====== Init Systems ======

**General Information**

Down and dirty quick info about three different init systems that Linux uses; SysV init, upstart, and systemd.

**Checklist**
  * Distro(s): Varies, depending upon init system.

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===== All Inits =====

This section will compare three init systems; sysv, upstart, and systemd.

==== Command Table ====

  * The service name "foo" will be used in all examples.
  * systemctl assumes ".service" appended to unit name if left off

^ Command ^ Sysv ^ Upstart ^ Systemd ^
| Status of Service | service foo status | initctl status foo | systemctl status foo |
| Start Service | service foo start | initctl start foo | systemctl start foo |
| Stop Service | service foo stop | initctl stop foo | systemctl stop foo |
| Restart Service | service foo restart | initctl restart foo | systemctl restart foo |
| Reload config file | service foo reload | initctl reload foo | systemctl reload foo |
| List services | ls /etc/init.d/ | initctl list | systemctl (or)\\ systemctl list-unit-files (or)\\ systemctl list-unit-files --type=service |
| Enable Service At Boot | chkconfig foo on | Edit /etc/init/<jobname>.conf "start on" field | systemctl enable foo |
| Disable Service At Boot | chkconfig foo off | Edit /etc/init/<jobname>.conf "start on" field | systemctl disable foo |
| List services and enabled run levels | chkconfig --list | initctl show-config | systemctl list-unit-files --type=service (or)\\ ls /etc/systemd/system/*.wants/ |

----

==== All Inits Run Levels ====

^ Description ^ Sysv ^ Upstart ^ Systemd ^
| Halt system | 0 | runlevel [0] | poweroff.target |
| Single user mode | 1,s,single | runlevel [1] | rescue.target |
| User defined. Same as 3 by default. | 2,4 | runlevel [24] | multi-user.target |
| Multi-user, non-graphical | 3 | runlevel [3] | multi-user.target |
| Multi-user, graphical | 5 | runlevel [5] | graphical.target |
| Reboot | 6 | runlevel [6] | reboot.target |
| Emergency shell | emergency | ? | emergency.target |

  * Note: Upstart has many different "events" besides runlevels that can trigger jobs to start, such as "start on file-system" or "start on startup"

----

===== SysV Init (init) =====

Sysv init details not contained in All Inits section.

__Distributions__: RHEL/CentOS <= 6 (RHEL 6 uses an init/upstart combination), Debian <= 7

==== Defualt run level ====
  * Set in file: /etc/inittab

Example with run level 5 as default:
<code bash>
id:5:initdefault
</code>

==== Master Copy of Startup Scripts ====
/etc/rc.d/init.d/

==== rc#.d directories ====
The rc#.d directories contain symbolic links to the scripts in /etc/rc.d/init.d.

# = the run level, ie rc0.d, rc1.d, rc2.d, rc3.d, rc4.d, rc5.d, rc6.d

__Locations__
  * /etc/rc.d/rc#.d or
  * /etc/rc#.d

__Script Names__
  * Scripts are named for if they are going to Start(S) or Kill(K) the script, a sequence number, and then the name of the script.
  * /etc/rc2.d/S80cups  = This would Start(S) cups after any scripts numbered S01 to S79
  * /etc/rc0.d/K80cups  = This would Stop/Kill (K) cups after any scripts numbered K01 to K79

==== Manage Services ====

  * service command: RedHat and Debian (service status/stop/start/restart)
  * chkconfig: Red Hat based systems (enable/disable services)
  * update-rc.d: Debian/Ubuntu (enable/disable services)

----

===== Upstart =====

Upstart details not contained in All Inits section.

__Distributions__: Ubuntu 9.10 - Current (migrating to systemd), RHEL/CentOS 6 (some scripts)

Upstart uses event driven jobs, instead of run levels to executes scripts.
It does have events for rc0-6 to maintain compatibility with init.

  * Job Definition Files Location: /etc/init/
  * Job control: initctl

----

===== Systemd =====

Systemd details not contained in All Inits section.

__Distributions__: RHEL/CentOS 7, Debian 8, Ubuntu 15.4?

==== Location of Unit Scripts ====

  * All of systemd unit scripts are located here (and sub folders): /usr/lib/systemd/
  * Systemd uses this directory of symlinks to run those scripts: /etc/systemd/system/
  * Other systemd configuration: /etc/systemd/
  * Unit control: systemctl

==== Laptop: Disable Suspend ====

On systemd based systems, the laptop suspend on lid closed can be disabled by changing the logind config.

1) Edit /etc/systemd/logind.conf

2) Uncomment "HandleLidSwitch" and set to ignore
<code bash>
HandleLidSwitch=ignore
</code>