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:

       o   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 "*".

       o   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:

       o   mymachine_7.5.13.raw

       o   mymachine_7.5.14.raw

       o   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:

       o   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.

       o   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.

       o   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:

       o   NAME_VERSION.SUFFIX

       o   NAME_VERSION_ARCHITECTURE.SUFFIX

       o   NAME_VERSION+LEFT.SUFFIX

       o   NAME_VERSION+LEFT-DONE.SUFFIX

       o   NAME_VERSION_ARCHITECTURE+LEFT.SUFFIX

       o   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:

       o   mymachine_7.5.13.raw

       o   mymachine_7.5.14_x86-64.raw

       o   mymachine_7.6.0_arm64.raw

       o   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(1)

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 256.7                                                     SYSTEMD.V(7)