network addresses for a networking interface).
For example, in the past a system administrator could quickly change the
system to maintenance or single-user mode by using the telinit command
with its S option, like this:
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matthew@seymour:~$ sudo telinit S
Today, the same thing would be done using this systemd command:
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matthew@seymour:~$ sudo systemctl rescue
The telinit command uses the init command to change runlevels and
shut down currently running services. However, under systemd, telinit
is deprecated.
After booting to single-user mode, you used to then return to multiuser mode,
like this:
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matthew@seymour:~$ sudo telinit 2
Today, the same thing would be done using this systemd command:
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matthew@seymour:~$ systemctl default
TIP
Linux is full of shortcuts. For example, if you don’t want to use the
systemctl default command, you can just click Ctrl+D. If you exit
the single-user shell by typing exit at the prompt, you go back to the
default runlevel without having to use telinit or systemctl.Troubleshooting Runlevel Problems
Reordering or changing system services during a particular runlevel is rarely
necessary when using Ubuntu unless some disaster occurs. But system
administrators should have a basic understanding of how Linux boots and
how services are controlled in order to perform troubleshooting or diagnose
problems. By using additional utilities such as the dmesg | less
command to read kernel output after booting or by examining system logging
with cat /var/log/messages | less, it is possible to gain a bit
more detail about what is going on when faced with troublesome drivers or