RHEL: Services basic management

RHEL: Services basic management - systemd

# Tested on CentOS 7

# systemd is the new Fedora init system adopted by Red Hat from RHEL 7 on.

# It is backwards compatible with SysV init scripts, enhances the administrative process
# and provides new features such as parallel startup of system services at boot time or
# dependency-based service control, for instance.

# systemd introduces the concept of systemd units. These units are represented by unit
# configuration files and encapsulate information about system services and other objects
# that are relevant to the init system.
#
# systemd unit locations:
#
# /usr/lib/systemd/system   Systemd units distributed with installed RPM packages.
#
# /run/systemd/system       Systemd units created at run time. This directory takes
#                           precedence over the directory with installed service units.
#
# /etc/systemd/system       Systemd units created and managed by the system administrator.
#                           This directory takes precedence over the directory with runtime
#                           units.

# "systemctl" command shows the state of all services. It queries the state of services,
# both systemd native and SysV/LSB services. It shows for each service, whether it managed
# to start up or failed (time-out, non-zero exit code, abnormal termination):

systemctl
UNIT                                  LOAD   ACTIVE SUB       DESCRIPTION
[...]
auditd.service                        loaded active running   Security Auditing Service
chronyd.service                       loaded active running   NTP client/server
crond.service                         loaded active running   Command Scheduler
dbus.service                          loaded active running   D-Bus System Message Bus
getty@tty1.service                    loaded active running   Getty on tty1
* kdump.service                         loaded failed failed    Crash recovery kernel arming
kmod-static-nodes.service             loaded active exited    Create list of required static device nodes for the current kernel
lvm2-monitor.service                  loaded active exited    Monitoring of LVM2 mirrors, snapshots etc. using dmeventd or progress polling
network.service                       loaded active exited    LSB: Bring up/down networking
[...]
local-fs-pre.target                   loaded active active    Local File Systems (Pre)
local-fs.target                       loaded active active    Local File Systems
multi-user.target                     loaded active active    Multi-User System
network-online.target                 loaded active active    Network is Online
network.target                        loaded active active    Network
paths.target                          loaded active active    Paths
remote-fs.target                      loaded active active    Remote File Systems
slices.target                         loaded active active    Slices
sockets.target                        loaded active active    Sockets
swap.target                           loaded active active    Swap
sysinit.target                        loaded active active    System Initialization
timers.target                         loaded active active    Timers
systemd-tmpfiles-clean.timer          loaded active waiting   Daily Cleanup of Temporary Directories

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.

109 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'.

# To have a quick view of system status, run "systemctl status":

systemctl status
   * myserver
       State: running
        Jobs: 0 queued
      Failed: 0 units
       Since: Tue 2016-02-02 09:13:06 CET; 1h 11min ago
      CGroup: /
              |-1 /usr/lib/systemd/systemd --switched-root --system --deserialize 21
              |-user.slice
              | |-user-0.slice
              |   |-session-3.scope
              |   | |-2162 /usr/sbin/anacron -s
              |   |-session-2.scope
              |   | |-2114 sshd: root@pts/1
              |   | |-2116 -bash
              |   | |-2212 man systemd-cgls
              |   | |-2221 less -s
              |   |-session-1.scope
              |     |-2068 sshd: root@pts/0
              |     |-2070 -bash
              |     |-2323 systemctl status
              |     |-2324 less
              |-system.slice
                |-mysqld.service
                | |-1014 /usr/sbin/mysqld --daemonize --pid-file=/var/run/mysqld/mysqld.pid
                |-tuned.service
                | |-939 /usr/bin/python -Es /usr/sbin/tuned -l -P
              [...]
                |-system-getty.slice
                  |-getty@tty1.service
                    |-764 /sbin/agetty --noclear tty1 linux

# To have a little bit more information about a service, use "systemctl status <service>".

# systemd tracks and remembers whether the service started up successfully or not
# both during start-up and runtime.

# Example of a service that failed to stay up, when it ran as PID 2148, and indicates that
# the process failed with exit status of 1:

systemctl status kdump
* kdump.service - Crash recovery kernel arming
   Loaded: loaded (/usr/lib/systemd/system/kdump.service; enabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Mon 2016-01-25 14:11:55 CET; 1h 50min ago
Main PID: 2148 (code=exited, status=1/FAILURE)

Jan 25 14:11:21 myserver systemd[1]: Starting Crash recovery kernel arming...
Jan 25 14:11:55 myserver kdumpctl[2148]: No memory reserved for crash kernel.
Jan 25 14:11:55 myserver kdumpctl[2148]: Starting kdump: [FAILED]
Jan 25 14:11:55 myserver systemd[1]: kdump.service: main process exited, code=exited, status=1/FAILURE
Jan 25 14:11:55 myserver systemd[1]: Failed to start Crash recovery kernel arming.
Jan 25 14:11:55 myserver systemd[1]: Unit kdump.service entered failed state.
Jan 25 14:11:55 myserver systemd[1]: kdump.service failed.

# Managing of system services with "systemctl"
# ------------------------------------------------------------------------------------------

# Start a service
systemctl start <service>

# Stop a service
systemctl stop <service>

# Restart a service
systemctl restart <service>

# Restart a service only if it is running
systemctl try-restart <service>

# Reload configuration
systemctl reload <service>

# Check if a service is running
systemctl status <service>
systemctl is-active <service>

# Display the status of all services
systemctl list-units --type service
systemctl list-units --all

# Enable a service
systemctl enable <service>

# Disable a service
systemctl disable <service>

# Check if a service is enabled
systemctl status <service>
systemctl is-enabled <service>

# List all services and check if they are enabled
systemctl list-unit-files --type service

# List services that are ordered to start before the specified unit
systemctl list-dependencies --after [<service>]

# List services that are ordered to start after the specified unit
systemctl list-dependencies --before [<service>]

# SYSTEMD TARGETS
# ------------------------------------------------------------------------------------------

# In RHEL 7, the concept of runlevels has been replaced with systemd "targets".

# Systemd targets are represented by target units. Target units end with the .target file
# extension and their only purpose is to group together other systemd units through a chain
# of dependencies.

# RHEL 7 has a number of predefined targets similar to the standard set of runlevels from
# the previous releases.

# systemd Targets

runlevel0.target, poweroff.target        shut down and power off the system
runlevel1.target, rescue.target          set up a rescue shell
runlevel2.target, multi-user.target      set up a non-graphical multi-user system
runlevel3.target, multi-user.target      set up a non-graphical multi-user system
runlevel4.target, multi-user.target      set up a non-graphical multi-user system
runlevel5.target, graphical.target       set up a graphical multi-user system
runlevel6.target, reboot.target          shut down and reboot the system

# List currently loaded target units:
systemctl list-units --type target

# Change the current target:
systemctl isolate <name.target>

# Verify / modify the default target:
systemctl get-default
systemctl set-default <name.target>

# The last command will replace the /etc/systemd/system/default.target file with a
# symbolic link to /usr/lib/systemd/system/<name.target>:

systemctl set-default graphical.target
   Removed symlink /etc/systemd/system/default.target.
   Created symlink from /etc/systemd/system/default.target to /usr/lib/systemd/system/graphical.target.

# Switching to default/rescue/emergency mode

systemctl default      # Enter default mode. Equivalent to systemctl isolate default.target

systemctl rescue                 # or systemctl isolate rescue.target
systemctl --no-wall rescue       # prevent sending informative message to users

# In emergency mode, the system only mounts the root file system only for reading and
# starts a few essential services. In RHEL 7, emergency mode requires the root password.

systemctl emergency              # or systemctl isolate emergency.target
systemctl --no-wall emergency    # prevent sending informative message to users

# Halting, powering off and rebooting system

# In RHEL 7, systemctl replaces power management commands; these commands are available in
# the system for compatibility reasons but it is recommended to use systemctl when
# possible:

ll /usr/sbin/halt /usr/sbin/poweroff /usr/sbin/shutdown /usr/sbin/reboot
   lrwxrwxrwx. 1 root root 16 Jan 15 18:33 /usr/sbin/halt -> ../bin/systemctl
   lrwxrwxrwx. 1 root root 16 Jan 15 18:33 /usr/sbin/poweroff -> ../bin/systemctl
   lrwxrwxrwx. 1 root root 16 Jan 15 18:33 /usr/sbin/reboot -> ../bin/systemctl
   lrwxrwxrwx. 1 root root 16 Jan 15 18:33 /usr/sbin/shutdown -> ../bin/systemctl

# Halt the system
systemctl halt

# Power off the system
systemctl poweroff

# Restart the system
systemctl reboot