Thursday, November 20, 2014

Systemd - Systemctl In Rhel7/Centos7

Systemd is a system and service manager for Linux, compatible with SysV and LSB init scripts. systemd provides aggressive parallelization capabilities, uses socket and D-Bus activation for starting services, offers on-demand starting of daemons, keeps track of processes using Linux cgroups, supports snapshotting and restoring of the system state, maintains mount and automount points and implements an elaborate transactional dependency-based service control logic. It can work as a drop-in replacement for sysvinit.

Boot process

Systemd primary task is to manage the boot process and provides informations about it.

To get the boot process duration, type:

>> systemd-analyze
Startup finished in 422ms (kernel) + 2.722s (initrd) + 9.674s (userspace) = 12.820s
To get the time spent by each task during the boot process, type:

>> systemd-analyze blame
7.029s network.service
2.241s plymouth-start.service
1.293s kdump.service
1.156s plymouth-quit-wait.service
1.048s firewalld.service
632ms postfix.service
621ms tuned.service
460ms iprupdate.service
446ms iprinit.service
344ms accounts-daemon.service
7ms systemd-update-utmp-runlevel.service
5ms systemd-random-seed.service
5ms sys-kernel-config.mount
To get the list of the dependencies, type:

>> systemctl list-dependencies
│ ├─firewalld.service
│ ├─microcode.service
│ ├─getty@tty1.service
│ └─serial-getty@ttyS0.service
Note: You will find additional information on this point in the Lennart Poettering’s blog.

Journal analysis

In addition, Systemd handles the system event log, a syslog daemon is not mandatory any more.
To get the content of the Systemd journal, type:

>> journalctl
To get all the events related to the crond process in the journal, type:

>> journalctl /sbin/crond
Note: You can replace /sbin/crond by `which crond`.

To get all the events since the last boot, type:

>> journalctl -b
To get all the events that appeared today in the journal, type:

>> journalctl --since=today
To get all the events with a syslog priority of err, type:

>> journalctl -p err
To get the 10 last events and wait for any new one (like “tail -f /var/log/messages“), type:

>> journalctl -f
Note: You will find additional information on this point in the Lennart Poettering’s blog or Lennart Poettering’s video (44min: the first ten minutes are very interesting concerning security issues).

Control groups

Systemd organizes tasks in control groups. For example, all the processes started by an apache webserver will be in the same control group, CGI scripts included.

To get the full hierarchy of control groups, type:

>> systemd-cgls
│ └─user-1000.slice
│ └─session-1.scope
│ ├─2889 gdm-session-worker [pam/gdm-password]
│ ├─2899 /usr/bin/gnome-keyring-daemon --daemonize --login
│ ├─2901 gnome-session --session gnome-classic
. .
└─785 /sbin/iprupdate --daemon
To get the list of control group ordered by CPU, memory and disk I/O load, type:

>> systemd-cgtop
Path Tasks %CPU Memory Input/s Output/s
/ 213 3.9 829.7M - -
/system.slice 1 - - - -
/system.slice/ModemManager.service 1 - - - -
To kill all the processes associated with an apache server (CGI scripts included), type:

>> systemctl kill httpd
To put resource limits on a service (here 500 CPUShares), type:

>> systemctl set-property httpd.service CPUShares=500
Note1: The change is written into the service unit file. Use the –runtime option to avoid this behavior.
Note2: By default, each service owns 1024 CPUShares. Nothing prevents you from giving a value smaller or bigger.

To get the current CPUShares service value, type:

>> systemctl show -p CPUShares httpd.service
On this topic, you can additionally watch Georgios’ Magklaras demo (24min).

Sources: New control group interface, Systemd 205 announcement.

Service management

Systemd deals with all the aspects of the service management. The systemctl command replaces the chkconfig and the service commands. The old commands are now a link to the systemctl command.

To activate the NTP service at boot, type:

>> systemctl enable ntpd
Note1: You should specify ntpd.service but by default the .service suffix will be added.
Note2: If you specify a path, the .mount suffix will be added.
Note3: If you mention a device, the .device suffix will be added.

To deactivate it, start it, stop it, restart it, reload it, type:

>> systemctl disable ntpd
>> systemctl start ntpd
>> systemctl stop ntpd
>> systemctl restart ntpd
>> systemctl reload ntpd
Note: It is also possible to mask and unmask a service. Masking a service prevents it from being started manually or by another service.

To know if the NTP service is activated at boot, type:

>> systemctl is-enabled ntpd
To know if the NTP service is running, type:

>> systemctl is-active ntpd
To get the status of the NTP service, type:

>> systemctl status ntpd
   Loaded: not-found (Reason: No such file or directory)
   Active: inactive (dead)
If you change a service configuration, you will need to reload it:

>> systemctl daemon-reload
To get the list of all the units (services, mount points, devices) with their status and description, type:

>> systemctl
To get a more readable list, type:

>> systemctl list-unit-files
To get the list of services that failed at boot, type:

>> systemctl --failed
To get the status of a process (here httpd) on a remote server (here, type:

>> systemctl -H status httpd.service
Run levels

Systemd also deals with run levels. As everything is represented by files in Systemd, target files replace run levels.

To move to single user mode, type:

>> systemctl rescue
To move to the level 3 (equivalent to the previous level 3), type:

>> systemctl isolate

>> systemctl isolate
To move to the graphical level (equivalent to the previous level 5), type:

>> systemctl isolate
To set the default run level to non-graphical mode, type:

>> systemctl set-default
To set the default run level to graphical mode, type:

>> systemctl set-default
To get the current default run level, type:

>> systemctl get-default
To stop a server, type:

>> systemctl poweroff
Note: You can still use the poweroff command, a link to the systemctl command has been created (the same thing is true for the halt and reboot commands).

To reboot a server, suspend it or put it into hibernation, type:

>> systemctl reboot
>> systemctl suspend
>> systemctl hibernate
Linux standardization

Systemd‘s authors have decided to help Linux standardization among distributions. Through Systemd, changes happen in the localization of some configuration files.


To get the server hostnames, type:

>> hostnamectl
Static hostname:
Icon name: computer-laptop
Chassis: laptop
Machine ID: asdasdasdasdsadas9aa37e54a422938d
Boot ID: adasdasdasdasdac4a82fef4ac26d0
Operating System: Centos
CPE OS Name: cpe:/o:rCentos
Kernel: Linux 3.10.0-54.0.1.el7.x86_64
Architecture: x86_64
Note: There are three kinds of hostnames: static, pretty, and transient.
“The static host name is the traditional hostname, which can be chosen by the user, and is stored in the /etc/hostname file. The “transient” hostname is a dynamic host name maintained by the kernel. It is initialized to the static host name by default, whose value defaults to “localhost”. It can be changed by DHCP or mDNS at runtime. The pretty hostname is a free-form UTF8 host name for presentation to the user.” Source: Centos 7 Networking Guide.

To assign the test hostname permanently to the server, type:

>> hostnamectl set-hostname test
Note: With this syntax all three hostnames (static, pretty, and transient) take the test value at the same time. However, it is possible to set the three hostnames separately by using the –pretty, –static, and –transient options.

To get the current locale, virtual console keymap and X11 layout, type:

>> localectl
System Locale: LANG=en_US.UTF-8
VC Keymap: en_US
X11 Layout: en_US
To assign the en_GB.utf8 value to the locale, type:

>> localectl set-locale LANG=en_GB.utf8
To assign the en_GB value to the virtual console keymap, type:

>> localectl set-keymap en_GB
To assign the en_GB value to the X11 layout, type:

>> localectl set-x11-keymap en_GB
To get the current date and time, type:

>> timedatectl
Local time: Fri 2014-01-24 22:34:05 CET
Universal time: Fri 2014-01-24 21:34:05 UTC
RTC time: Fri 2014-01-24 21:34:05
Timezone: Europe/Madrid (CET, +0100)
NTP enabled: yes
NTP synchronized: yes
RTC in local TZ: no
DST active: no
Last DST change: DST ended at
Sun 2013-10-27 02:59:59 CEST
Sun 2013-10-27 02:00:00 CET
Next DST change: DST begins (the clock jumps one hour forward) at
Sun 2014-03-30 01:59:59 CET
Sun 2014-03-30 03:00:00 CEST
To set the current date, type:

>> timedatectl set-time YYYY-MM-DD
To set the current time, type:

>> timedatectl set-time HH:MM:SS
To get the list of time zones, type:

>> timedatectl list-timezones
To change the time zone to America/New_York, type:

>> timedatectl set-timezone America/New_York
To get the users’ list, type:

>> loginctl list-users
42 gdm
1000 tom
0 root
To get the list of all current user sessions, type:

>> loginctl list-sessions
1 1000 tom seat0

1 sessions listed.
To get the properties of the user tom, type:

>> loginctl show-user tom
Timestamp=Fri 2014-01-24 21:53:43 CET

Sources: Archlinux wiki, Freedesktop wiki, Gentoo wiki, RHEL 7 System Administration Guide, Fedora wiki.

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