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I am using ttyS0 as a serial console to connect to a redhat linux, which is setup using /etc/init/ttyS0.conf.

When I am connected to serial console, could not find agetty process however sees a bash shell on ttyS0. Whereas with SSH, I could see agetty process in process listing. How this work, does bash kill agetty and get itself on ttyS0 when connecting over console.

# /etc/init/ttyS0.conf

#
 stop on runlevel [S016]
 start on runlevel [23]
#
 respawn
instance /dev/ttyS0
 exec agetty -h -L -w /dev/ttyS0 115200 vt102
#

no agetty process when connecting over console, however I see bash process with ttyS0

virsh # console --domain 1
Connected to domain LMS
Escape character is ^]
login: root
Password:
Last login: Fri Nov  9 03:33:11 from 192.168.122.1
[root@lmm ~]# ps -eaf | grep -i agetty
root      8397  8376  0 03:37 ttyS0    00:00:00 grep -i agetty
[root@lmm ~]#

[root@lmm ~]# ps -eaf | grep -i ttyS0
root      8376  5585  0 03:37 ttyS0    00:00:00 -bash

agetty process when connecting over SSH to machine

[root@lmm ~]# ps -eaf | grep -i agetty
root      5585     1  0 03:32 ttyS0    00:00:00 agetty -h -L -w /dev/ttyS0 115200 vt102
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When you connect using a terminal managed by agetty, which is the case here when you connect over the serial port, agetty replaces itself with login without forking (see the implementation in util-linux). So once you’ve logged in, there is no agetty process any more.

See also When will login terminate?

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