Introduction to boot targets

The concept of boot targets is a whole new ball game. Boot targets are used when working with systemd. We can see increased performance, because only requests for a particular socket are started as needed. Also, systemd emulates init for compatibility, while in the background systemd is doing the work. When we use boot targets, we work with units. For a given boot target, a number of daemons reside. Let's see the available boot targets in an Ubuntu distribution:

root@ubuntu:/home/philip# systemctl list-units --type target
 UNIT           LOAD    ACTIVE   SUB  DESCRIPTION
 basic.target      loaded active active Basic System
 cryptsetup.target loaded active active Encrypted Volumes
 getty.target      loaded active active Login Prompts
 graphical.target  loaded active active Graphical Interface
 local-fs-pre.target loaded active active Local File Systems (Pre)
 local-fs.target   loaded active active Local File Systems
 multi-user.target loaded active active Multi-User System
 network.target    loaded active active Network
 nss-user-lookup.target loaded active active User and Group Name Lookups
 paths.target     loaded active active Paths
 remote-fs-pre.target loaded active active Remote File Systems (Pre)
 remote-fs.target loaded active active Remote File Systems
 slices.target   loaded active active Slices
 sockets.target  loaded active active Sockets
 sound.target    loaded active active Sound Card
 swap.target     loaded active active Swap
 sysinit.target  loaded active active System Initialization
 time-sync.target loaded active active System Time Synchronized
 timers.target   loaded active active Timers
LOAD = Reflects whether the unit definition was properly loaded.
 ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
 SUB = The low-level unit activation state, values depend on unit type.
19 loaded units listed. Pass --all to see loaded but inactive units, too.
 To show all installed unit files use 'systemctl list-unit-files'.
 root@ubuntu:/home/philip#

From the preceding output, only the targets that are currently loaded will be displayed. The graphical.target is similar to runlevel 5 in init. To see all of the boot targets, we would do this:

root@ubuntu:/home/philip# systemctl list-units --type target --all
 UNIT             LOAD ACTIVE SUB DESCRIPTION
 basic.target       loaded active active Basic System
 cryptsetup.target  loaded active active Encrypted Volumes
 emergency.target   loaded inactive dead Emergency Mode
 failsafe-graphical.target loaded inactive dead Graphical failsafe fallback
 final.target       loaded inactive dead Final Step
 getty.target       loaded active active Login Prompts
 graphical.target   loaded active active Graphical Interface
 halt.target        loaded inactive dead Halt
 local-fs-pre.target loaded active active Local File Systems (Pre)
 local-fs.target    loaded active active Local File Systems
 multi-user.target    loaded active active Multi-User System
 network-online.target loaded inactive dead Network is Online
 network-pre.target    loaded inactive dead Network (Pre)
 network.target            loaded active active Network
 nss-user-lookup.target    loaded active active User and Group Name Lookups
 paths.target                loaded active active Paths
 reboot.target               loaded inactive dead Reboot
 remote-fs-pre.target        loaded active active

From the preceding output, we can see the boot targets that are active and also those that are inactive.

Now, let's say we want to see the actual daemons associated with a given target. We would run the following command:

root@ubuntu:/home/philip# systemctl list-dependencies graphical.target

From the preceding output, we can see a number of daemons in graphical.target. One such daemon is the NetworkManager.service, which is used for networking within our system. The way to read this would be:

• Green circle: Indicates that the service is currently running
• Red circle: Indicates that the service is not presently running