systemd for Administrators, Part X

Post Syndicated from Lennart Poettering original

Here’s the tenth installment
my ongoing series

Instantiated Services

Most services on Linux/Unix are singleton services: there’s
usually only one instance of Syslog, Postfix, or running on a
specific system at the same time. On the other hand some select
services may run in multiple instances on the same host. For example,
an Internet service like the Dovecot IMAP service could run in
multiple instances on different IP ports or different local IP
addresses. A more common example that exists on all installations is
getty, the mini service that runs once for each TTY and
presents a login prompt on it. On most systems this service is
instantiated once for each of the first six virtual consoles
tty1 to tty6. On some servers depending on
administrator configuration or boot-time parameters an additional
getty is instantiated for a serial or virtualizer console. Another
common instantiated service in the systemd world is fsck, the
file system checker that is instantiated once for each block device
that needs to be checked. Finally, in systemd socket activated
per-connection services (think classic inetd!) are also implemented
via instantiated services: a new instance is created for each incoming
connection. In this installment I hope to explain a bit how systemd
implements instantiated services and how to take advantage of them as
an administrator.

If you followed the previous episodes of this series you are
probably aware that services in systemd are named according to the
pattern foobar.service, where foobar is an
identification string for the service, and .service simply a
fixed suffix that is identical for all service units. The definition files
for these services are searched for in /etc/systemd/system
and /lib/systemd/system (and possibly other directories) under this name. For
instantiated services this pattern is extended a bit: the service name becomes
[email protected] where foobar is the
common service identifier, and quux the instance
identifier. Example: [email protected] is the serial
getty service instantiated for ttyS2.

Service instances can be created dynamically as needed. Without
further configuration you may easily start a new getty on a serial
port simply by invoking a systemctl start command for the new

# systemctl start [email protected]

If a command like the above is run systemd will first look for a
unit configuration file by the exact name you requested. If this
service file is not found (and usually it isn’t if you use
instantiated services like this) then the instance id is removed from
the name and a unit configuration file by the resulting
template name searched. In other words, in the above example,
if the precise [email protected] unit file cannot
be found, [email protected] is loaded instead. This unit
template file will hence be common for all instances of this
service. For the serial getty we ship a template unit file in systemd
(/lib/systemd/system/[email protected]) that looks
something like this:

Description=Serial Getty on %I
After=dev-%i.device systemd-user-sessions.service

ExecStart=-/sbin/agetty -s %I 115200,38400,9600

(Note that the unit template file we actually ship along with
systemd for the serial gettys is a bit longer. If you are interested,
have a look at the actual
which includes additional directives for compatibility with
SysV, to clear the screen and remove previous users from the TTY
device. To keep simple I have shortened the unit file to the
relevant lines here.)

This file looks mostly like any other unit file, with one
distinction: the specifiers %I and %i are used at
multiple locations. At unit load time %I and %i are
replaced by systemd with the instance identifier of the service. In
our example above, if a service is instantiated as
[email protected] the specifiers %I and
%i will be replaced by ttyUSB0. If you introspect
the instanciated unit with systemctl status
[email protected] you will see these replacements
having taken place:

$ systemctl status [email protected]
[email protected] – Getty on ttyUSB0
Loaded: loaded (/lib/systemd/system/[email protected]; static)
Active: active (running) since Mon, 26 Sep 2011 04:20:44 +0200; 2s ago
Main PID: 5443 (agetty)
CGroup: name=systemd:/system/[email protected]/ttyUSB0
└ 5443 /sbin/agetty -s ttyUSB0 115200,38400,9600

And that is already the core idea of instantiated services in
systemd. As you can see systemd provides a very simple templating
system, which can be used to dynamically instantiate services as
needed. To make effective use of this, a few more notes:

You may instantiate these services on-the-fly in
.wants/ symbolic links in the file system. For example, to
make sure the serial getty on ttyUSB0 is started
automatically at every boot, create a symlink like this:

# ln -s /lib/systemd/system/[email protected] /etc/systemd/system/[email protected]

systemd will instantiate the symlinked unit file with the
instance name specified in the symlink name.

You cannot instantiate a unit template without specifying an
instance identifier. In other words systemctl start
[email protected] will necessarily since the instance
name was left unspecified.

Sometimes it is useful to opt-out of the generic template
for one specific instance. For these make use of the fact that
systemd always searches first for the full instance file name before
falling back to the template file name: make sure to place a unit file
under the fully instantiated name in /etc/systemd/system and
it will override the generic templated version for this specific

The unit file shown above uses %i at some places and
%I at others. You may wonder what the difference between
these specifiers are. %i is replaced by the exact characters
of the instance identifier. For %I on the other hand the
instance identifier is first passed through a simple unescaping
algorithm. In the case of a simple instance identifier like
ttyUSB0 there is no effective difference. However, if the
device name includes one or more slashes (“/”) this cannot be
part of a unit name (or Unix file name). Before such a device name can
be used as instance identifier it needs to be escaped so that “/”
becomes “-” and most other special characters (including “-“) are
replaced by “xAB” where AB is the ASCII code of the character in
hexadecimal notation[1]. Example: to refer to a USB serial port by its
bus path we want to use a port name like
serial/by-path/pci-0000:00:1d.0--0:1.4:1.1-port0. The
escaped version of this name is
serial-byx2dpath-pcix2d0000:00:1d.0x2dusbx2d0:1.4:1.1x2dport0. %I
will then refer to former, %i to the latter. Effectively this
means %i is useful wherever it is necessary to refer to other
units, for example to express additional dependencies. On the other
hand %I is useful for usage in command lines, or inclusion in
pretty description strings. Let’s check how this looks with the above unit file:

# systemctl start ‘[email protected]:00:1d.0x2dusbx2d0:1.4:1.1x2dport0.service’
# systemctl status ‘[email protected]:00:1d.0x2dusbx2d0:1.4:1.1x2dport0.service’
[email protected]:00:1d.0x2dusbx2d0:1.4:1.1x2dport0.service – Serial Getty on serial/by-path/pci-0000:00:1d.0-usb-0:1.4:1.1-port0
Loaded: loaded (/lib/systemd/system/[email protected]; static)
Active: active (running) since Mon, 26 Sep 2011 05:08:52 +0200; 1s ago
Main PID: 5788 (agetty)
CGroup: name=systemd:/system/[email protected]/serial-byx2dpath-pcix2d0000:00:1d.0x2dusbx2d0:1.4:1.1x2dport0
└ 5788 /sbin/agetty -s serial/by-path/pci-0000:00:1d.0-usb-0:1.4:1.1-port0 115200 38400 9600

As we can see the while the instance identifier is the escaped
string the command line and the description string actually use the
unescaped version, as expected.

(Side note: there are more specifiers available than just
%i and %I, and many of them are actually
available in all unit files, not just templates for service
instances. For more details see the man
which includes a full list and terse explanations.)

And at this point this shall be all for now. Stay tuned for a
follow-up article on how instantiated services are used for
inetd-style socket activation.


[1] Yupp, this escaping algorithm doesn’t really result in
particularly pretty escaped strings, but then again, most escaping
algorithms don’t help readability. The algorithm we used here is
inspired by what udev does in a similar case, with one change. In the
end, we had to pick something. If you’ll plan to comment on the
escaping algorithm please also mention where you live so that I can
come around and paint your bike shed yellow with blue stripes. Thanks!