Why does exec start a process with new PID?

Chowroc · 01-10-2006, 08:19 PM

for example:

Code:

#!/bin/sh

_pid=$$
echo $_pid
exec /sbin/syslogd -m 0

[root@LFS0 ~]# ./_test.sh     
7497
[root@LFS0 ~]# ps aux | grep syslogd
root      7498  0.0  0.4  3468  620 ?        Ss   10:43   0:00 /sbin/syslogd -m 0
root      7500  0.0  0.3  5596  516 pts/0    R+   10:43   0:00 grep syslogd

But the man says "If command is specified, it replaces the shell. No new process is created."

What's wrong?

Thanks.

cyberjun · 01-11-2006, 12:27 AM

Hi,
Nice question. I think I might be able to explain

The catch is the syslogd process. I am sure it forks to go into the background. To verify do the following:
1. Create a similar script like :

Code:

localuser@localhost ~ $ cat see.sh
#!/bin/sh

_pid=$$
echo $_pid
exec yes > /dev/null

2. The above script when executed will not fork into the background. So you will have to execute the "ps aux | grep yes" from another terminal.

Hopefully, this time the PIDs will match. I tried on my own machine and it does show the same PIDs.

cheers,
--cyberjun

bigearsbilly · 01-11-2006, 03:27 AM

It works OK!

Code:

billym.primadtpdev>echo $$
20726
billym.primadtpdev>exec
billym.primadtpdev>echo $$
20726
billym.primadtpdev>ksh
billym.primadtpdev>echo $$
20994
billym.primadtpdev>exec
billym.primadtpdev>echo $$
20994
billym.primadtpdev>exit
billym.primadtpdev>echo $$
20726

Chowroc · 01-11-2006, 04:26 AM

I think this is caused by the daemon. Commanly, a daemon (write by C) will fork() a new process before the exec().

ajs318 · 01-11-2006, 05:55 AM

The manpage is right: exec keeps the PID. However, standalone daemons typically fork a new process when they start. This is necessary so that the process can detach itself from STDIN, STDOUT and STDERR, and return control to the shell or script which launched it. After the fork, the parent process just exits cleanly; the child process is the one that does the real work.

You are just catching the state of the process table after the fork, which is correctly showing the PID of the child process.