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Unit Sections

All units can have two standard sections: [Unit] and [Install]. We should differ unit and [UNIT]; the first are a type of configuration file in systemd environment, the second one is a section inside the unit file. All type of units can also implement it's own sections, like [Socket] in socket units or [Mount] in mount units. In this section we will comment the sections of systemd and how to start this units manually.

Unit

Parameter Description
Description A string describing the unit
Documentation Space separated list of URI's referencing the documentation
Requires Space-separated list of dependence units to start when this unit starts. If one of this dependences fails, it won't start the unit
Wants Similar to requires, but if the dependence start fails it wont fail the unit start
Conflicts Dependence units to stop before start the unit
Before, After Set hierarchy of units
AllowIsolate Boolean, set the permission to isolate to this unit

The full documentation is in the man systemd.unit(5).

Install

Parameter Description
Alias Space-separated list of alias names of the unit
WantedBy, RequiredBy Creates a symlink on requires or wants directory. Same effect of Wants and Requires parameters on Unit section
Also Units to install/desinstall when this unit is installed/desinstalled

The full documentation is in the man systemd.unit(5).

Automount

Parameter Description Required
Where The absolute path of the automount point. If not exists is created in the mount order Yes
DirectoryMode If the directory is created the filesystem access No
TimeoutIdleSec Time to wait the mount, after this time, systemd tries to umount it No

The unit must be named like the mount point, for example a unit with Where=/mnt/disk-0 should be called mnt-disk-0.automount. A mount unit with the same name must exists.

Mount

Parameter Description Required
What Defines the path of the device to mount Yes
Where Defines the mount point. Creates if not exists Yes
Type A string containing the filesystem type No
Options Coma separated list of mount options No
SloppyOptions Boolean value, on true unknown options in Options will be tolerated No
DirectoryMode When automatically creating mount point directory denotes it's permissions in octal mode. Default is 0755 No
TimeoutSec Time to wait the mount, after this time sends a SIGTERM, after another delay of this time sends a SIGKILL No

The unit must be named like the mount point, for example a unit with Where=/mnt/disk-0 should be called mnt-disk-0.mount. You can manually execute this systemd units. Some examples:

Simple mount file:

[Mount]
What=/dev/loop0
Where=/mnt/disk-0

In bash:

mount /dev/loop0 /mnt/disk-0

In C:

mount("/dev/loop0", "/mnt/disk-0", "ext4", 0, NULL);
/* You must specify the filesystem
   in this case is a ext4*/

Mount with a filesystem:

[Mount]
What=/dev/loop0
Where=/mnt/disk-0
Type=ext2

In bash:

mount /dev/loop0 /mnt/disk-0 -t ext2

In C:

mount("/dev/loop0", "/mnt/disk-0", "ext2", 0, NULL);

Mount with options:

[Mount]
What=/dev/loop0
Where=/mnt/disk-0
Type=ext2
Options=rw,users

In bash:

mount /dev/loop0 /mnt/disk-0 -t ext2 -o rw,users

In C:

mount("/dev/loop0", "/mnt/disk-0", "ext2", "rw,users", NULL);

Mount with sloppy options:

[Mount]
What=/dev/loop0
Where=/mnt/disk-0
Type=ext2
Options=rw,users
SloppyOptions=true

In bash:

mount /dev/loop0 /mnt/disk-0 -t ext2 -s -o rw,users

Mount with timeout:

[Mount]
What=/dev/loop0
Where=/mnt/disk-0
TimeoutSec="3min 10s"

In bash:

# To mount with a timeout we must use a order to timeout
# a process
maxtime "mount /dev/loop0 /mnt/disk-0" 190

# When the time is exceeded it sends a kill signal to the mount

Service

Parameter Description
Type Sets the process type: simple, forking, oneshot, dbus, notify, idle
RemainAfterExit Boleean to define if the unit should be active after process exited
PIDFile The file containing the PID
BusName D-Bus name. Required for type dbus
ExecStart Command to execute the process
ExecStartPre, ExecStartPost Commands to execute before or after the process
ExecReload Commands to execute reload on the service
ExecStop Commands to stop the service

The full documentation is in the man systemd.service(5).

Socket

Parameter Description
ListenStream Defines a SOCK_STREAM address to listen: sequenced, reliable, two-way, connection-based.
ListenDatagram Defines a SOCK_DGRAM address to listen: datagram connectionless, unreliable messages of maximum length.
ListenSequentialPacket Defines a SOCK_SEQPACKET address to listen: datagram sequenced, reliable, two-way, connection-based, messages with maximum length.
ListenFIFO Defines a system FIFO to listen.
ListenSpecial Defines a special file to listen, such as character device nodes.
ListenNetlink Defines a Netlink family to listen.
ListenMessageQueue Defines a POSIX message queue to listen (See mqueue).
ListenUSBFunction Defines a USB Endpoint to listen.
Service Defines the service to start when the socket accepts the connection

The full documentation is in the man systemd.socket(5).

Execute order when socket receives message

[Socket]
ListenStream=4111

In bash

This example is NOT efficient.

while true
        do
                # Create the socket for only one connection
                nc -l 4111 > /dev/null
                # Execute the order
                echo "Doing some orders!" >> /tmp/socket-activation.log
        done

Swap

Parameter Description Required
What Absolute path of a node to use for paging Yes
Priority An integer to set the priority No
Options An option string for the swap device No
TimeoutSec Time to wait the swapon, after this time, systemd will kill it No

The unit must be named like the swap path, just like the mount unit.

Timer

Parameter Description
OnCalendar Defines a time based on calendar
Unit Defines the unit to start when the timer stops

The full documentation is in the man systemd.timer(5).

Bibliography: