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Linux in a Nutshell

This directory of Linux commands is from Linux in a Nutshell, 5th Edition.

Click on any of the 687 commands below to get a description and list of available options. All links in the command summaries point to the online version of the book on Safari Bookshelf.

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init [bootflags] [runlevel]

System administration command. Initialize system. Usually run from the boot loader—e.g., lilo or grub.

Boot flags


Set the AUTOBOOT environment variable to yes. The boot loader will do this automatically when booting with the default command line.


Boot directly into a single-user shell for emergency recovery.


Single-user mode.


Boot into single-user mode but do not run any other startup scripts.

-z characters

The specified characters are ignored, but will make the command line take up a bit more room on the stack. init uses the extra space to show the current runlevel when running the ps command.


init is the first process run by any Unix machine at boot time. It verifies the integrity of all filesystems and then creates other processes, using fork and exec, as specified by /etc/inittab. Which processes may be run is controlled by runlevel. All process terminations are recorded in /var/run/utmp and /var/log/wtmp. When the runlevel changes, init sends SIGTERM and then, after 20 seconds, SIGKILL to all processes that cannot be run in the new runlevel.


The current runlevel may be changed by telinit, which is often just a link to init. The default runlevels vary from distribution to distribution, but these are standard:


Halt the system.

1, s, S

Single-user mode.


Multiuser mode, console login. This is commonly used in server configurations.


Full graphical mode. This is a common default for desktop configurations.


Reboot the system. Never set the default runlevel to 6.

q, Q

Reread /etc/inittab.

Check the /etc/inittab file for runlevels on your system.

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