SYSTEMD.V(7) systemd.v SYSTEMD.V(7)
NAME
systemd.v - Directory with Versioned Resources
DESCRIPTION
In various places systemd components accept paths whose trailing
components have the ".v/" suffix, pointing to a directory. These
components will then automatically look for suitable files inside the
directory, do a version comparison and open the newest file found (by
version). Available since version v256. Specifically, two expressions
are supported:
• When looking for files with a suffix .SUFFIX, and a path
...PATH/NAME.SUFFIX.v/ is specified, then all files
...PATH/NAME.SUFFIX.v/NAME_*.SUFFIX are enumerated, filtered, sorted
and the newest file used. The primary sorting key is the variable
part, here indicated by the wildcard "*".
• When a path ...PATH.v/NAME___.SUFFIX is specified (i.e. the
penultimate component of the path ends in ".v" and the final
component contains a triple underscore), then all files
...PATH.v/NAME_*.SUFFIX are enumerated, filtered, sorted and the
newest file used (again, by the variable part, here indicated by the
wildcard "*").
To illustrate this in an example, consider a directory
/var/lib/machines/mymachine.raw.v/, which is populated with three files:
• mymachine_7.5.13.raw
• mymachine_7.5.14.raw
• mymachine_7.6.0.raw
Invoke a tool such as systemd-nspawn(1) with a command line like the
following:
# systemd-nspawn --image=/var/lib/machines/mymachine.raw.v --boot
Then this would automatically be resolved to the equivalent of:
# systemd-nspawn --image=/var/lib/machines/mymachine.raw.v/mymachine_7.6.0.raw --boot
Much of systemd's functionality that expects a path to a disk image or
OS directory hierarchy support the ".v/" versioned directory mechanism,
for example systemd-nspawn(1), systemd-dissect(1) or the
RootDirectory=/RootImage= settings of service files (see
systemd.exec(5)).
Use the systemd-vpick(1) tool to resolve ".v/" paths from the command
line, for example for usage in shell scripts.
FILTERING AND SORTING
The variable part of the filenames in the ".v/" directories are filtered
and compared primarily with a version comparison, implementing Version
Format Specification[1]. However, additional rules apply:
• If the variable part is suffixed by one or two integer values
("tries left" and "tries done") in the formats +LEFT or +LEFT-DONE,
then these indicate usage attempt counters. The idea is that each
time before a file is attempted to be used, its "tries left" counter
is decreased, and the "tries done" counter increased (simply by
renaming the file). When the file is successfully used (which for
example could mean for an OS image: successfully booted) the
counters are removed from the file name, indicating that the file
has been validated to work correctly. This mechanism mirrors the
boot assessment counters defined by Automatic Boot Assessment[2].
Any filenames with no boot counters or with a non-zero "tries left"
counter are sorted before filenames with a zero "tries left"
counter.
• Preceding the use counters (if they are specified), an optional CPU
architecture identifier may be specified in the filename (separated
from the version with an underscore), as defined in the architecture
vocabulary of the ConditionArchitecture= unit file setting, as
documented in systemd.unit(5). Files whose name indicates an
architecture not supported locally are filtered and not considered
for the version comparison.
• The rest of the variable part is the version string.
Or in other words, the files in the ".v/" directories should follow one
of these naming structures:
• NAME_VERSION.SUFFIX
• NAME_VERSION_ARCHITECTURE.SUFFIX
• NAME_VERSION+LEFT.SUFFIX
• NAME_VERSION+LEFT-DONE.SUFFIX
• NAME_VERSION_ARCHITECTURE+LEFT.SUFFIX
• NAME_VERSION_ARCHITECTURE+LEFT-DONE.SUFFIX
EXAMPLE
Here's a more comprehensive example, further extending the one described
above. Consider a directory /var/lib/machines/mymachine.raw.v/, which is
populated with the following files:
• mymachine_7.5.13.raw
• mymachine_7.5.14_x86-64.raw
• mymachine_7.6.0_arm64.raw
• mymachine_7.7.0_x86-64+0-5.raw
Now invoke the following command on an x86-64 machine:
$ systemd-vpick --suffix=.raw /var/lib/machines/mymachine.raw.v/
This would resolve the specified path to
/var/lib/machines/mymachine.raw.v/mymachine_7.5.14_x86-64.raw.
Explanation: even though mymachine_7.7.0_x86-64+0-5.raw has the newest
version, it is not preferred because its tries left counter is zero. And
even though mymachine_7.6.0_arm64.raw has the second newest version it
is also not considered in this case, because we operate on an x86_64
system and the image is intended for arm64 CPUs. Finally, the
mymachine_7.5.13.raw image is not considered because it is older than
mymachine_7.5.14_x86-64.raw.
SEE ALSO
systemd(1), systemd-vpick(1), systemd-nspawn(1), systemd-dissect(1),
systemd.exec(5), systemd-sysupdate(8)
NOTES
1. Version Format Specification
https://uapi-group.org/specifications/specs/version_format_specification/
2. Automatic Boot Assessment
https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT/
systemd 257.9 SYSTEMD.V(7)
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