init?
In Linux, init is a abbreviation for Initialization. The init is a daemon process which starts as soon as the computer starts and continue running till, it is shutdown
If somehow init daemon could not start, no process will be started and the system will reach a stage called “Kernel Panic“. init is most commonly referred to as System V init.
- The SysV and LSB mainstay
- calls the scripts for each runlevel
/etc/init.d/rc
-followed by a number for the runlevel
Each runlevel has a directory of scripts
-When entering the runlevel, start S scripts
-When existing the runlevel, stop the K scripts
Distros using SysV init would have several directories under
/etc
such as rc0.d
, rc1.d
, through rc6.d
for each runlevel.Under each of the directories, you'd have a set of symbolic links that
point back to scripts that include startup and shutdown directives for
services like Apache and the SSH daemon. Each link has a name like KNNsshd
or SNNcups
, where the NN
determines the order in which services are killed (K) or started (S). Upstart
A replacement for SysV init
- Developed by ubuntu,used i other distros
Advantages over init
- better support for hotplug devices
- cleaner and faster service management
/etc/init
- configuration files
initctl
- this is the script that control the application.
After months of often bitter debate,Ubuntu will ditch and adopt systemd
Systemd
- Improve handling of dependencies
- Concurrent processing
- Reduce computational overhead
/etc/systemd
- configuration files
systemctl
- Control Services
To switch between targets :
#systemctl isolate muti-user.target
To get the default target : #systemctl get-default
To set the target : #systemctl set-default graphical.target
Locations of systemd files #/etc/systemd/system/
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