Linux administration

• A Linux system runs in one of several runlevels — modes providing different features and levels of functionality

• Linux systems normally have seven runlevels, numbered from 0–6:

  • Three are mandatory (0 = halt, 6 = reboot, 1 = single-user)
  • Four are user-defined (2–5)

• No consensus between administrators or distributions about how to organise the user-defined runlevels

  • Some rely (partly) on runlevels to define which major subsystems are running
  • Others prefer the flexibility of starting and stopping subsystems individually, without changing runlevel
  • In every common distribution, there is at least one user-defined runlevel which has the same services as another

• Many Linux distributions use a program called sulogin to restrict access to single-user mode
• sulogin is run when the system enters single-user mode
• It requires the root password to be typed on the console before switching to single-user mode
  • If the password is not typed, sulogin returns the system to the normal runlevel
• Why is sulogin needed?
  • Untrusted users may have access to the system's keyboard during bootup
  • In many configurations, this would enable them to boot the system up in single-user mode

• To safely shut down the system, run the halt command as root. This is a safe shutdown: it stops all services, disables all network interfaces, and unmounts all filesystems

• To safely reboot the system, run reboot as root. Most systems also let you hit Ctrl+Alt+Del on the console

• Alternatively, the shutdown command allows you to schedule the power-down or reboot, to give users warning to save their work

• Halt at 6pm:

# shutdown -h 18:00

• Reboot thirty minutes from now:

# shutdown -r +30

• The system's default runlevel on bootup is configured in /etc/inittab
• To configure a default runlevel of 3, /etc/inittab should contain the line:

id:3:initdefault

• There should be only one initdefault line in /etc/inittab

• Most bootloaders, including GRUB, give you the ability to type in a kernel command line
• Naming a runlevel on the kernel command line selects that runlevel for use on system startup
• To start in single-user mode:

linux 1

• To start in emergency mode:

linux -b

Emergency mode provides nothing beyond a shell to type into — useful for repairing serious system corruption

• The runlevel command prints the system's previous and current runlevels:

$ /sbin/runlevel
N 3

• If there is no previous runlevel (for example, if the runlevel hasn't been changed from the default), N is printed instead

• The system has a process named init, with pid 1, which is the ultimate ancestor of all other processes

• init is responsible for controlling runlevels, so switching runlevels involves telling init to do something. As root, run

# telinit 1

to switch into a given runlevel

• You can alternatively use init itself, with the same syntax:

# init 5

• Obviously, changing runlevels should not be undertaken lightly. In particular, changing runlevel can terminate important system services, or affect users' ability to log in

• /etc contains an init.d directory, and an rcN.d directory for each runlevel N. Some distributions (notably Red Hat) put all these directories in /etc/rc.d, not directly under /etc

• init.d contains an init script for each service that can be started

• The rcN.d directories contain symbolic links to the init scripts

  • These symbolic links control which services are available in each runlevel

• Symbolic links in the rcN.d directory are either start links or stop links

  • Start links are named SNNservice, where NN is a number and service is the name of a service
  • Stop links are named KNNservice

• The start links for a runlevel directory indicate which services should be started when that runlevel is entered

• Correspondingly, the stop links indicate which services should be stopped when the runlevel is entered

• The rc shell script (usually /etc/rc.d/rc or /etc/init.d/rc) runs the relevant init script appropriately for start links and stop links

• You can also start or stop an individual service without changing runlevel
• An init script always takes an argument of start or stop to start or stop the relevant service
• For example, if the MySQL database server has an init script /etc/init.d/mysql, you can start MySQL with

# /etc/init.d/mysql start

or stop it with

# /etc/init.d/mysql stop

• Some init scripts also accept an argument of restart (stop and then re-start) or reload (reload the service's configuration file)

• a. Look in /etc/init.d or /etc/rc.d/init.d to see what services can be started by init.
• b. Try running the script for crond, and use it to stop the cron service, and then start it up again.
• c. Take a quick look at the program in a text editor (it's a small shell script) to get a rough idea of what it
• d. Look in the rc3.d directory to see what services are killed and started when switching to runlevel 3.
• e. Use telinit to change to single-user mode.
• f. Once in single-user mode, use top to see what processes are left running.

• a. Reboot the machine by changing to runlevel 6.
• b. When the GRUB prompt appears, type Tab to see a list of operating systems to boot. Type the name of the one you want followed by a space and the number 1, to indicate that you want to boot straight into single-user mode.
• c. Change back to runlevel 3.