BootInitramfs

Runtime

If want to know how it works in general please read this document. It describes the basic idea of determining the root.

Boot order

• kernel calls /init. In the initramfs /init is the simplest script which saves the environment variables and starts/stops services on runlevel 3.
• When the root file system found init goes to runlevel 2. Last service on runlevel 2 asks init to restart. When the SysV init receives request to reboot himself uses predefined path (/sbin/init).
• /sbin/init is another script which removes initramfs, restore kernel environment and runs real init.

Services

The features may install one or more initialization scripts (or init scripts). An init script shall be installed in /etc/init.d (which may be a symbolic link to another location). The initramfs uses SysV-style init scripts.

Run Levels

Basically run levels are specified for use by the Default-Start and Default-Stop actions defined in the init scripts. The LSB standard defines them as:

• 0 halt;
• 1 single user mode;
• 2 multiuser with no network services exported;
• 3 normal/full multiuser;
• 4 reserved for local use, default is normal/full multiuser;
• 5 multiuser with a display manager or equivalent;
• 6 reboot;

but implementations are not required to provide these exact run levels or give them the meanings described here. The initramfs uses following run levels:

• ...
• 2 is used to switch to real system. The switch to this level occurs only after the rootfs is found and mounted.
• 3 default initramfs runtime.
• 9 exec real system in the /root.
• ...

See also

• The article about boot parameters.
• About initramfs services.