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System Init

/sbin/init is the first user process (pid=1) run on the system and runs until shutdown. All processes (aside from kernel-related once) are children to init.

init has a few jobs:

  • Coordinates later stages of the boot process.
  • Configures all aspects of the environment.
  • Starts processes needed for logging into the system.
  • Works closely with the kernel in cleaning up processes when they are terminated.

The popular schemes for system startup are:

  • Upstart (phased out by Ubuntu in favor of systemd)
  • systemd (most modern)
  • SysVinit (for old, single processor computers)

systemd​

systemd is a software suite that provides an array of system components for Linux operating systems.

It provides utilities such as systemctl to manage services. The commands only control whether the service will be stopped/started during boot time.

# show all available services
systemctl list-units -t service --all

# show all active services
systemctl list-units -t service

# start/stop service
sudo systemctl start foo
sudo systemctl start foo.service
sudo systemctl start /path/to/foo.service
sudo systemctl stop foo

# enable/disable service
sudo systemctl enable sshd.service
sudo systemctl disable sshd.service

All services which run on start-up are listed in:

ls /etc/init.d

To check for a particular service status:

sudo service network-manager status
sudo service --status-all