Linux Server Hacks, Volume Two

Hack #33. Keep Processes Running After a Shell Exits

Process-control commands such as nohup and disown make it easy for you to start long-running processes and keep them running even after you log out.

Suppose you're running a troubleshooting or monitoring tool on your server or compiling a very large program, and the process is going to need to run for hours, days, or longer. What if you need that process to keep running even after you've logged out or if your shell session ends before you meant it to? You can make this happen with the nohup and disown commands.

When you run a shell session, all the processes you run at the command line are child processes of that shell. If you log out or your session crashes or otherwise ends unexpectedly, SIGHUP (signal to hang up) kill signals will be sent to its child processes to end them too.

You can get around this by telling the process(es) that you want kept alive to ignore SIGHUP signals. There are two ways to do this: by using the nohup ("no hangup") command to run the command in an environment where it will ignore certain termination signals or by using the bash shell's disown command to make a specified background job independent of the current shell.

Using nohup to Execute Commands

The nohup command provides a quick and easy mechanism for keeping a process running regardless of whether its parent process is still active. To take advantage of this capability, run your favorite command, preceded by the nohup command:

	$ nohup command

This ...