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systemd management using systemctl and journalctl to check systemd logs

systemd is like the service manager for your Centos and other modern Linux distributions (including Debian/Mint/Ubuntu) allows you to enable services, stop them, restart them, check their status and even reboot your system.

The key commands or arguments you will use with systemctl are the following:

Unit Commands:
  list-units [PATTERN...]         List loaded units
  list-sockets [PATTERN...]       List loaded sockets ordered by address
  list-timers [PATTERN...]        List loaded timers ordered by next elapse
  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 PATTERN...            Check whether units are active
  is-failed PATTERN...            Check whether units are failed
  status [PATTERN...|PID...]      Show runtime status of one or more units
  show [PATTERN...|JOB...]        Show properties of one or more
                                  units/jobs or the manager
  cat PATTERN...                  Show files and drop-ins of one or more units
  set-property NAME ASSIGNMENT... Sets one or more properties of a unit
  help PATTERN...|PID...          Show manual for one or more units
  reset-failed [PATTERN...]       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

 

check the state of all running services

systemctl list-units --type service --all --state failed

systemctl list-units --type service --all --state running

systemctl list-units --type service --all --state active



How Do You Enable or Disable Services

systemctl enable sshd

systemctl disable crond

Note that disabling does NOT stop them if they are currently running.

 systemctl disable crond
Removed symlink /etc/systemd/system/multi-user.target.wants/crond.service.
[root@bcitcourse ~]# systemctl status crond
● crond.service - Command Scheduler
   Loaded: loaded (/usr/lib/systemd/system/crond.service; disabled; vendor preset: enabled)
   Active: active (running) since Sat 2018-11-03 19:12:14 UTC; 2 days ago
 Main PID: 139 (crond)
   CGroup: /system.slice/crond.service
           └─139 /usr/sbin/crond -n

Sep 18 07:19:54 bcitcourse systemd[1]: Starting Command Scheduler...
Sep 18 07:19:54 bcitcourse crond[31554]: (CRON) INFO (RANDOM_DELAY will be scaled with factor 34% if used.)
Sep 18 07:19:54 bcitcourse crond[31554]: (CRON) INFO (running with inotify support)
Sep 18 07:19:54 bcitcourse crond[31554]: (CRON) INFO (@reboot jobs will be run at computer's startup.)
Sep 18 07:23:54 bcitcourse systemd[1]: Stopping Command Scheduler...
Sep 18 07:23:54 bcitcourse systemd[1]: Started Command Scheduler.
Sep 18 07:23:54 bcitcourse systemd[1]: Starting Command Scheduler...
Sep 18 07:23:54 bcitcourse crond[32128]: (CRON) INFO (RANDOM_DELAY will be scaled with factor 44% if used.)
Sep 18 07:23:54 bcitcourse crond[32128]: (CRON) INFO (running with inotify support)
Sep 18 07:23:54 bcitcourse crond[32128]: (CRON) INFO (@reboot jobs will be run at computer's startup.)
Nov 01 18:20:58 bcitcourse systemd[1]: Stopping Command Scheduler...
Nov 01 18:20:58 bcitcourse systemd[1]: Stopped Command Scheduler.
Nov 01 18:21:26 bcitcourse systemd[1]: Started Command Scheduler.
Nov 01 18:21:26 bcitcourse systemd[1]: Starting Command Scheduler...
Nov 01 18:21:26 bcitcourse crond[156]: (CRON) INFO (RANDOM_DELAY will be scaled with factor 72% if used.)
Nov 01 18:21:26 bcitcourse crond[156]: (CRON) INFO (running with inotify support)
Nov 02 17:24:11 bcitcourse systemd[1]: Stopping Command Scheduler...
Nov 02 17:24:11 bcitcourse systemd[1]: Stopped Command Scheduler.
Nov 02 17:25:08 bcitcourse systemd[1]: Started Command Scheduler.
Nov 02 17:25:08 bcitcourse systemd[1]: Starting Command Scheduler...
Nov 02 17:25:08 bcitcourse crond[135]: (CRON) INFO (RANDOM_DELAY will be scaled with factor 37% if used.)
Nov 02 17:25:08 bcitcourse crond[135]: (CRON) INFO (running with inotify support)
Nov 02 19:26:01 bcitcourse crond[135]: (wwwmaster) BAD FILE MODE (/var/spool/cron/wwwmaster)
Nov 02 19:56:01 bcitcourse crond[135]: (wwwmaster) BAD FILE MODE (/var/spool/cron/wwwmaster)
Nov 03 19:11:56 bcitcourse systemd[1]: Stopping Command Scheduler...
Nov 03 19:11:56 bcitcourse systemd[1]: Stopped Command Scheduler.
Nov 03 19:12:14 bcitcourse systemd[1]: Started Command Scheduler.
Nov 03 19:12:14 bcitcourse systemd[1]: Starting Command Scheduler...
Nov 03 19:12:14 bcitcourse crond[139]: (CRON) INFO (RANDOM_DELAY will be scaled with factor 66% if used.)
Nov 03 19:12:14 bcitcourse crond[139]: (wwwmaster) BAD FILE MODE (/var/spool/cron/wwwmaster)
Nov 03 19:12:14 bcitcourse crond[139]: (CRON) INFO (running with inotify support)
[root@bcitcourse ~]# ps aux|grep crond
root       139  0.0  0.0  22700  1496 ?        Ss   Nov03   0:00 /usr/sbin/crond -n
root      5833  0.0  0.0 112660   952 pts/3    S+   19:11   0:00 grep --color=auto crond

 

How do targets work and what are they?

 

It is similar to the traditional runtimes where generally normal non GUI operation would be considered "runlevel 3" and the GUI would be "runlevel 5".

Instead, with systemctl you have targets for these that are based on names.

A good example is when installing your GNOME GUI:

You can do that, type startx and get your GUI.  But upon reboot you'll still be at the CLI.  Why?

You need to set your default target to the graphical target like below.

systemctl set-default graphical.target

 

What targets are available on my system?

ls  /usr/lib/systemd/system
arp-ethers.service                       finger@.service                    nfs-blkmap.service                 rhel-configure.service     sockets.target.wants                    systemd-poweroff.service
auth-rpcgss-module.service               finger.socket                      nfs-client.target                  rhel-dmesg.service         sound.target                            systemd-quotacheck.service
autovt@.service                          firewalld.service                  nfs-config.service                 rhel-domainname.service    sshd-keygen.service                     systemd-random-seed.service
basic.target                             fstrim.service                     nfs-idmapd.service                 rhel-import-state.service  sshd.service                            systemd-readahead-collect.service
basic.target.wants                       fstrim.timer                       nfs-idmap.service                  rhel-loadmodules.service   sshd@.service                           systemd-readahead-done.service
blk-availability.service                 garbd.service                      nfs-lock.service                   rhel-readonly.service      sshd.socket                             systemd-readahead-done.timer
bluetooth.target                         geoclue.service                    nfslock.service                    rpcbind.service            suspend.target                          systemd-readahead-drop.service
brandbot.path                            getty-pre.target                   nfs-mountd.service                 rpcbind.socket             swap.target                             systemd-readahead-replay.service
brandbot.service                         getty@.service                     nfs-rquotad.service                rpcbind.target             sys-fs-fuse-connections.mount           systemd-reboot.service
canberra-system-bootup.service           getty.target                       nfs-secure.service                 rpc-gssd.service           sysinit.target                          systemd-remount-fs.service
canberra-system-shutdown-reboot.service  graphical.target                   nfs-server.service                 rpcgssd.service            sysinit.target.wants                    systemd-rfkill@.service
canberra-system-shutdown.service         graphical.target.wants             nfs.service                        rpcidmapd.service          sys-kernel-config.mount                 systemd-shutdownd.service
console-getty.service                    gssproxy.service                   nfs-utils.service                  rpc_pipefs.target          sys-kernel-debug.mount                  systemd-shutdownd.socket
console-shell.service                    halt-local.service                 nmb.service                        rpc-rquotad.service        syslog.socket                           systemd-suspend.service
container-getty@.service                 halt.target                        nscd.service                       rpc-statd-notify.service   syslog.target.wants                     systemd-sysctl.service
crond.service                            hibernate.target                   nscd.socket                        rpc-statd.service          systemd-ask-password-console.path       systemd-timedated.service
cryptsetup-pre.target                    htcacheclean.service               nss-lookup.target                  rsyncd.service             systemd-ask-password-console.service    systemd-tmpfiles-clean.service
cryptsetup.target                        httpd.service                      nss-user-lookup.target             rsyncd@.service            systemd-ask-password-wall.path          systemd-tmpfiles-clean.timer
ctrl-alt-del.target                      hybrid-sleep.target                ntalk.service                      rsyncd.socket              systemd-ask-password-wall.service       systemd-tmpfiles-setup-dev.service
dbus-org.freedesktop.hostname1.service   initrd-cleanup.service             ntalk.socket                       rsyslog.service            systemd-backlight@.service              systemd-tmpfiles-setup.service
dbus-org.freedesktop.import1.service     initrd-fs.target                   paths.target                       runlevel0.target           systemd-binfmt.service                  systemd-udevd-control.socket
dbus-org.freedesktop.locale1.service     initrd-parse-etc.service           polkit.service                     runlevel1.target           systemd-bootchart.service               systemd-udevd-kernel.socket
dbus-org.freedesktop.login1.service      initrd-root-fs.target              poweroff.target                    runlevel1.target.wants     systemd-firstboot.service               systemd-udevd.service
dbus-org.freedesktop.machine1.service    initrd-switch-root.service         poweroff.target.wants              runlevel2.target           systemd-fsck-root.service               systemd-udev-settle.service
dbus-org.freedesktop.timedate1.service   initrd-switch-root.target          printer.target                     runlevel2.target.wants     systemd-fsck@.service                   systemd-udev-trigger.service
dbus.service                             initrd.target                      proc-fs-nfsd.mount                 runlevel3.target           systemd-halt.service                    systemd-update-done.service
dbus.socket                              initrd.target.wants                proc-sys-fs-binfmt_misc.automount  runlevel3.target.wants     systemd-hibernate-resume@.service       systemd-update-utmp-runlevel.service
dbus.target.wants                        initrd-udevadm-cleanup-db.service  proc-sys-fs-binfmt_misc.mount      runlevel4.target           systemd-hibernate.service               systemd-update-utmp.service
debug-shell.service                      ip6tables.service                  quotaon.service                    runlevel4.target.wants     systemd-hostnamed.service               systemd-user-sessions.service
default.target                           iptables.service                   rc-local.service                   runlevel5.target           systemd-hwdb-update.service             systemd-vconsole-setup.service
default.target.wants                     kexec.target                       rdisc.service                      runlevel5.target.wants     systemd-hybrid-sleep.service            system.slice
dev-hugepages.mount                      kmod-static-nodes.service          rdma-hw.target                     runlevel6.target           systemd-importd.service                 system-update.target
dev-mqueue.mount                         local-fs-pre.target                rdma-load-modules@.service         saslauthd.service          systemd-initctl.service                 tcsd.service
dracut-cmdline.service                   local-fs.target                    rdma-ndd.service                   sendmail.service           systemd-initctl.socket                  timers.target
dracut-initqueue.service                 local-fs.target.wants              rdma.service                       serial-getty@.service      systemd-journal-catalog-update.service  timers.target.wants
dracut-mount.service                     machine.slice                      reboot.target                      shutdown.target            systemd-journald.service                time-sync.target
dracut-pre-mount.service                 machines.target                    reboot.target.wants                shutdown.target.wants      systemd-journald.socket                 tmp.mount
dracut-pre-pivot.service                 messagebus.service                 remote-cryptsetup.target           sigpwr.target              systemd-journal-flush.service           umount.target
dracut-pre-trigger.service               multi-user.target                  remote-fs-pre.target               sleep.target               systemd-kexec.service                   user.slice
dracut-pre-udev.service                  multi-user.target.wants            remote-fs.target                   -.slice                    systemd-localed.service                 var-lib-nfs-rpc_pipefs.mount
dracut-shutdown.service                  named.service                      rescue.service                     slices.target              systemd-logind.service                  vzfifo.service
ebtables.service                         named-setup-rndc.service           rescue.target                      smartcard.target           systemd-machined.service                vzreboot.service
emergency.service                        network-online.target              rescue.target.wants                smb.service                systemd-machine-id-commit.service       xinetd.service
emergency.target                         network-pre.target                 rhel-autorelabel-mark.service      sm-client.service          systemd-modules-load.service
final.target                             network.target                     rhel-autorelabel.service           sockets.target             systemd-nspawn@.service

 

We can check by symlink to see what the default target is:

lrwxrwxrwx 1 root root 16 Sep 18 07:18 /usr/lib/systemd/system/default.target -> graphical.target

Checking to see what services start with a target

systemctl list-dependencies default.target
default.target
● ├─display-manager.service
● ├─mysqld.service
● ├─network.service
● ├─quotaon.service
● ├─systemd-readahead-collect.service
● ├─systemd-readahead-replay.service
● ├─systemd-update-utmp-runlevel.service
● ├─vzfifo.service
● └─multi-user.target
●   ├─brandbot.path
●   ├─dbus.service
●   ├─httpd.service
●   ├─mysqld.service
●   ├─network.service
●   ├─rhel-configure.service
●   ├─rsyslog.service
●   ├─saslauthd.service
●   ├─sendmail.service
●   ├─sm-client.service
●   ├─sshd.service
●   ├─systemd-ask-password-wall.path
●   ├─systemd-logind.service
●   ├─systemd-update-utmp-runlevel.service
●   ├─systemd-user-sessions.service
●   ├─xinetd.service
●   ├─basic.target
●   │ ├─iptables.service
●   │ ├─rhel-dmesg.service
●   │ ├─paths.target
●   │ ├─slices.target
●   │ │ ├─-.slice
●   │ │ └─system.slice
●   │ ├─sockets.target
●   │ │ ├─dbus.socket
●   │ │ ├─rpcbind.socket
●   │ │ ├─systemd-initctl.socket
●   │ │ ├─systemd-journald.socket
●   │ │ ├─systemd-shutdownd.socket
●   │ │ ├─systemd-udevd-control.socket
●   │ │ └─systemd-udevd-kernel.socket
●   │ ├─sysinit.target
●   │ │ ├─dev-hugepages.mount
●   │ │ ├─dev-mqueue.mount
●   │ │ ├─kmod-static-nodes.service
●   │ │ ├─proc-sys-fs-binfmt_misc.automount
●   │ │ ├─rhel-autorelabel-mark.service
●   │ │ ├─rhel-autorelabel.service
●   │ │ ├─rhel-domainname.service
●   │ │ ├─rhel-import-state.service
●   │ │ ├─rhel-loadmodules.service
●   │ │ ├─sys-fs-fuse-connections.mount
●   │ │ ├─sys-kernel-config.mount
●   │ │ ├─sys-kernel-debug.mount
●   │ │ ├─systemd-ask-password-console.path
●   │ │ ├─systemd-binfmt.service
●   │ │ ├─systemd-firstboot.service
●   │ │ ├─systemd-hwdb-update.service
●   │ │ ├─systemd-journal-catalog-update.service
●   │ │ ├─systemd-journal-flush.service
●   │ │ ├─systemd-journald.service
●   │ │ ├─systemd-machine-id-commit.service
●   │ │ ├─systemd-modules-load.service

 

How can we see what switchable "isolate"able targets are available?

ls -al /usr/lib/systemd/system/|grep ^d
drwxr-xr-x 22 root root 16384 Nov  3 22:59 .
drwxr-xr-x 13 root root  4096 Sep 18 07:18 ..
drwxr-xr-x  2 root root  4096 Sep 18 07:23 basic.target.wants
drwxr-xr-x  2 root root  4096 Aug 16 18:34 dbus.target.wants
drwxr-xr-x  2 root root  4096 Aug 16 18:34 default.target.wants
drwxr-xr-x  2 root root  4096 Sep 18 07:18 graphical.target.wants
drwxr-xr-x  2 root root  4096 Sep 18 07:18 initrd.target.wants
drwxr-xr-x  2 root root  4096 Sep 18 07:23 local-fs.target.wants
drwxr-xr-x  2 root root  4096 Sep 18 07:23 multi-user.target.wants
drwxr-xr-x  2 root root  4096 Sep 18 07:18 poweroff.target.wants
drwxr-xr-x  2 root root  4096 Sep 18 07:18 reboot.target.wants
drwxr-xr-x  2 root root  4096 Sep 18 07:18 rescue.target.wants
drwxr-xr-x  2 root root  4096 Sep 18 07:18 runlevel1.target.wants
drwxr-xr-x  2 root root  4096 Sep 18 07:18 runlevel2.target.wants
drwxr-xr-x  2 root root  4096 Sep 18 07:18 runlevel3.target.wants
drwxr-xr-x  2 root root  4096 Sep 18 07:18 runlevel4.target.wants
drwxr-xr-x  2 root root  4096 Sep 18 07:18 runlevel5.target.wants
drwxr-xr-x  2 root root  4096 Sep 18 07:18 shutdown.target.wants
drwxr-xr-x  2 root root  4096 Sep 18 07:18 sockets.target.wants
drwxr-xr-x  2 root root  4096 Sep 18 07:18 sysinit.target.wants
drwxr-xr-x  2 root root  4096 Aug 16 18:34 syslog.target.wants
drwxr-xr-x  2 root root  4096 Sep 18 07:18 timers.target.wants

How do we switch targets without rebooting?

systemctl isolate targetname

systemctl isolate poweroff

systemctl sockets

systemctl list-sockets

LISTEN                      UNIT                         ACTIVATES
/dev/log                    systemd-journald.socket      systemd-journald.service
/run/dbus/system_bus_socket dbus.socket                  dbus.service
/run/systemd/initctl/fifo   systemd-initctl.socket       systemd-initctl.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/rpcbind.sock       rpcbind.socket               rpcbind.service
kobject-uevent 1            systemd-udevd-kernel.socket  systemd-udevd.service

9 sockets listed.
Pass --all to see loaded but inactive sockets, too.

journalctl (systemd logging)

journalctl -u sshd

Aug 07 06:22:45 bcitcourse sshd[595]: Received disconnect from 59.63.188.32: 11:  [preauth]
Aug 07 06:23:00 bcitcourse sshd[597]: Received disconnect from 221.194.47.221: 11:  [preauth]
Aug 07 06:23:15 bcitcourse sshd[599]: Received disconnect from 59.63.188.32: 11:  [preauth]
Aug 07 06:23:16 bcitcourse sshd[601]: Received disconnect from 221.194.44.232: 11:  [preauth]
Aug 07 06:23:42 bcitcourse sshd[603]: Received disconnect from 59.63.188.32: 11:  [preauth]
Aug 07 06:24:14 bcitcourse sshd[607]: Received disconnect from 59.63.188.32: 11:  [preauth]
Aug 07 06:24:46 bcitcourse sshd[609]: Received disconnect from 59.63.188.32: 11:  [preauth]
Jul 22 10:21:09 bcitcourse sshd[867]: reverse mapping checking getaddrinfo for 88.150-182-91.adsl-dyn.isp.belgacom.be [91.182.150.88] failed - POSSIBLE BREAK-IN ATTEMPT!
Jul 22 10:21:09 bcitcourse sshd[867]: Invalid user pi from 91.182.150.88
Jul 22 10:21:09 bcitcourse sshd[867]: input_userauth_request: invalid user pi [preauth]
Jul 22 10:21:09 bcitcourse sshd[865]: reverse mapping checking getaddrinfo for 88.150-182-91.adsl-dyn.isp.belgacom.be [91.182.150.88] failed - POSSIBLE BREAK-IN ATTEMPT!
Jul 22 10:21:09 bcitcourse sshd[865]: Invalid user pi from 91.182.150.88
Jul 22 10:21:09 bcitcourse sshd[865]: input_userauth_request: invalid user pi [preauth]
Jul 22 10:21:09 bcitcourse sshd[867]: pam_unix(sshd:auth): check pass; user unknown
Jul 22 10:21:09 bcitcourse sshd[867]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=91.182.150.88
Jul 22 10:21:09 bcitcourse sshd[865]: pam_unix(sshd:auth): check pass; user unknown
Jul 22 10:21:09 bcitcourse sshd[865]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=91.182.150.88
Jul 22 10:21:11 bcitcourse sshd[867]: Failed password for invalid user pi from 91.182.150.88 port 46846 ssh2
Jul 22 10:21:11 bcitcourse sshd[865]: Failed password for invalid user pi from 91.182.150.88 port 46842 ssh2

What's the difference?

journalctl -b 2 and journalctl -b -2

It is the fact the first "2" is a positive and the second "-2" is a negative.

Thus, 1 means the first boot found in the
           journal in chronological order, 2 the second and so on; while -0 is
           the last boot, -1 the boot before last, and so on.

 


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