Overview
By the end of this article you should be able to answer the following questions:
Announcement
I have released my new course on Udemy, Kubernetes By Example. Sign up now to get free lifetime access!$ rpm -ql httpd | grep “.service$”
$ systemctl –-help
$ systemctl status httpd.service
$ systemctl start httpd.service
$ systemctl enable httpd.service
What is a service
A service is one of the dozen types of unit that systemd manages.
In Linux, a service is basically an application that is continuously running in the background waiting to be used. A common example of a service, is a web server, for example the “httpd” (aka Apache HTTP web server):
A service is represented by a *.service file, so if you want to check what services that this package has installed, you can do:
$ rpm -ql httpd | grep ".service$" /usr/lib/systemd/system/htcacheclean.service /usr/lib/systemd/system/httpd.service
As you can see, this application comes with 2 services, of which httpd.service is the main service. We’ll cover more about *.service files later.
To manage a service, e.g. start/stop a service, we use the “systemctl” command:
$ systemctl --help
systemctl [OPTIONS...] {COMMAND} ...
Query or send control commands to the systemd manager.
-h --help Show this help
--version Show package version
-t --type=TYPE List only units of a particular type
--state=STATE List only units with particular LOAD or SUB or ACTIVE state
-p --property=NAME Show only properties by this name
-a --all Show all loaded units/properties, including dead/empty
ones. To list all units installed on the system, use
the 'list-unit-files' command instead.
--reverse Show reverse dependencies with 'list-dependencies'
-l --full Don't ellipsize unit names on output
--fail When queueing a new job, fail if conflicting jobs are
pending
--irreversible When queueing a new job, make sure it cannot be implicitly
cancelled
--ignore-dependencies
When queueing a new job, ignore all its dependencies
--show-types When showing sockets, explicitly show their type
-i --ignore-inhibitors
When shutting down or sleeping, ignore inhibitors
--kill-who=WHO Who to send signal to
-s --signal=SIGNAL Which signal to send
-H --host=[USER@]HOST
Show information for remote host
-P --privileged Acquire privileges before execution
-q --quiet Suppress output
--no-block Do not wait until operation finished
--no-wall Don't send wall message before halt/power-off/reboot
--no-reload When enabling/disabling unit files, don't reload daemon
configuration
--no-legend Do not print a legend (column headers and hints)
--no-pager Do not pipe output into a pager
--no-ask-password
Do not ask for system passwords
--system Connect to system manager
--user Connect to user service manager
--global Enable/disable unit files globally
--runtime Enable unit files only temporarily until next reboot
-f --force When enabling unit files, override existing symlinks
When shutting down, execute action immediately
--root=PATH Enable unit files in the specified root directory
-n --lines=INTEGER Number of journal entries to show
-o --output=STRING Change journal output mode (short, short-monotonic,
verbose, export, json, json-pretty, json-sse, cat)
--plain Print unit dependencies as a list instead of a tree
Unit Commands:
list-units List loaded units
list-sockets List loaded sockets ordered by address
start [NAME...] Start (activate) one or more units
stop [NAME...] Stop (deactivate) one or more units
reload [NAME...] Reload one or more units
restart [NAME...] Start or restart one or more units
try-restart [NAME...] Restart one or more units if active
reload-or-restart [NAME...] Reload one or more units if possible,
otherwise start or restart
reload-or-try-restart [NAME...] Reload one or more units if possible,
otherwise restart if active
isolate [NAME] Start one unit and stop all others
kill [NAME...] Send signal to processes of a unit
is-active [NAME...] Check whether units are active
is-failed [NAME...] Check whether units are failed
status [NAME...|PID...] Show runtime status of one or more units
show [NAME...|JOB...] Show properties of one or more
units/jobs or the manager
set-property [NAME] [ASSIGNMENT...]
Sets one or more properties of a unit
help [NAME...|PID...] Show manual for one or more units
reset-failed [NAME...] Reset failed state for all, one, or more
units
list-dependencies [NAME] Recursively show units which are required
or wanted by this unit or by which this
unit is required or wanted
Unit File Commands:
list-unit-files List installed unit files
enable [NAME...] Enable one or more unit files
disable [NAME...] Disable one or more unit files
reenable [NAME...] Reenable one or more unit files
preset [NAME...] Enable/disable one or more unit files
based on preset configuration
is-enabled [NAME...] Check whether unit files are enabled
mask [NAME...] Mask one or more units
unmask [NAME...] Unmask one or more units
link [PATH...] Link one or more units files into
the search path
get-default Get the name of the default target
set-default NAME Set the default target
Job Commands:
list-jobs List jobs
cancel [JOB...] Cancel all, one, or more jobs
Snapshot Commands:
snapshot [NAME] Create a snapshot
delete [NAME...] Remove one or more snapshots
Environment Commands:
show-environment Dump environment
set-environment [NAME=VALUE...] Set one or more environment variables
unset-environment [NAME...] Unset one or more environment variables
Manager Lifecycle Commands:
daemon-reload Reload systemd manager configuration
daemon-reexec Reexecute systemd manager
System Commands:
default Enter system default mode
rescue Enter system rescue mode
emergency Enter system emergency mode
halt Shut down and halt the system
poweroff Shut down and power-off the system
reboot Shut down and reboot the system
kexec Shut down and reboot the system with kexec
exit Request user instance exit
switch-root [ROOT] [INIT] Change to a different root file system
suspend Suspend the system
hibernate Hibernate the system
hybrid-sleep Hibernate and suspend the system
I have highlighted some of systemctl’s most commonly used sub-commands above.
systemctl doesn’t just manage service. It can manage all kinds of other things that can have a state (e.g). These things are referred to as “units”.
Here’s a list of the different types of units that systemctl can manage:
$ systemctl --type=help Available unit types: service socket target device mount automount snapshot timer swap path slice scope
Each unit is represented in the form of a file. All unit files are stored in /usr/lib/systemd/system.
Starting/Stopping/Enabling Services
To check the status of a service we do:
$ systemctl status httpd httpd.service - The Apache HTTP Server Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled) Active: inactive (dead) Jun 06 10:03:18 centos7.codingbee.dyndns.org systemd[1]: Starting The Apache HTTP Server... Jun 06 10:03:18 centos7.codingbee.dyndns.org systemd[1]: Started The Apache HTTP Server. Jun 06 10:44:49 centos7.codingbee.dyndns.org systemd[1]: Stopping The Apache HTTP Server... Jun 06 10:44:51 centos7.codingbee.dyndns.org systemd[1]: Stopped The Apache HTTP Server. Jun 06 10:44:57 centos7.codingbee.dyndns.org systemd[1]: Starting The Apache HTTP Server... Jun 06 10:44:57 centos7.codingbee.dyndns.org systemd[1]: Started The Apache HTTP Server. Jun 06 10:44:59 centos7.codingbee.dyndns.org systemd[1]: Stopping The Apache HTTP Server... Jun 06 10:45:00 centos7.codingbee.dyndns.org systemd[1]: Stopped The Apache HTTP Server.
This shows that the httpd service is not currently running (i.e. inactive) and it wont start up by default when you’re booting up the machine (i.e. it is disabled). This command also provides a short log history.
So to start a service, we do:
$ systemctl start httpd $
This doesn’t give a success output, so let’s check the status again to see if this has been successful:
$ systemctl status httpd
httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled)
Active: active (running) since Sat 2015-06-06 11:08:52 BST; 47s ago
Main PID: 4336 (httpd)
Status: "Total requests: 0; Current requests/sec: 0; Current traffic: 0 B/sec"
CGroup: /system.slice/httpd.service
├─4336 /usr/sbin/httpd -DFOREGROUND
├─4337 /usr/sbin/httpd -DFOREGROUND
├─4338 /usr/sbin/httpd -DFOREGROUND
├─4339 /usr/sbin/httpd -DFOREGROUND
├─4340 /usr/sbin/httpd -DFOREGROUND
└─4341 /usr/sbin/httpd -DFOREGROUND
Jun 06 11:08:52 centos7.codingbee.dyndns.org systemd[1]: Starting The Apache HTTP Server...
Jun 06 11:08:52 centos7.codingbee.dyndns.org systemd[1]: Started The Apache HTTP Server.
Here we can see that the service is now running now. However the service is still disabled. This means that this service won’t be automatically started when you reboot the machine. You can therefore enable this service:
$ systemctl enable httpd ln -s '/usr/lib/systemd/system/httpd.service' '/etc/systemd/system/multi-user.target.wants/httpd.service'
Note: This essentially creates a soft link in the “multi-user.target.wants” folder. We’ll cover more about this later.
Let’s now check the status again:
$ systemctl status httpd
httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled)
Active: active (running) since Sat 2015-06-06 11:08:52 BST; 5min ago
Main PID: 4336 (httpd)
Status: "Total requests: 0; Current requests/sec: 0; Current traffic: 0 B/sec"
CGroup: /system.slice/httpd.service
├─4336 /usr/sbin/httpd -DFOREGROUND
├─4337 /usr/sbin/httpd -DFOREGROUND
├─4338 /usr/sbin/httpd -DFOREGROUND
├─4339 /usr/sbin/httpd -DFOREGROUND
├─4340 /usr/sbin/httpd -DFOREGROUND
└─4341 /usr/sbin/httpd -DFOREGROUND
Jun 06 11:08:52 centos7.codingbee.dyndns.org systemd[1]: Starting The Apache HTTP Server...
Jun 06 11:08:52 centos7.codingbee.dyndns.org systemd[1]: Started The Apache HTTP Server.
Now it is enabled. If we now stop the service and then reboot the machine, by default this service will now automatically start up again during the boot process.
Viewing services
To view all currently running services, we do:
$ 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-xorg.service loaded active running ABRT Xorg log watcher abrtd.service loaded active running ABRT Automated Bug Reporting Tool accounts-daemon.service loaded active running Accounts Service alsa-state.service loaded active running Manage Sound Card State (restore and store) atd.service loaded active running Job spooling tools | | |..etc
To view all services irrespective of whether they are running or not, we do:
$ systemctl list-unit-files --type=service UNIT FILE STATE abrt-ccpp.service enabled abrt-oops.service enabled abrt-pstoreoops.service disabled abrt-vmcore.service enabled abrt-xorg.service enabled abrtd.service enabled accounts-daemon.service enabled alsa-restore.service static alsa-state.service static alsa-store.service static . . ...etc
This is similar to the old chkconfig command, since it shows which services are currently enabled so that they autostart at boot time. However unlike chkconfig, it only shows the info for the current target (which is a bit like a runlevel).
Masking services
If you want to have a service enabled+started but you want to temporarily disable it for troubleshooting purposes, then you can mask the service:
$ systemctl mask httpd Created symlink from /etc/systemd/system/httpd.service to /dev/null.
By masking it, it means that you are prevented from running the commands:
[root@target ~]# systemctl stop httpd [root@target ~]# systemctl status httpd ● httpd.service Loaded: masked (/dev/null; bad) Active: inactive (dead) since Fri 2018-03-02 16:32:25 UTC; 1s ago Main PID: 20219 (code=exited, status=0/SUCCESS) Status: "Total requests: 0; Current requests/sec: 0; Current traffic: 0 B/sec" $ systemctl start httpd Failed to start httpd.service: Unit is masked. $ systemctl disable httpd $ systemctl enable httpd Failed to execute operation: Cannot send after transport endpoint shutdown
Notice you can still stop+disable a masked service. But you prevented from taking any action that could let the deamon starts up.
Also notice that if a service is running at the time of masking then it will continue to run until it is stopped.
To reverse the masking you do:
$ systemctl unmask httpd