Introduction to systemd
systemd
systemd es un demonio de administración de sistema diseñado exclusivamente para la API del núcleo Linux. systemd fue desarrollado para reemplazar el sistema de inicio (init) heredado de los sistemas operativos System V y Berkeley Software Distribution (BSD). En el proceso de arranque en Linux, es el primer proceso que se ejecuta en el espacio de usuario, por lo tanto, también es el proceso padre de todos los procesos hijos en el espacio de usuario.
Está hecho para proveer un mejor framework para expresar las dependencias del servicio, permite hacer más trabajo paralelamente al inicio del sistema y reducir la sobrecarga del shell.
El nombre viene del sufijo system daemon (procesos en segundo plano) con la letra d.[4]
systemd fue escrito por Lennart Poettering[1] y publicado como software libre bajo los términos de la GNU Lesser General Public License versión 2.1 ó posterior.[
Comparado con el init de System V, systemd puede tomar ventaja de nuevas técnicas:
Systemd is a system and service manager for Linux operating systems. It is designed to be backwards compatible with SysV init scripts, and provides a number of features such as parallel startup of system services at boot time, on-demand activation of daemons, support for system state snapshots, or dependency-based service control logic. In Red Hat Enterprise Linux 7, systemd replaces Upstart as the default init system.
Está hecho para proveer un mejor framework para expresar las dependencias del servicio, permite hacer más trabajo paralelamente al inicio del sistema y reducir la sobrecarga del shell.
El nombre viene del sufijo system daemon (procesos en segundo plano) con la letra d.[4]
systemd fue escrito por Lennart Poettering[1] y publicado como software libre bajo los términos de la GNU Lesser General Public License versión 2.1 ó posterior.[
Comparado con el init de System V, systemd puede tomar ventaja de nuevas técnicas:
- Los servicios de activación de sockets y la activación de buses, que conduce a una mejor paralelización de servicios independientes.
- cgroups se utilizan para realizar un seguimiento de los procesos de servicio, en lugar de PIDs. Esto significa que los demonios no pueden “escapar” de systemd aunque estén doblemente-bifurcados.
Systemd is a system and service manager for Linux operating systems. It is designed to be backwards compatible with SysV init scripts, and provides a number of features such as parallel startup of system services at boot time, on-demand activation of daemons, support for system state snapshots, or dependency-based service control logic. In Red Hat Enterprise Linux 7, systemd replaces Upstart as the default init system.
Systemd introduces the concept of systemd units. These units are represented by unit configuration files located in one of the directories listed in Table 6.2, “Systemd Unit Locations”, and encapsulate information about system services, listening sockets, saved system state snapshots, and other objects that are relevant to the init system. For a complete list of available systemd unit types, see Table 6.1, “Available systemd Unit Types”.
Managing System Services
Previous versions of Red Hat Enterprise Linux, which were distributed with SysV init or Upstart, used init scripts located in the
/etc/rc.d/init.d/ directory. These init scripts were typically written in Bash, and allowed the system administrator to control the state of services and daemons in their system. In Red Hat Enterprise Linux 7, these init scripts have been replaced with service units.
Service units end with the
.service file extension and serve a similar purpose as init scripts. To view, start, stop, restart, enable, or disable system services, use the systemctl command as described in Table 6.3, “Comparison of the service Utility with systemctl ”, Table 6.4, “Comparison of the chkconfig Utility with systemctl”, and in the section below. The service and chkconfig commands are still available in the system and work as expected, but are only included for compatibility reasons and should be avoided.
Note
For clarity, all examples in the rest of this section use full unit names with the
.service file extension, for example: ~]# systemctl stop bluetooth.service
When working with system services, it is possible to omit this file extension to reduce typing: when the
systemctl utility encounters a unit name without a file extension, it automatically assumes it is a service unit. The following command is equivalent to the one above: ~]# systemctl stop bluetooth
Table 6.3. Comparison of the service Utility with systemctl
| service | systemctl | Description |
|---|---|---|
service name start | systemctl start name.service | Starts a service. |
service name stop | systemctl stop name.service | Stops a service. |
service name restart | systemctl restart name.service | Restarts a service. |
service name condrestart | systemctl try-restart name.service | Restarts a service only if it is running. |
service name reload | systemctl reload name.service | Reloads configuration. |
service name status | systemctl status name.service systemctl is-active name.service | Checks if a service is running. |
service --status-all | systemctl list-units --type service --all | Displays the status of all services. |
Table 6.4. Comparison of the chkconfig Utility with systemctl
| chkconfig | systemctl | Description |
|---|---|---|
chkconfig name on | systemctl enable name.service | Enables a service. |
chkconfig name off | systemctl disable name.service | Disables a service. |
chkconfig --list name | systemctl status name.service systemctl is-enabled name.service | Checks if a service is enabled. |
chkconfig --list | systemctl list-unit-files --type service | Lists all services and checks if they are enabled. |
6.2.1. Listing Services
To list all currently loaded service units, type the following at a shell prompt:
systemctl list-units --type service
For each service unit, this command displays its full name (
UNIT) followed by a note whether the unit has been loaded (LOAD), its high-level (ACTIVE) and low-level (SUB) unit activation state, and a short description (DESCRIPTION).
By default, the
systemctl list-units command displays only active units. If you want to list all loaded units regardless of their state, run this command with the --all or -a command line option: systemctl list-units --type service --all
You can also list all available service units to see if they are enabled. To do so, type:
systemctl list-unit-files --type service
For each service unit, this command displays its full name (
UNIT FILE) followed by information whether the service unit is enabled or not (STATE). For information on how to determine the status of individual service units, see Section 6.2.2, “Displaying Service Status”.
Example 6.1. Listing Services
To list all currently loaded service units, run the following command:
~]$ systemctl list-units --type service
UNIT LOAD ACTIVE SUB DESCRIPTION
abrt-ccpp.service loaded active exited Install ABRT coredump hook
abrt-oops.service loaded active running ABRT kernel log watcher
abrt-vmcore.service loaded active exited Harvest vmcores for ABRT
abrt-xorg.service loaded active running ABRT Xorg log watcher
abrtd.service loaded active running ABRT Automated Bug Reporting Tool
...
systemd-vconsole-setup.service loaded active exited Setup Virtual Console
tog-pegasus.service loaded active running OpenPegasus CIM Server
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.
46 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'
To list all installed service units to determine if they are enabled, type:
~]$ systemctl list-unit-files --type service
UNIT FILE STATE
abrt-ccpp.service enabled
abrt-oops.service enabled
abrt-vmcore.service enabled
abrt-xorg.service enabled
abrtd.service enabled
...
wpa_supplicant.service disabled
ypbind.service disabled
208 unit files listed.
6.2.2. Displaying Service Status
To display detailed information about a service unit that corresponds to a system service, type the following at a shell prompt:
systemctl status name.service
Replace name with the name of the service unit you want to inspect (for example,
gdm). This command displays the name of the selected service unit followed by its short description, one or more fields described in Table 6.5, “Available Service Unit Information”, and if it is executed by the root user, also the most recent log entries.
Table 6.5. Available Service Unit Information
| Field | Description |
|---|---|
Loaded | Information whether the service unit has been loaded, the absolute path to the unit file, and a note whether the unit is enabled. |
Active | Information whether the service unit is running followed by a time stamp. |
Main PID | The PID of the corresponding system service followed by its name. |
Status | Additional information about the corresponding system service. |
Process | Additional information about related processes. |
CGroup | Additional information about related Control Groups. |
To only verify that a particular service unit is running, run the following command:
systemctl is-active name.service
Similarly, to determine whether a particular service unit is enabled, type:
systemctl is-enabled name.service
Note that both
systemctl is-active and systemctl is-enabled return an exit status of 0 if the specified service unit is running or enabled. For information on how to list all currently loaded service units, see Section 6.2.1, “Listing Services”.
Example 6.2. Displaying Service Status
The service unit for the GNOME Display Manager is named
gdm.service. To determine the current status of this service unit, type the following at a shell prompt: ~]# systemctl status gdm.service
gdm.service - GNOME Display Manager
Loaded: loaded (/usr/lib/systemd/system/gdm.service; enabled)
Active: active (running) since Thu 2013-10-17 17:31:23 CEST; 5min ago
Main PID: 1029 (gdm)
CGroup: /system.slice/gdm.service
├─1029 /usr/sbin/gdm
├─1037 /usr/libexec/gdm-simple-slave --display-id /org/gno...
└─1047 /usr/bin/Xorg :0 -background none -verbose -auth /r...
Oct 17 17:31:23 localhost systemd[1]: Started GNOME Display Manager.
6.2.3. Starting a Service
To start a service unit that corresponds to a system service, type the following at a shell prompt as
root: systemctl start name.service
Replace name with the name of the service unit you want to start (for example,
gdm). This command starts the selected service unit in the current session. For information on how to enable a service unit to be started at boot time, see Section 6.2.6, “Enabling a Service”. For information on how to determine the status of a certain service unit, see Section 6.2.2, “Displaying Service Status”.
Example 6.3. Starting a Service
The service unit for the Apache HTTP Server is named
httpd.service. To activate this service unit and start the httpd daemon in the current session, run the following command as root: ~]# systemctl start httpd.service
6.2.4. Stopping a Service
To stop a service unit that corresponds to a system service, type the following at a shell prompt as
root: systemctl stop name.service
Replace name with the name of the service unit you want to stop (for example,
bluetooth). This command stops the selected service unit in the current session. For information on how to disable a service unit and prevent it from being started at boot time, see Section 6.2.7, “Disabling a Service”. For information on how to determine the status of a certain service unit, see Section 6.2.2, “Displaying Service Status”.
Example 6.4. Stopping a Service
The service unit for the
bluetoothd daemon is named bluetooth.service. To deactivate this service unit and stop the bluetoothd daemon in the current session, run the following command as root: ~]# systemctl stop bluetooth.service
6.2.5. Restarting a Service
To restart a service unit that corresponds to a system service, type the following at a shell prompt as
root: systemctl restart name.service
Replace name with the name of the service unit you want to restart (for example,
httpd). This command stops the selected service unit in the current session and immediately starts it again. Importantly, if the selected service unit is not running, this command starts it too. To tell systemd to restart a service unit only if the corresponding service is already running, run the following command as root: systemctl try-restart name.service
Certain system services also allow you to reload their configuration without interrupting their execution. To do so, type as
root: systemctl reload name.service
Note that system services that do not support this feature ignore this command altogether. For convenience, the
systemctl command also supports the reload-or-restart and reload-or-try-restart commands that restart such services instead. For information on how to determine the status of a certain service unit, see Section 6.2.2, “Displaying Service Status”.
Example 6.5. Restarting a Service
In order to prevent users from encountering unnecessary error messages or partially rendered web pages, the Apache HTTP Server allows you to edit and reload its configuration without the need to restart it and interrupt actively processed requests. To do so, type the following at a shell prompt as
root: ~]# systemctl reload httpd.service
6.2.6. Enabling a Service
To configure a service unit that corresponds to a system service to be automatically started at boot time, type the following at a shell prompt as
root: systemctl enable name.service
Replace name with the name of the service unit you want to enable (for example,
httpd). This command reads the [Install] section of the selected service unit and creates appropriate symbolic links to the /usr/lib/systemd/system/name.service file in the /etc/systemd/system/ directory and its subdirectories. This command does not, however, rewrite links that already exist. If you want to ensure that the symbolic links are re-created, use the following command as root: systemctl reenable name.service
This command disables the selected service unit and immediately enables it again. For information on how to determine whether a certain service unit is enabled to start at boot time, see Section 6.2.2, “Displaying Service Status”. For information on how to start a service in the current session, see Section 6.2.3, “Starting a Service”.
Example 6.6. Enabling a Service
To configure the Apache HTTP Server to start automatically at boot time, run the following command as
root: ~]# systemctl enable httpd.service
ln -s '/usr/lib/systemd/system/httpd.service' '/etc/systemd/system/multi-user.target.wants/httpd.service'
6.2.7. Disabling a Service
To prevent a service unit that corresponds to a system service from being automatically started at boot time, type the following at a shell prompt as
root: systemctl disable name.service
Replace name with the name of the service unit you want to disable (for example,
bluetooth). This command reads the [Install] section of the selected service unit and removes appropriate symbolic links to the /usr/lib/systemd/system/name.service file from the /etc/systemd/system/ directory and its subdirectories. In addition, you can mask any service unit to prevent it from being started manually or by another service. To do so, run the following command as root: systemctl mask name.service
This command replaces the
/etc/systemd/system/name.service file with a symbolic link to /dev/null, rendering the actual unit file inaccessible to systemd. To revert this action and unmask a service unit, type as root: systemctl unmask name.service
For information on how to determine whether a certain service unit is enabled to start at boot time, see Section 6.2.2, “Displaying Service Status”. For information on how to stop a service in the current session, see Section 6.2.4, “Stopping a Service”.
Example 6.7. Disabling a Service
Example 6.4, “Stopping a Service” illustrates how to stop the
bluetooth.service unit in the current session. To prevent this service unit from starting at boot time, type the following at a shell prompt as root: ~]# systemctl disable bluetooth.service
rm '/etc/systemd/system/dbus-org.bluez.service'
rm '/etc/systemd/system/bluetooth.target.wants/bluetooth.service'
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