What's the difference between nohup and ampers

Question

Both nohup myprocess.out & or myprocess.out & set myprocess.out to run in the background. After I shutdown the terminal, the process is still running. What's the difference between them?

Answer 1

nohup catches the hangup signal (see man 7 signal) while the ampersand doesn't (except the shell is confgured that way or doesn't send SIGHUP at all).

Normally, when running a command using & and exiting the shell afterwards, the shell will terminate the sub-command with the hangup signal (kill -SIGHUP <pid>). This can be prevented using nohup, as it catches the signal and ignores it so that it never reaches the actual application.

In case you're using bash, you can use the command shopt | grep hupon to find out whether your shell sends SIGHUP to its child processes or not. If it is off, processes won't be terminated, as it seems to be the case for you. More information on how bash terminates applications can be found here.

There are cases where nohup does not work, for example when the process you start reconnects the SIGHUP signal, as it is the case here.

Answer 2

Using the ampersand (&) will run the command in a child process (child to the current bash session). However, when you exit the session, all child processes will be killed.

using nohup + ampersand (&) will do the same thing, except that when the session ends, the parent of the child process will be changed to "1" which is the "init" process, thus preserving the child from being killed.

Answer 3

Most of the time we login to remote server using ssh. If you start a shell script and you logout then the process is killed. Nohup helps to continue running the script in background even after you log out from shell.

Nohup command name &
eg: nohup sh script.sh &

Nohup catches the HUP signals. Nohup doesn't put the job automatically in the background. We need to tell that explicitly using &

Answer 4

The nohup command is a signal masking utility and catches the hangup signal. Where as ampersand doesn’t catch the hang up signals. The shell will terminate the sub command with the hang up signal when running a command using & and exiting the shell. This can be prevented by using nohup, as it catches the signal. Nohup command accept hang up signal which can be sent to a process by the kernel and block them. Nohup command is helpful in when a user wants to start long running application log out or close the window in which the process was initiated. Either of these actions normally prompts the kernel to hang up on the application, but a nohup wrapper will allow the process to continue. Using the ampersand will run the command in a child process and this child of the current bash session. When you exit the session, all of the child processes of that process will be killed. The ampersand relates to job control for the active shell. This is useful for running a process in a session in the background.

Answer 5

myprocess.out & would run the process in background using a subshell. If the current shell is terminated (say by logout), all subshells are also terminated so the background process would also be terminated. The nohup command ignores the HUP signal and thus even if the current shell is terminated, the subshell and the myprocess.out would continue to run in the background. Another difference is that & alone doesn't redirect the stdout/stderr so if there are any output or error, those are displayed on the terminal. nohup on the other hand redirect the stdout/stderr to nohup.out or $HOME/nohup.out.

Answer 6

Correct me if I'm wrong

  nohup myprocess.out &

nohup catches the hangup signal, which mean it will send a process when terminal closed.

 myprocess.out &

Process can run but will stopped once the terminal is closed.

nohup myprocess.out

Process able to run even terminal closed, but you are able to stop the process by pressing ctrl + z in terminal. Crt +z not working if & is existing.