Wednesday, 22 June 2011

nohup: Execute Commands After You Exit From a Shell Prompt

Most of the time you login into remote server via ssh. If you start a shell script or command and you exit (abort remote connection), the process / command will get killed. Sometime job or command takes a long time. If you are not sure when the job will finish, then it is better to leave job running in background. However, if you logout the system, the job will be stopped. What do you do?

nohup command

Answer is simple, use nohup utility which allows to run command./process or shell script that can continue running in the background after you log out from a shell:

nohup Syntax:

nohup command-name &
  • command-name : is name of shell script or command name. You can pass argument to command or a shell script.
  • & : nohup does not automatically put the command it runs in the background; you must do that explicitly, by ending the command line with an & symbol.

nohup command examples

1) Login to remote server
$ ssh
2) Execute script called
# nohup &
Type exit or press CTRL + D exit from remote server.
# exit
3) Find all programs and scripts with setuid bit set on, enter:
# nohup find / -xdev -type f -perm +u=s -print > out.txt &
Type exit or press CTRL + D exit from remote server.
# exit
Please note that nohup does not change the scheduling priority of COMMAND; use nice for that:
# nohup nice -n -5 ls / > out.txt &
As you can see nohup keep processes running after you exit from a shell. Read man page of nohup and nice command for more information. Please note that nohup is almost available on Solaris/BSD/Linux/UNIX variant.
# 1: As pointed out by Jason you can use at command to queue a job for later execution. For example, you can run script to queue (one minute) later execution
$ echo "" | at now + 1 minute
# 2: You can also use screen command for same. Brock pointed out disown shell internal command for same purpose. Here is how you can try it out:
$ &
$ disown -h
$ exit

This is a complete copy from the source. 

Aditional Info: less Nohup.out

Provides you with the output of the <command&>. This is some thing similar to tail -f on a live log file. 

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