nohup – Run the program ignoring hangup signals

The nohup command is a Linux command that allows users to run a command or a script, and ignore the hangup (HUP) signal sent by the system when the user logs out or closes the terminal. This command is particularly useful when running long-term processes that need to continue running even after the user logs out or the terminal is closed.

Overview

The syntax for the nohup command is as follows:

nohup COMMAND [ARG]...

where COMMAND is the command or script that you want to run, and ARG are the arguments that you want to pass to the command. When you run a command with nohup, any output that would normally be sent to the terminal is redirected to a file called nohup.out in the current directory.

Here is an example of how to use nohup to run a script called myscript.sh:

nohup ./myscript.sh &

In this example, the & symbol is used to run the command in the background. This means that you can continue to use the terminal for other tasks while the script is running.

Specific Use Cases

Here are some specific use cases for the nohup command:

Running a Script in the Background

Suppose you have a long-running script that you want to run in the background, and you don’t want it to be terminated when you log out of the system. You can use nohup to accomplish this. For example:

nohup ./long_running_script.sh &

This will start the script in the background and redirect all output to nohup.out.

Running a Command on a Remote Server

If you are working on a remote server and need to run a command that will take a long time to complete, you can use nohup to ensure that the command continues to run even if your connection to the server is lost. For example:

nohup ./long_running_command &

Running a Command on Boot

If you want to run a command or script when your system boots up, you can add it to the system’s startup scripts and use nohup to ensure that it continues to run even after you log out. For example:

sudo nano /etc/rc.local

Add the following line to the end of the file:

nohup /path/to/command &

Save and exit the file. This will ensure that the command is run at startup and continues to run even after you log out.

Options

The following table lists the available options for the nohup command:

Option	Description
-n	Do not overwrite the nohup.out file. Append to it instead.
-p	Specify the process ID (PID) of a running process that you want to attach to nohup.

Troubleshooting Tips

Here are some troubleshooting tips for the nohup command:

Check the nohup.out File for Errors

If you are experiencing issues with a command or script that you are running with nohup, you can check the nohup.out file for any error messages or other output. This file should be located in the same directory where you ran the nohup command.

Check the Process Status

If you are unsure if a process is still running, you can use the ps command to check its status. For example:

ps -ef | grep [PROCESS_NAME]

This will show you all processes that match the specified name.

Kill a Running Process

If you need to stop a process that is running with nohup, you can use the kill command to send a signal to the process. For example:

kill [PID]

where PID is the process ID of the process that you want to stop.

Notes

• The nohup command is not a replacement for a proper process management system like systemd or supervisord. It is intended for use in specific cases where a long-running process needs to continue running even after the user logs out or the terminal is closed.
• When using nohup, it is important to redirect all input and output to files or to /dev/null to ensure that the process does not hang waiting for input or output.