systemd

# Basics of units/unit files:
• These are the basic objects that systemd manages.
• Unit types vary and are typically designated by their suffixes.  
   ◦ .service for service files.
   ◦ .target for sync point between units when booting up or state changes.
   ◦ .mount for mount points.
   ◦ .timer for timers that can be used similarly to cron jobs.
   ◦ .socket, .device, .automount, .swap, .path, .snapshot, .slice, .scope, ...
• Unit files are broken down to section designations followed by key-value
   pairs, for example:
   ◦ `[section]
     Directive01=value
     Directive02=value
     ...`
• Boolean values are flexible.
   ◦ 1, true, on, yes
   ◦ 0, false, off, no
• Unit sections are clearly defined.
   ◦ See `man systemd.unit` for general unit file details. 
   ◦ See `man systemd.service`, `man systemd.timer`, etc. for specific details.

# Working with units:
• Listing units:
   ◦ `systemctl` or `systemctl list-units` # This will list all systemd units.
   ◦ `systemctl -t ` # This will limit list to service of ‘’.  
• Status of unit:
   ◦ `systemctl status ` 
• Starting unit:
   ◦ `systemctl start ` 
• Stopping unit:
   ◦ `systemctl stop ` 
• Restarting unit:
   ◦ `systemctl restart ` 
• Reloading unit configuration: 
   ◦ `systemctl reload ` 
• Enabling unit at boot time:
   ◦ `systemctl enable ` 
• Disabling unit at boot time:
   ◦ `systemctl disable ` 
• Check if unit is enabled at boot time (status shows this as well):
   ◦ `systemctl is-enabled ` # returns 0/1 (prints enabled/disabled)
• Mask unit preventing it from starting:
   ◦ `systemctl mask ` 
• Unmask unit:
   ◦ `systemctl unmask ` 
• Reload systemd (rescan for new/changed units):
   ◦ `systemctl daemon-reload` 

# Working with targets:
• Set default target (similar to SysV runlevels):
   ◦ `systemctl set-default ` 
      ◦ e.g. `systemctl set-default graphical` 
      ◦ might require ‘-f’ to override previous default
• Change current target (equivalent to telinit ):
   ◦ `systemctl isolate `

# Working with cgroups:
• Show control group contents recursively
   ◦ `systemd-cgls` 

# Troubleshooting:
• Listing failed units:
   ◦ `systemctl --failed` 
• Finding info about failed/problem units:
   ◦ `journalctl _PID=` # Unit PID can be found using → `systemctl status `
   ◦ see Journaling below for more tips.

# Journaling:
• To simply invoke the journal:
   ◦ `journalctl` 
• Show boot messages: 
   ◦ current boot:
      ◦ `journalctl -b` 
   ◦ specific boot:
      ◦ `journalctl -b ` # Where ‘’ can be 0 for current or -n for previous nth boot
   ◦ list all boots:
      ◦ `journalctl --list-boots` 
• List all messages from unit
   ◦ `journalctl -u ` 
• Show kernel ring buffer (`dmesg`)
   ◦ `journalctl -k` 
• Show messages related to specific executable: 
   ◦ `journalctl /path/to/executable` 
• Follow most recent entries (and forthcoming entries)
   ◦ `journalctl -f`
   ◦ can be combined with -u to follow a unit and new entries
      ◦ `journalctl -f -u `