allowing the process to continue running even if the terminal is closed.
• command: This represents the executable or script you wish to run.
• argument1 argument2 ...: These are any arguments the command might require.

Example output:

nohup: ignoring input and appending output to 'nohup.out'

This output indicates that the command has been successfully detached from the terminal. The standard output and standard error are redirected to a file named nohup.out in the current directory unless you specify otherwise.

Use case 2: Launching nohup in background mode

Code:

nohup command argument1 argument2 ... &

Motivation:

Running commands in the background is ideal when you need the command line to be freed up for more tasks while the process continues running. This is common when managing multiple long-running processes or multitasking in scripts, allowing you to continue working without delay.

Explanation:

• The construction here is similar to the previous example with the addition of &, which signals the shell to run the command in the background.
• &: This is the shell operator that tells the terminal to run the task as a background job. It doesn’t wait for the command to complete and returns control to the user immediately.

Example output:

[1] 1234
nohup: ignoring input and appending output to 'nohup.out'

The [1] 1234 output shows a job number and a unique process ID, which can be used to check on or manipulate the background task.

Use case 3: Running a shell script that can live beyond the terminal

Code:

nohup path/to/script.sh &

Motivation:

There are scenarios where an entire script containing numerous commands needs to be robust against interruptions. Perhaps you’re updating a remote server with a critical update script and cannot afford for it to terminate mid-process—even if your SSH session accidentally drops.

Explanation:

• path/to/script.sh: This specifies the relative or absolute path to the shell script you wish to run. The & is used to run it in the background just like in previous examples.

Example output:

nohup: ignoring input and appending output to 'nohup.out'
[1] 2567

This confirms that your script is running in the background, capturing any output in the default nohup.out file or a specified log file.

Use case 4: Running a process and writing the output to a specific file

Code:

nohup command argument1 argument2 ... > path/to/output_file &

Motivation:

Storing command output in a specific file can be crucial for logging purposes, debugging, or simply reviewing the output later. This is particularly beneficial in monitoring systems or batch processing tasks where output tracking may be required for compliance or analysis.

Explanation:

• >: This redirection operator is used to write the standard output to the specified file.
• path/to/output_file: The path where the output should be saved. It can be a newly created file or an existing one that nohup will append to.

Example output:

[2] 3456

Here, instead of redirecting the output to nohup.out, the output is recorded in the specified output_file, and the job number and process ID are presented to handle the task further if necessary.

Conclusion:

The nohup command is a robust tool within Unix-like operating systems that offers a simple yet effective solution to keep processes running even when a terminal session is terminated. From long-running processes to batch job execution, nohup provides the reliability needed in both personal and professional environments, ensuring that processes are immune to any disconnects or session closures. Employing nohup in combination with background operation or output management expands its practicality further, allowing for seamless multitasking and comprehensive logging.