How To Use Systemctl On CentOS 7.x or RHEL Linux 7

Centos 7LinuxRed Hat

In this article we will learn how to use ‘systemctl’ command, ‘systemctl’ is a new command which is available in the new version of Linux distributions which is used to control the system and services, we will explore some possible ways to use the ‘systemctl’ command in Linux.

Checking Services Status

We can verify the status of the services using this command. Below is the command to check the service.

# systemctl status httpd
httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
Active: active (running) since Thu 2016-06-02 06:48:58 EDT; 27s ago
   Docs: man:httpd(8)
   man:apachectl(8)
Main PID: 10011 (httpd)
Status: "Total requests: 0; Current requests/sec: 0; Current traffic: 0 B/sec"
CGroup: /system.slice/httpd.service
      ├─10011 /usr/sbin/httpd -DFOREGROUND
      ├─10012 /usr/sbin/httpd -DFOREGROUND
      ├─10013 /usr/sbin/httpd -DFOREGROUND
      ├─10014 /usr/sbin/httpd -DFOREGROUND
      ├─10015 /usr/sbin/httpd -DFOREGROUND
      └─10016 /usr/sbin/httpd -DFOREGROUND
Jun 02 06:48:57 centos-linux-1.shared systemd[1]: Starting The Apache HTTP Server...
Jun 02 06:48:58 centos-linux-1.shared systemd[1]: Started The Apache HTTP Server.

To Show the Services

# systemctl show httpd
Type=notify
Restart=no
NotifyAccess=main
RestartUSec=100ms
TimeoutStartUSec=1min 30s
TimeoutStopUSec=1min 30s
WatchdogUSec=0
WatchdogTimestamp=Thu 2016-06-02 06:48:58 EDT
WatchdogTimestampMonotonic=2588490928
StartLimitInterval=10000000StartLimitBurst=5
StartLimitAction=none
….
.
.
.
.
AssertResult=yes
ConditionTimestamp=Thu 2016-06-02 06:48:57 EDT
ConditionTimestampMonotonic=2587709137
AssertTimestamp=Thu 2016-06-02 06:48:57 ED
AssertTimestampMonotonic=2587709138
Transient=no
This will display all the status information about http (web servers services)
Verifying the Services active or enabled
Here we will verify that the servers are active and they are enabled at system startup
To verify the services active or not we needed to run the below command
# systemctl is-active httpd.service
active

To enable the service to run at the system start-up or after the machine is Rebooted

# systemctl enable httpd
Created symlink from /etc/systemd/system/multi-use.rtarget.wants/httpd.service to /usr/lib/systemd/system/httpd.service.

To Check whether the services are enabled or not, run the below command

# systemctl is-enabled httpd
enabled

To start the services manually or restart the services manually, we can use the below commands

# systemctl start httpd
# systemctl restart httpd

To list out the dependencies we can use the below command

# systemctl list-dependencies httpd
httpd.service
├─-.mount
├─system.slice
└─basic.target
├─microcode.service
├─rhel-autorelabel-mark.service
├─rhel-autorelabel. service
├─rhel-configure.service
├─rhel-dmesg.service
├─rhel-loadmodules.service
├─paths.target
├─slices.target
│ ├─-.slice
│ └─system.slice…
…
.
..

To list all the jobs currently running in the background

# systemctl list-jobs
No jobs running.

To list all the installed unit files

# systemctl list-unit files

To List all the installed Unit Files for a particular Service

# systemctl list-unit-files | grep httpd
httpd.service enabled

To List all the Installed Sockets and these Types

# systemctl list-sockets --show-types
LISTEN                          TYPE                          UNIT                                      ACTIVATES
/dev/log                         Datagram                     systemd-journald.socket                  systemd-j
/run/dmeventd-client             FIFO                         dm-event.socket                          dm-event.
/run/dmeventd-server             FIFO                         dm-event.socket                          dm-event.
/run/lvm/lvmetad.socket          Stream                       lvm2-lvmetad.socket                      lvm2-lvme
/run/lvm/lvmpolld.socket         Stream                       lvm2-lvmpolld.socket                     lvm2-lvmp
/run/systemd/initctl/fifo        FIFO                         systemd-initctl.socket                   systemd-i
/run/systemd/journal/socket      Datagram                     systemd-journald.socket                  systemd-j
/run/systemd/journal/stdout      Stream                       systemd-journald.socket                  systemd-j
/run/systemd/shutdownd           Datagram                     systemd-shutdownd.socket                 systemd-s
/run/udev/control                SequentialPacket             systemd-udevd-control.socket             systemd-u
/var/run/dbus/system_bus_socket  Stream                       dbus.socket                              dbus.serv
kobject-uevent 1                 Netlink                      systemd-udevd-kernel.socket              systemd-u
12 sockets listed.
Pass --all to see loaded but inactive sockets, too.

To Setup/list the Default Target (mean Run Level)

# systemctl set-default multi-user.target
Removed symlink /etc/systemd/system/default.target.
Created symlink from /etc/systemd/system/default.target to /usr/lib/systemd/system/multi-user.target.
# systemctl get-default
multi-user.target

Masking & Unmasking the Services

In some situations, especially in big companies – one of the administrators had stoped the servers and other will start the services, to avoid this type of situations we can mask and unmask the services, if some services are stoped without unmasking we cannot start the services again

# systemctl mask httpd
Created symlink from /etc/systemd/system/httpd.service to /dev/null.

To see the Status of the Services after Masking

# httpd.service
Loaded: masked (/dev/null)
Active: active (running) since Thu 2016-06-02 07:03:27 EDT; 10min ago
Main PID: 10128 (httpd)
Status: "Total requests: 0; Current requests/sec: 0; Current traffic: 0 B/sec"
CGroup: /system.slice/httpd.service
├─10128 /usr/sbin/httpd -DFOREGROUND
├─10129 /usr/sbin/httpd -DFOREGROUND
├─10130 /usr/sbin/httpd -DFOREGROUND
├─10131 /usr/sbin/httpd -DFOREGROUND
├─10132 /usr/sbin/httpd -DFOREGROUND
└─10133 /usr/sbin/httpd -DFOREGROUND
Jun 02 07:03:27 centos-linux-1.shared systemd[1]: Starting The Apache HTTP Server...
Jun 02 07:03:27 centos-linux-1.shared systemd[1]: Started The Apache HTTP Server.
Jun 02 07:13:47 centos-linux-1.shared systemd[1]: httpd.service: Got notification m....
Jun 02 07:13:57 centos-linux-1.shared systemd[1]: httpd.service: Got notification m....
Jun 02 07:14:07 centos-linux-1.shared systemd[1]: httpd.service: Got notification m....
Jun 02 07:14:17 centos-linux-1.shared systemd[1]: httpd.service: Got notification m....
Hint: Some lines were ellipsized, use -l to show in full.

If we try to Start the Masked Services it will display Errors

# systemctl start httpd
Failed to start httpd.service: Unit httpd.service is masked.

To unmask the services to Start, we need to run the below command

# systemctl unmask httpd
Removed symlink /etc/systemd/system/httpd.service.

After unmasking of the service, we can start the services again back

# systemctl start httpd
# systemctl status httpd
httpd.service - The Apache HTTP Server
Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
Active: active (running) since Thu 2016-06-02 07:03:27 EDT; 18min ago
Docs: man:httpd(8)
man:apachectl(8)
Main PID: 10128 (httpd)
Status: "Total requests: 0; Current requests/sec: 0; Current traffic: 0 B/sec"
CGroup: /system.slice/httpd.service
├─10128 /usr/sbin/httpd -DFOREGROUND
├─10129 /usr/sbin/httpd -DFOREGROUND
├─10130 /usr/sbin/httpd -DFOREGROUND
├─10131 /usr/sbin/httpd -DFOREGROUND
├─10132 /usr/sbin/httpd -DFOREGROUND
└─10133 /usr/sbin/httpd -DFOREGROUND

We can create a snapshot of the services on the Linux Machine

A snapshot refers to a saved status of the system manager. The Snapshot is useful only to save an restore the units running or stopped.

To take a snapshot of the service we can use

# systemctl snapshot httpd
ht.tpdsnapshot

To see the status of the service Snapshot

# systemctl status httpd.snapshot
httpd.snapshot
Loaded: loaded
Active: inactive (dead)
Jun 02 07:26:31 centos-linux-1.shared systemd[1]: Created snapshot httpd.snapshot.

To delete the Snapshot and Check the status of the service snapshot

# systemctl delete httpd.snapshot
# systemctl status httpd.snapshot
httpd.snapshot
Loaded: not-found (Reason: No such file or directory)
Active: inactive (dead)
Jun 02 07:26:31 centos-linux-1.shared systemd[1]: Created snapshot httpd.snapshot.
Jun 02 07:29:05 centos-linux-1.shared systemd[1]: Removing snapshot httpd.snapshot.

After this article we can now able to control the services in the new or the latest version of Linux, where the ‘systemctl’ is the replacement of service command in the older versions of Linux. Hope this will help in doing your jobs.

raja
Published on 20-Jan-2020 12:56:10
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