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linuxubuntusystemdinit

Why Ubuntu 18.04 use `/sbin/init` instead of `systemd`?

First of all, Here is my environment of system:

# cat /proc/version
Linux version 4.15.0-52-generic (buildd@lgw01-amd64-051) (gcc version 7.3.0 (Ubuntu 7.3.0-16ubuntu3)) #56-Ubuntu SMP Tue Jun 4 22:49:08 UTC 2019

# cat /etc/issue
Ubuntu 18.04.2 LTS \n \l

Refer to this Ubuntu Wiki, ubuntu has used Systemd by default since 15.04 and Systemd runs with PID 1 as /sbin/init. However, I found the different result on my ubuntu 18.04:

# ps aux | awk '$2==1{print $0}'
root         1  0.0  0.8 159692  8784 ?        Ss   Oct24   0:21 /sbin/init noibrs splash

# lsof -p 1 | grep txt
systemd   1 root  txt       REG              252,1  1595792     927033 /lib/systemd/systemd

So, my question is that:

  1. Why Ubuntu 18.04 use /sbin/init instead of /lib/systemd/systemd?

  2. Why lsof -p 1 | grep txt return /lib/systemd/systemd while the process of PID 1 is /sbin/init?

Solution

  • /sbin/init is a symbolic link to /lib/systemd/systemd

    Take a look at the output of stat /sbin/init or readlink /sbin/init

    This is what they mean by systemd "running as /sbin/init". The systemd binary is linked as /sbin/init and started by that link name.

    Update

    To further explain the difference between the ps and lsof output: ps is showing the command that started the process, while lsof is showing which files a process has opened.

    When systemd was started, it was called by /sbin/init noibrs splash, the file system resolved the link to the file /lib/systemd/systemd which was then read from disk and executed.