Ubuntu and Fedora Replace init with Upstart
Init: the Old Way
Juliet Kemp
Monday, May 4, 2009 03:46:12 PM
init is the program which is used on most Linuxes to generate all
other processes on bootup. The BSD style (which runs from the
/etc/rc script) is slightly different from the SysV style: SySV is
probably the one you're familiar with, as it's used by Debian, SuSE, and until
recently, Redhat. With this setup, the system checks /etc/inittab to
find a default runlevel. This file also defines what to do for various
runlevels. A runlevel is a way of specifying a particular set of processes to
run.
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The important point is that init is level-driven. This means that you
allocate each service to a runlevel, and services are started up in blocks
based on which runlevel you boot into (or move into once booted). You can of
course manually start and stop services using the same scripts, but the basic
idea is one of sets of services which correspond to particular usage
situations.
Within the runlevels, the names of the start and stop links govern the
timing of the script. Each /etc/rcx.d directory has a collection of
softlinks to the start/stop service scripts in /etc/init.d/. These
look like this:
K20service -> /etc/init.d/service
S35service -> /etc/init.d/service
If the link starts with a K, the service is stopped. If it starts with an S,
it is started. This is done with the logic in /etc/init.d/rc.
The numbers control the order, to avoid dependency problems: lower numbers are run first.
With modern setups, increasingly this static system has disadvantages:
- Plug-and-play hardware problems: init cannot handle hardware
that's plugged in after bootup. Instead, modern Linux distros which use init have to handle this via the
kernel.
- Simlarly, it may have problems with networked filesystems which may not be available on boot, or may only
be available at certain stages.
- We may have daemons which we'd like only to run when the hardware is
available.
So, given these issues: how does upstart work, how does it improve
matters, and how can you start using it?
Next: Upstart: the New Way »
