systemctl command with all possible
options rhel7 and centos7
systemctl is a command available in
new version of Linux. Systemctl is used to control the systemd and service
manager. To control services we have to use lot many options along with
systemctl command. It is changed dramatically in new version of Linux. In this
article we are going to explore as many possible ways as to use systemctl
command in Linux.
Let’s start with checking the service
status.
Service Status Check and show service units
below is the command used to verify
the service status
[root@Linuxforfreshers ~]# systemctl
status network.service
network.service - LSB: Bring up/down
networking
Loaded: loaded (/etc/rc.d/init.d/network)
Active: active (exited) since Thu 2016-04-28 13:39:38 IST; 32min ago
Process: 5239 ExecReload=/etc/rc.d/init.d/network reload (code=killed,
signal=TERM)
Process: 5313 ExecStart=/etc/rc.d/init.d/network start (code=exited,
status=0/SUCCESS)
[root@linuxforfreshers ~]# systemctl
show crond.service
Id=crond.service
Names=crond.service
Requires=basic.target
Wants=system.slice
Conflicts=shutdown.target
Before=shutdown.target
Verify service is active and
enabled
If service is active it means service
is running without any issues. As a
example we verify web service is running OR not.
[root@Linuxforfreshers ~]# systemctl
is-active httpd.service
active
Enabling service means we are ensuring
that service should start when server is rebooted. In older versions chkconfig
command.
[root@linuxforfreshers ~]# systemctl
enable crond.service
[root@linuxforfreshers ~]# systemctl
is-enabled crond.service
enabled
Start and restart service using
systemctl
As simple as starting the services and
restarting the services is very easy, Here we have little more than that in New
version.
[root@linuxforfreshers ~]# systemctl
start crond.service
[root@linuxforfreshers ~]# systemctl
restart crond.service
as we say it is more than start and
restart we have systemctl try-restart now your thinking about what is the
difference between restart and try-restart..?
option restart will restart the
service if it is in stopped state also.
option try-restart Restart one or more
units specified on the command line if the units are running. This does nothing
if units are not running. Note that, for compatibility with Red Hat init
scripts, condrestart is equivalent to this command.
[root@linuxforfreshers ~]# systemctl
try-restart crond.service
Listing dependencies, jobs,
sockets, unit-files and Units
Listing dependencies means what are
the services we have to start before starting this required service this before
version this feature was not there.
Shows required and wanted units of the
specified unit. If no unit is specified, default.target is implied. Target
units are recursively expanded. When –all is passed, all other units are
recursively expanded as well
[root@linuxforfreshers ~]# systemctl
list-dependencies crond.service
crond.service
├─system.slice
└─basic.target
├─alsa-restore.service
├─alsa-state.service
├─firewalld.service
├─microcode.service
├─rhel-autorelabel-mark.service
├─rhel-autorelabel.service
├─rhel-configure.service
├─rhel-dmesg.service
├─rhel-loadmodules.service
├─paths.target
├─slices.target
list-jobs will show what are jobs
running currently in background
[root@linuxforfreshers ~]# systemctl
list-jobs
No jobs running.
Listing installed unit files
[root@linuxforfreshers ~]# systemctl
list-unit-files |grep sshd
anaconda-sshd.service static
sshd-keygen.service static
sshd.service enabled
sshd@.service static
sshd.socket disabled
List all available sockets
[root@linuxforfreshers ~]# systemctl
list-sockets
LISTEN UNIT ACTIVATES
/dev/initctl systemd-initctl.socket
systemd-initctl.service
/dev/log systemd-journald.socket
systemd-journald.service
/run/dmeventd-client dm-event.socket
dm-event.service
/run/dmeventd-linuxforfreshers
dm-event.socket dm-event.service
/run/lvm/lvmetad.socket
lvm2-lvmetad.socket lvm2-lvmetad.service
/run/systemd/journal/socket
systemd-journald.socket systemd-journald.service
/run/systemd/journal/stdout
systemd-journald.socket systemd-journald.service
/run/systemd/shutdownd
systemd-shutdownd.socket systemd-shutdownd.service
/run/udev/control systemd-udevd-control.socket
systemd-udevd.service
/var/run/avahi-daemon/socket
avahi-daemon.socket avahi-daemon.service
/var/run/cups/cups.sock cups.socket
cups.service
/var/run/dbus/system_bus_socket
dbus.socket dbus.service
/var/run/rpcbind.sock rpcbind.socket
rpcbind.service
@ISCSIADM_ABSTRACT_NAMESPACE
iscsid.socket iscsid.service
@ISCSID_UIP_ABSTRACT_NAMESPACE
iscsiuio.socket iscsiuio.service
kobject-uevent 1
systemd-udevd-kernel.socket systemd-udevd.service
16 sockets listed.
Pass --all to see loaded but inactive
sockets, too.
Note: because the addresses might
contains spaces, this output is not suitable for programmatic consumption.
[root@linuxforfreshers ~]# systemctl
list-sockets --show-types
LISTEN TYPE UNIT ACTIVATES
/dev/initctl FIFO
systemd-initctl.socket systemd-initctl.service
/dev/log Datagram
systemd-journald.socket systemd-journald.service
/run/dmeventd-client FIFO
dm-event.socket dm-event.service
/run/dmeventd-linuxforfreshers FIFO
dm-event.socket dm-event.service
/run/lvm/lvmetad.socket Stream
lvm2-lvmetad.socket lvm2-lvmetad.service
/run/systemd/journal/socket Datagram
systemd-journald.socket systemd-journald.service
/run/systemd/journal/stdout Stream
systemd-journald.socket systemd-journald.service
/run/systemd/shutdownd Datagram
systemd-shutdownd.socket systemd-shutdownd.service
/run/udev/control SequentialPacket
systemd-udevd-control.socket systemd-udevd.service
/var/run/avahi-daemon/socket Stream
avahi-daemon.socket avahi-daemon.service
/var/run/cups/cups.sock Stream
cups.socket cups.service
/var/run/dbus/system_bus_socket Stream
dbus.socket dbus.service
/var/run/rpcbind.sock Stream
rpcbind.socket rpcbind.service
@ISCSIADM_ABSTRACT_NAMESPACE Stream
iscsid.socket iscsid.service
@ISCSID_UIP_ABSTRACT_NAMESPACE Stream
iscsiuio.socket iscsiuio.service
kobject-uevent 1 Netlink
systemd-udevd-kernel.socket systemd-udevd.service
16 sockets listed.
Pass --all to see loaded but inactive
sockets, too.
[root@linuxforfreshers ~]# systemctl
list-sockets --failed
0 sockets listed.
Pass --all to see loaded but inactive
sockets, too.
Setting up default target (Older
version Run Level) and getting default target
We have to use set-default to set
default run level and we can see default run level using get-default option as
shown below example
[root@linuxforfreshers ~]# systemctl
set-default multi-user.target
rm
'/etc/systemd/system/default.target'
ln -s
'/usr/lib/systemd/system/multi-user.target'
'/etc/systemd/system/default.target'
[root@linuxforfreshers ~]# systemctl
get-default
multi-user.target
Masking service ans unmasking
service
What is mean by masking service, there
is situation that company will have multiple administrators working together
still there are times one administrator will stop the service which is not
required but another administrator will start the same service unfortunately Or
unknowingly , will lead to lot of problems to avoid this types of situations.
We have to disable, stop the service and mask it, when other administrator try
to start also the service will never start until unless explicitly unmask.
[root@linuxforfreshers ~]# systemctl
disable crond.service
rm
'/etc/systemd/system/multi-user.target.wants/crond.service'
[root@linuxforfreshers ~]# systemctl
stop crond.service
[root@linuxforfreshers ~]# systemctl
mask crond.service
ln -s '/dev/null' '/etc/systemd/system/crond.service'
[root@linuxforfreshers ~]# systemctl
status crond.service
crond.service
Loaded: masked (/dev/null)
Active: inactive (dead)
As shown in above command examples we
have stopped the service, disabled the service and masked the service. Now try
to start the service.
[root@linuxforfreshers ~]# systemctl
start crond.service
Failed to issue method call: Unit
crond.service is masked.
now unmask the service and start it
will start
[root@linuxforfreshers ~]# systemctl
unmask crond.service
rm '/etc/systemd/system/crond.service'
[root@linuxforfreshers ~]# systemctl
start crond.service
[root@linuxforfreshers ~]# systemctl
status crond.service
crond.service - Command Scheduler
Loaded: loaded
(/usr/lib/systemd/system/crond.service; disabled)
Active: active (running) since Thu
2016-04-28 22:45:12 IST; 9s ago
Main PID: 7521 (crond)
CGroup: /system.slice/crond.service
└─7521 /usr/sbin/crond -n
Reload and reset service status
Most of the administrator will still
have an question that what is the difference between service reload and
restart.
Service reload is used whenever we
changed something to the service and we would like to push the changes to the
service without interrupting the connected users. Reloading the service will
never change existing PID (Process Identity)
Service restart is used to restart the
service which means stop and start the service, whenever we run restart
existing users will disconnect and new PID will be created. It required little
downtime to the service changes.
Before reloading the service status
[root@linuxforfreshers ~]# systemctl
status sshd.service
sshd.service - OpenSSH server daemon
Loaded: loaded
(/usr/lib/systemd/system/sshd.service; enabled)
Active: active (running) since Thu
2016-04-28 05:56:52 IST; 16h ago
Process: 7228 ExecReload=/bin/kill
-HUP $MAINPID (code=exited, status=0/SUCCESS)
Main PID: 1601 (sshd)
CGroup: /system.slice/sshd.service
└─1601 /usr/sbin/sshd -D
After reloading the service
[root@linuxforfreshers ~]# systemctl
reload sshd.service
[root@linuxforfreshers ~]# systemctl status
sshd.service
sshd.service - OpenSSH server daemon
Loaded: loaded
(/usr/lib/systemd/system/sshd.service; enabled)
Active: active (running) since Thu
2016-04-28 05:56:52 IST; 16h ago
Process: 7243 ExecReload=/bin/kill
-HUP $MAINPID (code=exited, status=0/SUCCESS)
Main PID: 1601 (sshd)
CGroup: /system.slice/sshd.service
└─1601 /usr/sbin/sshd -D
if observe correctly PID before and
after reload not changed.
Reset the “failed” state of the
specified units
[root@linuxforfreshers ~]# systemctl
reset-failed sshd.service
Daemon reload option
After deleting the file or directory,
you should reload the systemd process so that it no longer attempts to
reference these files and reverts back to using the system copies. You can do
this by typing:
[root@linuxforfreshers ~]# systemctl
daemon-reload
Isolating Targets using systemctl
command
It is possible to start all of the
units associated with a target and stop all units that are not part of the
dependency tree. The command that we need to do this is called, appropriately,
isolate. This is similar to changing the runlevel in other init systems.
For instance, if you are operating in
a graphical environment with graphical.target active, you can shut down the
graphical system and put the system into a multi-user command line state by
isolating the multi-user.target. Since graphical.target depends on
multi-user.target but not the other way around, all of the graphical units will
be stopped.
You may wish to take a look at the
dependencies of the target you are isolating before performing this procedure
to ensure that you are not stopping vital services:
systemctl list-dependencies
multi-user.target
When you are satisfied with the units
that will be kept alive, you can isolate the target by typing:
systemctl isolate multi-user.target
Create service snapshot and delete
Create a snapshot. If a snapshot name
is specified, the new snapshot will be named after it. If none is specified, an
automatic snapshot name is generated. In either case, the snapshot name used is
printed to STDOUT, unless –quiet is specified.
A snapshot refers to a saved state of
the systemd manager. It is implemented itself as a unit that is generated
dynamically with this command and has dependencies on all units active at the
time. At a later time, the user may return to this state by using the isolate
command on the snapshot unit.
Snapshots are only useful for saving
and restoring which units are running or are stopped, they do not save/restore
any other state. Snapshots are dynamic and lost on reboot.
[root@linuxforfreshers ~]# systemctl
snapshot sshd.service
sshd.service.snapshot
[root@linuxforfreshers ~]# systemctl
status sshd.service.snapshot
sshd.service.snapshot
Loaded: loaded
Active: inactive (dead)
[root@linuxforfreshers ~]# systemctl
delete sshd.service.snapshot
[root@linuxforfreshers ~]# systemctl
status sshd.service.snapshot
sshd.service.snapshot
Loaded: not-found (Reason: No such
file or directory)
Active: inactive (dead)
