SysV

Runs scripts in /etc/init.d

runlevels

Part of SystemV, in /etc/rc.d/, based on the distro you have folders or files defining each runlevel

• rc for "run commands" defines what is run at each runlevel
• a runlevel is the point at which the system is entered, or the level at which the user begins accessing the computer
• 1 is low-level, 5 is high-level for GUIs

Systemd

systemd is a suite of basic building blocks for service management.

• runs as PID 1 and starts the rest of the system
• /usr/lib/systemd holds configurations

Upstart

Used on Ubuntu by default

Launchd

Used on macOS

TODO

runit -- TODO

is an init scheme for unix-like operating systems.

• initializes and supervises processes throughout the os
• features parallelization of the start up of system services
• is an init daemon, it is the acestor of all other processes (since it is the first process started during booting

Daemon

• a program that runs as a background process, not under direct control of an interactive user
• usually, parent process of a daemon is the init process
  • created by forking a child process and immediately exiting, causing init to adopt it as a child process
  • also created directly by init

systemd Unit Files -- source

.service files, in /usr/lib/systemd/system/ have unit types:

• simple does not block on start process, becoming "active" immediately after forking off the first process
  • hard to debug errors
• oneshot blocks on start operation until the first process exits. The status is reported as "activating"
  • once start process completes, "activating" transitions straight to "inactive"
• forking, process configured with ExecStart= will call fork() as part of its startup. PIDFile= is used so that systemd can identify the main process of the daemon
• Environment= specifies variables accessible by Exec commands
• ExecStart= specifies exactly one command with arguments that is run when the service is started

Basic service:

[Unit]
Description=Foo

[Service]
ExecStart=/usr/sbin/foo-daemon