The nohup (“no hangup”) command prevents a process from ending when the user logs off. For example, if an administrator launches a backup script, and then logs off the system, the script would stop running. By placing the nohup command in front of the normal command, the script would continue even after the administrator logged off.
Syntax
The syntax of the nohup command is:
# nohup {command/script} If you encounter the below error while running the nohup command:
nohup: command not found
you may try installing the below package as per your choice of distribution:
| Distribution | Command |
|---|---|
| OS X | brew install coreutils |
| Debian | apt-get install coreutils |
| Ubuntu | apt-get install coreutils |
| Alpine | apk add coreutils |
| Arch Linux | pacman -S coreutils |
| Kali Linux | apt-get install coreutils |
| CentOS | yum install coreutils |
| Fedora | dnf install coreutils |
| Raspbian | apt-get install coreutils |
Examples
1. Running nohup without specifying an output file
Let’s do an example and test that it actually does continue working after you have logged out. We’ll kick off a script called script.sh. The contect of the script is as follows:
# cat script.sh #!/bin/sh while(true) do echo "Hello World!" sleep 10 done
Lets run the script in backgroupd with nohup command:
# nohup script.sh & [1] 179 $ nohup: appending output to `nohup.out`
As you can see above we have not specified an output file. Thus the standard output and standard error by default goes into the nohup.out file in the directory from where the script has been invoked. Also the number 179 Specified the process ID(PID) of the job. This PID can be verified in the “ps -ef” command output as shown below.
Now exit the shell, log back in again, and run the following command:
$ ps x | grep script.sh 179 ? S N 0:01 sh /root/script.sh 506 p2 S 0:00 grep script.sh
The script is still working. Use “ps -ef | grep ps1” if your system does not support ps x. You can also check the output file nohup.out for more confirmation.
# cat nohup.out Hello World! Hello World! Hello World! Hello World! ...
Running nohup specifying an output file
Now the above script can also be run nohup command and specifying a custom output file as shown below:
# nohup script.sh > /tmp/script.out 2>&1 [1] 14235
Conclusion
If you are running a process and you don’t think it will be completed by the time you log out for the day, use the nohup command. Nohup will continue processing when you exit your account. Nohup means no hang up.