Introduction to systemd
System startup and server processes are managed by the systemd system and Service Manager.This program provide a mentod for activating system resources, Server Daemons and other processes, both on boot time and on running system.
Daemons are process that wait or run in the background performing various task.
To listen for connection Daemon uses a socket.
A Service often refers to one or more daemons,but starting or stopping a service may instead make a on-time change to the state of the system for example, to configure Network interfaces, which does not involve leaving a daemon process running afterward.
In RHEL 7 , Process ID 1 is systemd, the new init system.
In RHEL 6 and older system , Process ID 1 is init process
Few new feature of systemd
systemctl and systemd units
Daemons are process that wait or run in the background performing various task.
To listen for connection Daemon uses a socket.
A Service often refers to one or more daemons,but starting or stopping a service may instead make a on-time change to the state of the system for example, to configure Network interfaces, which does not involve leaving a daemon process running afterward.
In RHEL 7 , Process ID 1 is systemd, the new init system.
In RHEL 6 and older system , Process ID 1 is init process
Few new feature of systemd
- Paralleization capabilities, which increase the boot speed of a system
- On-Demand starting of Daemons without requiring a separate service.
- Automatic service dependency management prevents long timeouts, such as not starting a network service when the network is not active.
- A method of tracking related processes together using Linux Control groups.
systemctl and systemd units
The systemctl command is used to manage different type of systemd objects, called units. A list of available unit type can be displayed with systemctl -t help .
[root@server1 ~]# systemctl -t help Available unit types: service socket busname target snapshot device mount automount swap timer path slice scope
Some of the common unit type are listed as follows.
service unit have a .service extension and represent system services. This type of unit is used to start frequently accessed daemons, such as a web server
socket unit have a .socket extension and represent Inter process communication sockets. Control of the socket will be passed to a daemon or newly started service when a client connection is made.
Path unit have .path extension and are used to delay the activation of a service unitl a specific file system change occurs.This commonly used for service which use spool directories, such as a printing systems.
Checking Service status
List Unit files with systemctl
Query the state of all units to verify a system startup.
Both the command give the same output.
The --all option will add inactive units as well.
List only failed services
Viewing the enabled and disabled setting for all units. Optionally, Limit the type of unit.
service unit have a .service extension and represent system services. This type of unit is used to start frequently accessed daemons, such as a web server
socket unit have a .socket extension and represent Inter process communication sockets. Control of the socket will be passed to a daemon or newly started service when a client connection is made.
Path unit have .path extension and are used to delay the activation of a service unitl a specific file system change occurs.This commonly used for service which use spool directories, such as a printing systems.
Command
|
Task
|
systemctl status UNIT
|
View detailed information about a unit state
|
systemctl stop UNIT
|
Stop a service ona running system
|
systemctl start UNIT
|
Start a service on a running system
|
systemctl restart UNIT
|
Restart a service on a running system
|
systemctl reload UNIT
|
Reload configuration file of a running service.
|
systemctl mask UNIT
|
Completely disable a service from being started, both manually and at
boot.
|
systemctl unmask UNIT
|
Make a masked service available
|
systemctl enable UNIT
|
Configure a service to start at boot time
|
systemctl disable UNIT
|
Disable a service from starting at boot time
|
systemctl list-dependencies
UNIT
|
List units which are required and wanted by the specified unit.
|
systemctl is-active UNIT
|
Check whether the unit is in active state
|
systemctl is-enabled UNIT
|
Check whether the unit is enabled to
start automatically at boot time
|
Checking Service status
[root@server1 ~]# systemctl status sshd.service
● sshd.service - OpenSSH server daemon
Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled; vendor preset: enabled)
Active: active (running) since Mon 2019-12-16 16:17:07 IST; 4h 7min left
Docs: man:sshd(8)
man:sshd_config(5)
Process: 1164 ExecStart=/usr/sbin/sshd $OPTIONS (code=exited, status=0/SUCCESS)
Main PID: 1303 (sshd)
CGroup: /system.slice/sshd.service
└─1303 /usr/sbin/sshd
Dec 16 16:17:06 localhost.localdomain systemd[1]: Starting OpenSSH server daemon...
Dec 16 16:17:07 localhost.localdomain systemd[1]: PID file /var/run/sshd.pid not readable (yet?) after start.
Dec 16 16:17:07 localhost.localdomain sshd[1303]: Server listening on 0.0.0.0 port 22.
Dec 16 16:17:07 localhost.localdomain sshd[1303]: Server listening on :: port 22.
Dec 16 16:17:07 localhost.localdomain systemd[1]: Started OpenSSH server daemon.
Keyword:
|
Description:
|
loaded
|
Unit configuration file has been processed
|
active(running)
|
Running with one or more continuing processes
|
active(exited)
|
Sucessfully completed one-time configuration
|
active(waiting)
|
Running but waiting for a event
|
inactive
|
Not running
|
enabled
|
Will be started at boot time
|
disabled
|
Will not be started at boot time
|
static
|
Cannot be enabled, but may be started by an enabled unit.
|
List Unit files with systemctl
Query the state of all units to verify a system startup.
Both the command give the same output.
[root@Server1 ~]# systemctl list-units
[root@Server1 ~]# systemctl
UNIT LOAD ACTIVE SUB DESCRIPTION
auditd.service loaded active running Security Auditing Service
NetworkManager.service loaded active running Network Manager
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.
Query the status of only the service units
[root@Server1 ~]# systemctl list-units --type=service
[root@Server1 ~]# systemctl --type=service
The --all option will add inactive units as well.
[root@Server1 ~]# systemctl list-units -t service -all
[root@Server1 ~]# systemctl -t service -all
UNIT LOAD ACTIVE SUB DESCRIPTION
auditd.service loaded active running Security Auditing Service
auth-rpcgss-module.service loaded inactive dead Kernel Module supporting RPCSEC_GSS
List only failed services
[root@Server1 ~]# systemctl --failed --type=service 0 loaded units listed. Pass --all to see loaded but inactive units, too.
Viewing the enabled and disabled setting for all units. Optionally, Limit the type of unit.
[root@Server1 ~]# systemctl list-unit-files --type=service UNIT FILE STATE arp-ethers.service disabled auditd.service enabled auth-rpcgss-module.service static
We can also check whether the particular service enabled to start after reboot using is-enabled option
[root@Server1 ~]# systemctl is-enabled sshd
enabled
[root@Server1 ~]# systemctl is-active sshd active
Unit Dependencies
Service may be started as dependencies of other services. If a socket unit is enabled and the service unit with the same name is not, the service will automatically started when a request is made to that socket unit.
[root@Server1 ~]# systemctl list-dependencies multi-user.target multi-user.target ● ├─auditd.service ● ├─besclient.service ● ├─brandbot.path ● ├─choose_repo.service
--reverse show particular unit is required for which other units.
[root@Server1 ~]# systemctl list-dependencies multi-user.target --reverse multi-user.target ● └─graphical.target
Masking services
A system may have conflicting services installed for a certain function, such as firewalls (iptables and firewalld). To prevent an administrator from accidentally starting a service, a service may be masked.
[root@Server1 ~]# systemctl mask iptables ln -s '/dev/null' '/etc/systemd/system/iptables.service' [root@Server1 ~]# systemctl unmask iptables rm '/etc/systemd/system/iptables.service'
Enabling system daemons to start or stop at boot
Service are started at boot when links are created in the appropriate systemd configuration directories.
[root@server1 ~]# systemctl enable sshd Created symlink from /etc/systemd/system/multi-user.target.wants/sshd.service to /usr/lib/systemd/system/sshd.service.
Same way disable can be used to disable the service from starting during the boot.
[root@server1 ~]# systemctl disable sshd Removed symlink /etc/systemd/system/multi-user.target.wants/sshd.service.
