There are many available commands on Linux systems. Some you’ll use multiple times a day, while others are generally reserved for special uses. nohup is one such command. You won’t use it every day, but you’ll be grateful that it’s around when you need it. Here we show you what nohup is and how to use it.

Table of Contents

• What Is Nohup?
• Nohup.out
• How Is Nohup Different Than a Daemon?
• Running a Process With Nohup
• Running Multiple Commands With Nohup
• Frequently Asked Questions

What Is Nohup?

nohup is short for “No Hangups.” It’s not a command that you run by itself. nohup is a supplemental command that tells the Linux system not to stop a command once it has started. It’ll keep running until it’s done, even if the user that started it logs out. It serves a somewhat similar purpose to tmux or screen, allowing you to start a process and not worry about whether it’ll finish if you log out or get disconnected from your server.

The syntax for nohup is simple and looks something like this:

nohup sh your-script.sh &

Notice the & at the end of the command. That moves the command to the background, returning you to the prompt on the terminal you’re working on. Many times the command will still remain attached to the terminal, so you’ll terminate it the moment you close the open window and precede the command with nohup.

nohup works with just about any command that you run in the terminal. It can be run with custom scripts as well as standard system commands and command line utilities. Use it to start a security auditing script on a remote server so that you can disconnect and walk away or to run updates in the background and not have them stop for anything.

Tip: check out other ways to run bash commands in the background.

Nohup.out

Because nohup can keep running independent of the user that started it, the command needs somewhere to output any messages or errors. As there isn’t a terminal to associate with it, nohup logs everything to a “nohup.out” output file

By default, that file is located in whichever directory you started the command. “nohup.out” is somewhat unique, as it contains both the standard output and the error output. nohup redirects both to the same file by default.

You don’t necessarily need to use “nohup.out,” though – it’s just the default. You can specify a custom output when you run nohup and place it in a custom location.

nohup sh your-script.sh > /path/to/custom.out &

The custom output contains the same data as the standard “nohup.out” file and is nice for the security audit script mentioned above. You could name it “audit-script-output.txt” in some kind of mounted remote folder and view that server’s security posture from the comfort of your own laptop rather than looking through cat outputs all day.

Even for normal everyday users, specifying a custom output file could be useful, as it would allow you to have a clear distinction between outputs. If you use this command multiple times from your default (home) folder without specifying an output file, “nohup.out” will just append itself with all outputs, creating a bit of a mess. If you want to read command output in an organized manner, specifying custom file names for each output gives you a means to do that in your own way!

How Is Nohup Different Than a Daemon?

By this point, you’re probably wondering what sets nohup apart from a daemonized process. After all, they both do seem to serve relatively the same purpose but don’t really.

Daemons run continuously in the background. They’re best reserved for processes that you don’t want to ever exit, like servers. They require more work to program, too, so they’re not best for simple one-off scripts.

nohup is more of a single-use command. Think of it as a script that will take a long time to run but will still ultimately finish. Maybe there’s a long and complicated task that you run every now and then that takes hours to complete. You don’t want to leave a terminal open or a user logged in, so use nohup to keep it running in the background and put all of the output into whatever file you choose, whether the default “nohup.out” or a location of your choice.

Running a Process With Nohup

To run a process with nohup, put it before any command you run. This includes all normal programs you would run, such as Discord, Steam, your browser, a text editor, or any other application you can think of.

Remember, you must detach your process from the active prompt so that you can close your terminal without suddenly losing the process.

For example, to open Kate (my text editor) on my system and detach it from the terminal while keeping a log with “kate.log,” I would type this:

nohup kate > kate.log &

After closing the terminal, Kate is still running.

If there were a problem starting the application, I could simply pull up “kate.log” and figure out what’s wrong directly from the terminal with:

cat kate.log

This process is useful to determine whether there’s an issue with an application that refuses to start or behaves erratically. When you typically start a program like your browser, its error signals are often ignored. Starting them in the terminal with nohup and passing the output onto a logfile helps paint a clearer picture of what could be causing problems.

Running Multiple Commands With Nohup

Since you can pass any command through nohup, you can also pass a command sequence. To do this correctly, you’ll have to invoke a shell first and pass the commands along as a string, as most terminal emulators interpret every command after the first as not being preceded by nohup.

Every modern Linux distro has bash installed, so we are ignoring other custom shells and using that one for this example:

nohup bash -c 'command1 && command2' &

In this command, we invoked bash to execute a command and chained them with a double-ampersand (&&). We’re telling nohup to “Run bash with the -c (execute) flag and pass this string surrounded by single quotes to it.” Then we tell the terminal to free up the prompt while that entire sequence runs in the background.

Frequently Asked Questions

Screenshots for “What Is Nohup?” and “Nohup.out” by John Perkins, and those for “Running a Process With Nohup” by Miguel Leiva-Gomez.

The above is the detailed content of What Is Nohup and How Do You Use It? - Make Tech Easier. For more information, please follow other related articles on the PHP Chinese website!