Who does the daemonizing?

Question

There are various tricks to daemonize a linux process, i.e. to make a command running after the terminal is closed.

nohup is used for this purpose, and fork()/setsid() combination can be used in a C program to make itself a daemon process.

The above was my knowledge about linux daemon, but today I noticed that exiting the terminal doesn't really terminate processes started with & at the end of the command.

$ while :; do echo "hi" >> temp.log ; done &
[1] 11108
$ ps -ef | grep 11108
username   11108 11076 83 15:25 pts/0    00:00:05 /bin/sh
username   11116 11076  0 15:25 pts/0    00:00:00 grep 11108
$ exit
(after reconnecting)
$ ps -ef | grep 11108
username   11108     1 91 15:25 pts/0    00:00:17 /bin/sh
username   11130 11540  0 15:25 pts/0    00:00:00 grep 11108

So apparently, the process's PPID changed to 1, meaning that it got daemonized somehow.

This contradicts my knowledge, that & is not enough and one must use nohup or some other tricks to a process 'daemon'.

Does anyone know who is doing this daemonizing?

I'm using a CentOS 6.3 host and putty/cygwin/sshclient produced the same result.

http://stackoverflow.com/questions/958249/whats-the-difference-between-nohup-and-a-daemon — Grumpy, Dec 06 '12 at 07:02

score 4 · Accepted Answer · edited Dec 07 '12 at 01:48

4

You can daemonize a process if that doesn't respond to SIGHUP signal.

When bash shell is terminated while it is running background tasks, bash shell sends SIGHUP (hangup signal) to all tasks. However bash won't wait until child processes are completely terminated. If child process doesn't respond to SIGHUP signal, that process becomes an orphan process. (its parent pid is changed to 1 - init process - to prevent becoming a useless zombie process)

Subshell executions basically do not responds to SIGHUP signals, thus your command will still be running after logging out from the first shell.

edited Dec 07 '12 at 01:48

lyomi

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answered Dec 06 '12 at 16:47

Kevin

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Thanks for a clear explanation. What I was missing was that programs written in C/C++ has a default SIGHUP handler that terminates themselves, so I needed the `fork`/`setsid` thing. Whereas shell scripts ignores SIGHUP so I don't need daemonizing tricks other than `&`. – lyomi Dec 07 '12 at 01:42

Who does the daemonizing?

1 Answers1