Here's the thirteenth installment of my ongoing series on systemd for Administrators:

Log and Service Status

This one is a short episode. One of the most commonly used commands on a systemd system is systemctl status which may be used to determine the status of a service (or other unit). It always has been a valuable tool to figure out the processes, runtime information and other meta data of a daemon running on the system.

With Fedora 17 we introduced the journal, our new logging scheme that provides structured, indexed and reliable logging on systemd systems, while providing a certain degree of compatibility with classic syslog implementations. The original reason we started to work on the journal was one specific feature idea, that to the outsider might appear simple but without the journal is difficult and inefficient to implement: along with the output of systemctl status we wanted to show the last 10 log messages of the daemon. Log data is some of the most essential bits of information we have on the status of a service. Hence it it is an obvious choice to show next to the general status of the service.

And now to make it short: at the same time as we integrated the journal into systemd and Fedora we also hooked up systemctl with it. Here's an example output:

$ systemctl status avahi-daemon.service
avahi-daemon.service - Avahi mDNS/DNS-SD Stack
	  Loaded: loaded (/usr/lib/systemd/system/avahi-daemon.service; enabled)
	  Active: active (running) since Fri, 18 May 2012 12:27:37 +0200; 14s ago
	Main PID: 8216 (avahi-daemon)
	  Status: "avahi-daemon 0.6.30 starting up."
	  CGroup: name=systemd:/system/avahi-daemon.service
		  ├ 8216 avahi-daemon: running [omega.local]
		  └ 8217 avahi-daemon: chroot helper

May 18 12:27:37 omega avahi-daemon[8216]: Joining mDNS multicast group on interface eth1.IPv4 with address 172.31.0.52.
May 18 12:27:37 omega avahi-daemon[8216]: New relevant interface eth1.IPv4 for mDNS.
May 18 12:27:37 omega avahi-daemon[8216]: Network interface enumeration completed.
May 18 12:27:37 omega avahi-daemon[8216]: Registering new address record for 192.168.122.1 on virbr0.IPv4.
May 18 12:27:37 omega avahi-daemon[8216]: Registering new address record for fd00::e269:95ff:fe87:e282 on eth1.*.
May 18 12:27:37 omega avahi-daemon[8216]: Registering new address record for 172.31.0.52 on eth1.IPv4.
May 18 12:27:37 omega avahi-daemon[8216]: Registering HINFO record with values 'X86_64'/'LINUX'.
May 18 12:27:38 omega avahi-daemon[8216]: Server startup complete. Host name is omega.local. Local service cookie is 3555095952.
May 18 12:27:38 omega avahi-daemon[8216]: Service "omega" (/services/ssh.service) successfully established.
May 18 12:27:38 omega avahi-daemon[8216]: Service "omega" (/services/sftp-ssh.service) successfully established.

This, of course, shows the status of everybody's favourite mDNS/DNS-SD daemon with a list of its processes, along with -- as promised -- the 10 most recent log lines. Mission accomplished!

There are a couple of switches available to alter the output slightly and adjust it to your needs. The two most interesting switches are -f to enable follow mode (as in tail -f) and -n to change the number of lines to show (you guessed it, as in tail -n).

The log data shown comes from three sources: everything any of the daemon's processes logged with libc's syslog() call, everything submitted using the native Journal API, plus everything any of the daemon's processes logged to STDOUT or STDERR. In short: everything the daemon generates as log data is collected, properly interleaved and shown in the same format.

And that's it already for today. It's a very simple feature, but an immensely useful one for every administrator. One of the kind "Why didn't we already do this 15 years ago?".

Stay tuned for the next installment!