Log in and switch users in multiuser targets

I prefer to power off or reboot my machine with init <number> command rather than any command or clicking the menus. In this section we learn indepth about the <number> initiated with, it called runlevel and this is the explanation of each level that related to systemd initial mode.

Name	Runlevel	Description	Old Command	New Command
/lib/systemd/system/poweroff.target	0	Power off the machine	init 0	systemctl isolate poweroff.target
/lib/systemd/system/rescue.target	1	Boot into rescue mode (single user)	init 1	systemctl isolate rescue.target
/lib/systemd/system/runlevel2.target	2	Boot into single use with no network setup	init 2	systemctl isolate runlevel2.target
/lib/systemd/system/runlevel3.target	3	Boot text based multi-user	init 3	systemctl isolate runlevel3.target
/lib/systemd/system/runlevel4.target	4	reserved for special purposed text mode	init 4	systemctl isolate runlevel4.target
/lib/systemd/system/graphical.target	5	Boot into multi user graphical mode	init 5	systemctl isolate graphical.target
/lib/systemd/system/reboot.target	6	Reboot the machine	init 6	systemctl isolate reboot.target

Get the current run-level target

To get our currend run-level mode, we could use these commands

[rhcsa@localhost ~]$ runlevel 
N 5
[rhcsa@localhost ~]$ systemctl get-default 
graphical.target
[rhcsa@localhost ~]$ systemctl --type target|grep "multi\|rescue\|graph\|runlevel[0-6]"
  graphical.target       loaded active active Graphical Interface
  multi-user.target      loaded active active Multi-User System

We could also check the current run-level by looking at the default.target is content

[rhcsa@localhost ~]$ cat /etc/systemd/system/default.target
#  SPDX-License-Identifier: LGPL-2.1-or-later
#
#  This file is part of systemd.
#
#  systemd is free software; you can redistribute it and/or modify it
#  under the terms of the GNU Lesser General Public License as published by
#  the Free Software Foundation; either version 2.1 of the License, or
#  (at your option) any later version.

[Unit]
Description=Graphical Interface
Documentation=man:systemd.special(7)
Requires=multi-user.target
Wants=display-manager.service
Conflicts=rescue.service rescue.target
After=multi-user.target rescue.service rescue.target display-manager.service
AllowIsolate=yes

If we take a closer look at each run-level target files available in /lib/systemd/system, they are basically comprises of the file itself and its soft link, technically shows in column 3 with value 1 which mean the file has only one link to itself. A specific topic related soft link and hard link will be covers in the next article about Symbolic Link.

[root@localhost ~]# ls -li /lib/systemd/system/*.target|grep 'runlevel\|reboot\|rescue\|graphic\|multi\|poweroff'|sort -k2 -r
34787643 -rw-r--r--. 1 root root 607 Jan 18  2022 /lib/systemd/system/poweroff.target
34787616 -rw-r--r--. 1 root root 606 Jan 18  2022 /lib/systemd/system/graphical.target
34809600 -rw-r--r--. 1 root root 598 Jan 18  2022 /lib/systemd/system/reboot.target
34787636 -rw-r--r--. 1 root root 540 Jan 18  2022 /lib/systemd/system/multi-user.target
34809605 -rw-r--r--. 1 root root 500 Jan 18  2022 /lib/systemd/system/rescue.target
34809613 lrwxrwxrwx. 1 root root  17 Sep 23  2022 /lib/systemd/system/runlevel4.target -> multi-user.target
34809612 lrwxrwxrwx. 1 root root  17 Sep 23  2022 /lib/systemd/system/runlevel3.target -> multi-user.target
34809611 lrwxrwxrwx. 1 root root  17 Sep 23  2022 /lib/systemd/system/runlevel2.target -> multi-user.target
34809615 lrwxrwxrwx. 1 root root  16 Sep 23  2022 /lib/systemd/system/runlevel5.target -> graphical.target
34787604 lrwxrwxrwx. 1 root root  16 Sep 23  2022 /lib/systemd/system/default.target -> graphical.target
34809609 lrwxrwxrwx. 1 root root  15 Sep 23  2022 /lib/systemd/system/runlevel0.target -> poweroff.target
34809616 lrwxrwxrwx. 1 root root  13 Sep 23  2022 /lib/systemd/system/runlevel6.target -> reboot.target
34809610 lrwxrwxrwx. 1 root root  13 Sep 23  2022 /lib/systemd/system/runlevel1.target -> rescue.target
34787598 lrwxrwxrwx. 1 root root  13 Sep 23  2022 /lib/systemd/system/ctrl-alt-del.target -> reboot.target

Switch the run-level

To switch between run-level we can use the old command or new command as listed on the table above we could also use this command, for instance we wanted to switch to rescue.target

[root@localhost ~]# systemctl set-default rescue.target

Or we could also replace the symlink of default.target with run-level target that we wanted

[root@localhost ~]# ls -li /etc/systemd/system/default.target
34950082 lrwxrwxrwx. 1 root root 40 Feb 15 18:16 /etc/systemd/system/default.target -> /usr/lib/systemd/system/graphical.target
[root@localhost ~]# ln -s -f -v /lib/systemd/system/rescue.target /etc/systemd/system/default.target

Then reboot the machine

Log in and switch users

To switch between users, we use su command whether with option - or -l or not. I we use - option we will load the target user’s shell profile listed on .bash_profile file

[root@localhost ~]# su -V;su -h
su from util-linux 2.37.4

Usage:
 su [options] [-] [<user> [<argument>...]]

Change the effective user ID and group ID to that of <user>.
A mere - implies -l.  If <user> is not given, root is assumed.

Options:
 -m, -p, --preserve-environment      do not reset environment variables
 -w, --whitelist-environment <list>  don't reset specified variables

 -g, --group <group>             specify the primary group
 -G, --supp-group <group>        specify a supplemental group

 -, -l, --login                  make the shell a login shell
 -c, --command <command>         pass a single command to the shell with -c
 --session-command <command>     pass a single command to the shell with -c
                                   and do not create a new session
 -f, --fast                      pass -f to the shell (for csh or tcsh)
 -s, --shell <shell>             run <shell> if /etc/shells allows it
 -P, --pty                       create a new pseudo-terminal

 -h, --help                      display this help
 -V, --version                   display version

For more details see su(1).

[user1@localhost ~]$ echo "echo \"hi, this is from $USER .bash_profile\"" >> ~/.bash_profile 
[user1@localhost ~]$ exit
logout
[root@localhost ~]# su user1
[user1@localhost root]$ exit
exit
[root@localhost ~]# su - user1
hi, this is from user1 .bash_profile
[user1@localhost ~]$

This material is refers to the rhca objectives