Both Alibaba Cloud and AWS have released CentOS 7 images. When purchasing cloud servers and configuring the system, you now have another option.
However, CentOS 7 has adopted systemd as the default init system. With the adoption of systemd, the systemctl command is used instead of the traditional service command. Therefore, studying and learning about systemd services has become more urgent.
This article uses an Alibaba Cloud server running CentOS 7 as an example to explain systemd service management.
1. Systemd Service Management
To date, systemd defines service startup scripts in the form of Units. Managing services is equivalent to managing Units.
2. List of Active Units (list-units)
The default option for the systemctl command is systemctl list-units. Therefore, even without adding options, the results of list-units will be displayed.
Additionally, if you add the --no-pager option to the systemctl command, the entire result page will be displayed.
# systemctl list-units UNIT LOAD ACTIVE SUB DESCRIPTION proc-sys...t_misc.automount loaded active running Arbitrary Executable File Fo sys-devi...tty-ttyS0.device loaded active plugged /sys/devices/platform/serial sys-devi...tty-ttyS1.device loaded active plugged /sys/devices/platform/serial sys-devi...tty-ttyS2.device loaded active plugged /sys/devices/platform/serial sys-devi...tty-ttyS3.device loaded active plugged /sys/devices/platform/serial sys-devi...vdb-xvdb1.device loaded active plugged /sys/devices/vbd-51728/block sys-devi...lock-xvdb.device loaded active plugged /sys/devices/vbd-51728/block sys-devi...vda-xvda1.device loaded active plugged /sys/devices/vbd-768/block/x sys-devi...lock-xvda.device loaded active plugged /sys/devices/vbd-768/block/x sys-devi...-net-eth0.device loaded active plugged /sys/devices/vif-0/net/eth0 sys-devi...-net-eth1.device loaded active plugged /sys/devices/vif-1/net/eth1 sys-module-configfs.device loaded active plugged /sys/module/configfs （省略） 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. 88 loaded units listed. Pass --all to see loaded but inactive units, too. To show all installed unit files use 'systemctl list-unit-files'.
3. List of Installed Unit Files (list-unit-files)
[root@iZ2516ti6utZ ~]# systemctl list-unit-files UNIT FILE STATE proc-sys-fs-binfmt_misc.automount static dev-hugepages.mount static dev-mqueue.mount static proc-sys-fs-binfmt_misc.mount static sys-fs-fuse-connections.mount static sys-kernel-config.mount static sys-kernel-debug.mount static tmp.mount disabled brandbot.path disabled cups.path disabled （省略） 239 unit files listed.
For example, if you install Apache using yum install httpd, the following Unit file will be recorded.
# systemctl list-unit-files | grep httpd httpd.service disabled
4. Enabling a Unit (enable)
When enabled, it will appear as a startup service when the system boots. systemctl enable httpd is equivalent to chkconfig httpd on.
# systemctl enable httpd ln -s '/usr/lib/systemd/system/httpd.service' '/etc/systemd/system/multi-user.target.wants/httpd.service'
Additionally, multi-user is equivalent to runlevel 2, 3, or 4, with the following functions:
runlevel0 -> poweroff runlevel1 -> rescue runlevel2 -> multi-user runlevel3 -> multi-user runlevel4 -> multi-user runlevel5 -> graphical runlevel6 -> reboot
After enabling, previously disabled services will become enabled.
# systemctl list-unit-files --no-pager | grep httpd httpd.service enabled
5. Disabling a Unit (disable)
After disabling, it will be removed from the list of startup services when the system boots. systemctl disable httpd is equivalent to chkconfig httpd off.
# systemctl disable httpd rm '/etc/systemd/system/multi-user.target.wants/httpd.service'
After disabling, you can verify it with list-unit-files, which will show it as disabled.
# systemctl list-unit-files | grep httpd httpd.service disabled
6. Checking Unit Enable/Disable Status (is-enabled)
If enabled, it returns enabled; if disabled, it returns disabled.
# systemctl is-enabled httpd disabled # systemctl enable httpd ln -s '/usr/lib/systemd/system/httpd.service' '/etc/systemd/system/multi-user.target.wants/httpd.service' # systemctl is-enabled httpd enabled
7. Re-enabling a Unit (reenable)
If re-enabled, it changes from disabled to enabled, and the symbolic link for the startup script is recreated.
# systemctl reenable httpd rm '/etc/systemd/system/multi-user.target.wants/httpd.service' ln -s '/usr/lib/systemd/system/httpd.service' '/etc/systemd/system/multi-user.target.wants/httpd.service'
8. Starting a Unit (start)
With the service command, the startup process is displayed. However, with systemctl, no specific information is shown even after starting.
# systemctl start httpd # ps aux | grep httpd root 28023 0.0 0.4 209696 4784 ? Ss 15:19 0:00 /usr/sbin/httpd -DFOREGROUND apache 28024 0.0 0.2 211780 2964 ? S 15:19 0:00 /usr/sbin/httpd -DFOREGROUND apache 28025 0.0 0.2 211780 2964 ? S 15:19 0:00 /usr/sbin/httpd -DFOREGROUND apache 28026 0.0 0.2 211780 2964 ? S 15:19 0:00 /usr/sbin/httpd -DFOREGROUND apache 28027 0.0 0.2 211780 2964 ? S 15:19 0:00 /usr/sbin/httpd -DFOREGROUND apache 28028 0.0 0.2 211780 2964 ? S 15:19 0:00 /usr/sbin/httpd -DFOREGROUND root 28035 0.0 0.0 112640 964 pts/0 S+ 15:20 0:00 grep --color=auto httpd
9. Checking Unit Startup Status (status)
10. Viewing Detailed Unit Information (show)
To check detailed Unit information, viewing it directly via status is better, but for script processing, show is preferable.
# systemctl show httpd Id=httpd.service Names=httpd.service Requires=basic.target Wants=system.slice Conflicts=shutdown.target Before=shutdown.target After=network.target remote-fs.target nss-lookup.target systemd-journald.socket basic.target system.slice Description=The Apache HTTP Server LoadState=loaded ActiveState=active SubState=running …… ……
11. Stopping a Unit (stop)
Stopping a Unit is similar; no specific information is displayed.
# systemctl stop httpd # ps aux | grep httpd root 28054 0.0 0.0 112640 964 pts/0 S+ 15:29 0:00 grep --color=auto httpd
12. Reloading Unit Configuration Files (reload)
Whether it can be reloaded depends on the Unit.
# systemctl reload httpd
13. Restarting a Unit (restart)
Like start and stop, it does not display information.
# systemctl restart httpd
13-1. Attempting to Restart a Unit (try-restart)
If the Unit is running, it restarts; if stopped, no action is taken.
# systemctl try-restart httpd
14. Reloading or Restarting a Unit (reload-or-restart)
If the Unit can be reloaded, it performs a reload; otherwise, it performs a restart. If the Unit is stopped, it starts it.
# systemctl reload-or-restart httpd
Or if the Unit is stopped:
# systemctl stop httpd
# systemctl reload-or-restart httpd
# systemctl status httpd
httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled)
Active: active (running) since Mon 2015-03-23 15:44:18 CST; 19s ago
Process: 28113 ExecStop=/bin/kill -WINCH ${MAINPID} (code=exited, status=0/SUCCESS)
Process: 28081 ExecReload=/usr/sbin/httpd $OPTIONS -k graceful (code=exited, status=0/SUCCESS)
Main PID: 28121 (httpd)
……
……
14-1. Attempting to Reload or Restart a Unit (reload-or-try-restart)
Similar to reload-or-restart, but it does not start the Unit if it is stopped.
# systemctl reload-or-try-restart httpd # systemctl stop httpd # systemctl reload-or-try-restart httpd Job for httpd.service failed. See 'systemctl status httpd.service' and 'journalc
15. Forcefully Killing a Unit (kill)
# systemctl kill httpd
16. Masking/Unmasking a Unit (mask/unmask)
Even if a Unit is disabled (disable), the service might still start. Masking prevents the service from starting itself.
# systemctl mask httpd ln -s '/dev/null' '/etc/systemd/system/httpd.service' # systemctl start httpd Failed to issue method call: Unit httpd.service is masked.
In the masked state, is-enabled returns masked.
# systemctl is-enabled httpd masked
To unmask, execute unmask.
# systemctl unmask httpd rm '/etc/systemd/system/httpd.service'
17. Checking Unit Startup Status (is-active)
If is-active is used, it returns active if the unit is running. # systemctl start httpd # systemctl is-active httpd active
If it is not started, it returns unknown.
# systemctl stop httpd # systemctl is-active httpd Unknown
18. Checking Unit Failure Status (is-failed)
is-failed returns active if the unit started normally. # systemctl is-failed httpd Active If the Unit cannot start normally and encounters an error, it displays failed. # mv /etc/httpd/conf/httpd.conf /etc/httpd/conf/httpd.conf.org # systemctl restart httpd Job for httpd.service failed. See 'systemctl status httpd.service' and 'journalctl -xn' for details. # systemctl is-failed httpd failed
19. Resetting Unit Failure Status.
Reset the failure status recognized by systemd.
# systemctl is-failed httpd failed # systemctl reset-failed httpd # systemctl is-failed httpd unknown
20. Checking Unit Dependencies (list-dependencies)
A list of Units that this Unit depends on.