Universal Wayland Session Manager

Wraps standalone Wayland compositors into a set of Systemd units on the fly. This provides robust session management including environment, XDG autostart support, bi-directional binding with login session, and clean shutdown.

For compositors this is an opportunity to offload Systemd integration and session/XDG autostart management in Systemd-managed environments.

Important

This project is currently in a stable phase with a slow-burning refactoring. Although no drastic changes are planned, keep an eye for commits with breaking changes, indicated by an exclamation point (e.g. fix!: ..., chore!: ..., feat!: ..., etc.).

Important

v0.19.x introduced some breaking changes:

• Various resources (config, environment, and runtime files) are consolidated into uwsm subdirs. Warnings will be issued for encountered old paths, and v0.20.0 will drop them. Please migrate.
• Automatic finalization mechanism: it removes the necessity to run uwsm finalize in compositors which put variables to systemd activation environment themselves. v0.20.0 will stop preventing compositors from doing that. To test this behavior in v0.19.1 export UWSM_AUTOFINALIZE_TEST=true (in shell profile or environment.d). More about the algorithm in second installation section

Note

It is highly recommended to use dbus-broker as the D-Bus daemon implementation. Among other benefits, it reuses the systemd activation environment instead of having its own separate one. This simplifies environment management and allows proper cleanup. The separate activation environment of the reference D-Bus implementation doesn't allow unsetting vars, so they're set to an empty string instead, as a best effort cleanup. The only way to properly clean up the environment in this case is to run loginctl terminate-user "".

Concepts and features

Uses systemd units and dependencies for startup, operation, and shutdown.

• Binds to the basic structure of graphical-session-pre.target, graphical-session.target, xdg-desktop-autostart.target.
• Adds custom nested slices app-graphical.slice, background-graphical.slice, session-graphical.slice to put apps in and terminate them cleanly on exit.
• Provides convenient way of launching apps to those slices.

Systemd units are treated with hierarchy and universality in mind.

• Templated units with specifiers.
• Named from common to specific where possible.
• Allowing for high-level name-.d drop-ins.

Bi-directional binding between login session and graphical session.

Using waitpid utility (or a built-in shim) together with native systemd mechanisms, uwsm binds lifetime of a login session (session-N.scope system unit) to graphical session (a set of user units) and vice versa.

Compositor-specific behavior is adjustable by plugins.

Currently included:

• sway
• wayfire
• labwc
• hyprland

Idempotently (well, best-effort-idempotently) handles environment.

• On startup a specialized unit prepares environment by:
  • sourcing shell profile
  • sourcing uwsm/env, uwsm/env-${desktop} files from each dir of reversed sequence ${XDG_CONFIG_HOME}:${XDG_CONFIG_DIRS}:${XDG_DATA_DIRS} (in increasing priority), where ${desktop} is each item of ${XDG_CURRENT_DESKTOP}, lowercased
• Difference between environment state before and after preparation is exported into systemd user manager (and dbus activation environment if reference dbus implementation is used)
• On shutdown previously exported variables are unset from systemd user manager (activation environment of reference dbus daemon does not support unsetting, so those vars are emptied instead (!))
• Lists of variables for export and cleanup are determined algorithmically by:
  • comparing environment before and after preparation procedures
  • boolean operations with predefined lists
  • manually exported vars by uwsm finalize action

Summary of where to put a user-level var:

• For entire user's context: define in ${XDG_CONFIG_HOME}/environment.d/*.conf (see man 5 environment.d)
• For login session context and uwsm environment preloader, including plugins: export in ~/.profile (may have caveats, see your shell's manual)
• For uwsm-managed graphical session: export in ${XDG_CONFIG_HOME}/uwsm/env
• For uwsm-managed graphical session of specific compositor: export in ${XDG_CONFIG_HOME}/uwsm/env-${desktop}

Also for convenience environment preloader defines IN_UWSM_ENV_PRELOADER=true variable (not exported), which can be probed from shell profile to do things conditionally.

Can work with Desktop entries from `wayland-sessions` in XDG data hierarchy and/or be included in them.

• Actively select and launch compositor from Desktop entry (which is used as compositor instance ID):
  • Data taken from entry (Can be amended or overridden via cli arguments):
    • Exec for argument list
    • DesktopNames for XDG_CURRENT_DESKTOP and XDG_SESSION_DESKTOP
    • Name and Comment for unit Description
  • Entries can be overridden, masked or added in ${XDG_DATA_HOME}/wayland-sessions/
  • Optional interactive selector (requires whiptail), choice is saved in ${XDG_CONFIG_HOME}/uwsm/default-id, default is read from it, falling back all the way through ${XDG_CONFIG_DIRS}:${XDG_DATA_DIRS}
  • Desktop entry actions are supported
• Be launched via a Desktop entry by a login/display manager.

Can run with arbitrary compositor command line, or take it (along with other data) from desktop entries (saved as a unit drop-in).

wayland-wm-env@${compositor}.service.d/50_custom.conf
wayland-wm@${compositor}.service.d/50_custom.conf

Provides better control of XDG autostart apps.

• XDG autostart services (app-*@autostart.service units) are placed into app-graphical.slice that receives stop action before compositor is stopped.
• Can be mass-controlled via stopping and starting wayland-session-xdg-autostart@${compositor}.target

Tries best to shutdown session cleanly via a net of dependencies between units.

All managed transient files (in /run/user/${UID}/systemd/user):

background-graphical.slice
app-graphical.slice
session-graphical.slice
app-@autostart.service.d/slice-tweak.conf
wayland-session-pre@.target
wayland-session-shutdown.target
wayland-session-xdg-autostart@.target
wayland-session@.target
wayland-wm-app-daemon.service
wayland-wm-env@.service
wayland-wm-env@${compositor}.service.d/50_custom.conf
wayland-wm@.service
wayland-wm@${compositor}.service.d/50_custom.conf
wayland-session-bindpid@.service
wayland-session-waitenv.service

See Longer story section below for descriptions.

Provides helpers and tools for various operations.

• uwsm finalize: for explicitly exporting variables to activatoin environments and signal compositor's unit readiness (compositor service unit uses Type=notify)
• uwsm check may-start: for checking conditions for launch at login (for integration into login shell profile)
• uwsm app: for launching applications as scopes or services in proper slices
  • desktop entries or plain executables are supported
  • support for launching a terminal/in terminal (proposed xdg-terminal-exec)
  • flexible unit metadata support
• uwsm-app: a simple and fast shell client to app-daemon feature of uwsm, a drop-in replacement of uwsm app. The daemon (started on-demand) handles finding requested desktop entries, parsing and generation of commands for client to execute. This avoids the overhead of repeated python startup and increases app launch speed.
• uuctl: graphical (via dmenu-like menus) tool for managing user units.
• fumon: background service for notifying about failed units (disabled by default, enable with systemctl)

Installation

1. Building and installing

Checkout the last version-tagged commit. Untagged commits are WIP.

Building and installing the python project directly.

meson setup --prefix=/usr/local -Duuctl=enabled -Dfumon=enabled -Duwsm-app=enabled build
meson install -C build

The example enables optional tools uuctl, fumon, and uwsm-app available in this project (see helpers and tools spoiler in concepts section above).

Building and installing a deb package.

Read and run ./build-deb.sh -i

Alternatively,

IFS='()' read -r _ current_version _ < debian/changelog
sudo apt install devscripts
mk-build-deps
sudo apt install --mark-auto ./uwsm-build-deps_${current_version}_all.deb
dpkg-buildpackage -b -tc --no-sign
sudo apt install ../uwsm_${current_version}_all.deb

Arch AUR package.

https://aur.archlinux.org/packages/uwsm

NixOS package.

https://github.com/NixOS/nixpkgs/tree/master/pkgs/by-name/uw/uwsm/package.nix

Runtime dependencies:

• python modules:
  • xdg (pyxdg)
  • dbus (dbus_python)
• waitpid (optional, but recommended for resources; from util-linux or util-linux-extra package)
• whiptail (optional, for select feature; from whiptail or libnewt package)
• a dmenu-like menu (optional; for uuctl script), supported:
  • fuzzel
  • wofi
  • rofi
  • tofi
  • bemenu
  • wmenu
  • dmenu
• notify-send (optional, for feedback messages; libnotify-bin or libnotify package)

2. Service startup notification and vars set by compositor

Potentially tricky part.

TLDR; if your compositor puts WAYLAND_DISPLAY (and along with it DISPLAY, any other important or useful variables) into systemd activation environment, uwsm will make everything work automagically, proceed to section 3.

Otherwise configure compositor to run uwsm finalize command at the end of its startup. It will deal with putting WAYLAND_DISPLAY and DISPLAY (if set) variables into activation environments in the best possible ways and signal unit readiness to systemd.

If compositor is known to set useful vars but they are missing from activation environments.

List names of variable as arguments to uwsm finalize, or append them to whitespace-separated list in UWSM_FINALIZE_VARNAMES variable (do it beforehand, i.e. in env files or shell profile).

Example snippet for sway config (these vars are already covered by sway plugin via UWSM_FINALIZE_VARNAMES var and listed here just for clearness):

exec exec uwsm finalize SWAYSOCK I3SOCK XCURSOR_SIZE XCURSOR_THEME

Undefined variables will be are silently ignored.

If compositor signals unit readiness prematurely or puts other vars into activation environments later than `WAYLAND_DISPLAY`, too late for downstream units to get.

Append names of variables to whitespace-separated list in UWSM_WAIT_VARNAMES variable (do it beforehand, i.e. in env files or shell profile). This will make uwsm delay graphical session startup until those vars appear in systemd activation environment.

Depending on situation, combine this with with uwsm finalize command (see previous spoiler) to put more variables into activation environments and gain more control over delay mechanism of uwsm.

Be aware that uwsm finalize skips undefined vars, so be sure that all vars listed in UWSM_WAIT_VARNAMES are really being set, or use explicit assignment to serve as a marker. Example:

# in env file:
UWSM_WAIT_VARNAMES="${UWSM_WAIT_VARNAMES} FINALIZED"

# in compositor's autostart:
uwsm finalize FINALIZED="I'm here" SWAYSOCK I3SOCK XCURSOR_SIZE XCURSOR_THEME

You can also tweak UWSM_WAIT_VARNAMES_SETTLETIME (float, default: 0.2) to change pause duration after all expected vars are found.

Technical details

Inside wayland-wm@${compositor}.service before executing compositor itself, uwsm forks a process that probes systemd activation environment for WAYLAND_DISPLAY var and vars listed in UWSM_WAIT_VARNAMES variable (whitespace-separated). When all expected vars appear, it pauses for UWSM_WAIT_VARNAMES_SETTLETIME seconds (float, default: 0.2) and signals unit readiness. It also updates cleanup list with delta between states of activation environment at unit startup time and the end of settle pause. If classic dbus implementation is used, this delta is also synched to its activation environment.

A separate unit, wayland-session-waitenv.service is launched alongside compositor, with similar ordering after graphical-session-pre.target, before graphical-session.target. It also waits for the same variables in the same manner, then successfully exits (or times out). Its job is to dealy graphical-session.target activation in case compositor signals its readiness prematurely. Or to fail startup if expected vars do not appear.

uwsm finalize command fills systemd and dbus environments with essential vars set by compositor: WAYLAND_DISPLAY (mandatory) and DISPLAY (if present). Optional vars are taken by name from arguments and UWSM_FINALIZE_VARNAMES var, which is also pre-filled by plugins. Dbus implementation quirks are handled. Undefined vars are silently ignored. Any exported variables are also added to cleanup list.

Timeout for unit startup is 10 seconds.

3. Applications and Slices

To properly put applications in app-graphical.slice (or like), Configure application launching in compositor via:

uwsm app -- {executable|entry.desktop[:action]} [args ...]

When app launching is properly configured, compositor service itself can be placed in session.slice by either:

• Setting environment variable UWSM_USE_SESSION_SLICE=true before generating units. Best places to put this:
  • export in ~/.profile before uwsm invocation
  • put in ~/.config/environment.d/*.conf (see man environment.d)
• Adding -S argument to uwsm start subcommand.

Background and details

By default uwsm launhces compositor service in app.slice and all processes spawned by compositor will be a part of wayland-wm@${compositor}.service unit. This works, but is not an optimal solution.

Systemd documentation recommends running compositors in session.slice and launch apps as scopes or services in app.slice.

uwsm provides convenient way of handling this: it generates special nested slices that will also receive stop action ordered before wayland-wm@${compositor}.service shutdown:

• app-graphical.slice
• background-graphical.slice
• session-graphical.slice

app-*@autostart.service units are also modified to be started in app-graphical.slice.

To launch an app inside one of those slices, use:

uwsm app [-s a|b|s|custom.slice] [-t scope|service] -- your_app [with args]

Launching desktop entries via a valid ID is also supported, (optionally with an action ID appended via :):

uwsm app [-s a|b|s|custom.slice] [-t scope|service] -- your_app.desktop[:action] [with args]

In this case args must be supported by the entry or its selected action according to XDG Desktop Entry Specification.

Specifying paths to executables or desktop entry files is also supported.

Always use -- to disambiguate command line if any dashed arguments are intended for the app being launched.

Example snippets for sway config for launching apps:

Launch proposed default terminal:

bindsym --to-code $mod+t exec exec uwsm app -T

Fuzzel has a very handy launch-prefix option:

bindsym --to-code $mod+r exec exec fuzzel --launch-prefix='uwsm app --' --log-no-syslog --log-level=warning

Launch SpaceFM via a desktop entry:

bindsym --to-code $mod+e exec exec uwsm app spacefm.desktop

Featherpad desktop entry has "standalone-window" action:

bindsym --to-code $mod+n exec exec uwsm app featherpad.desktop:standalone-window

Unit type of launched apps can be controlled by -t service|scope argument or setting its default via UWSM_APP_UNIT_TYPE env var.

Operation

Syntax and behavior

-h|--help option is available for uwsm and all of its subcommands.

Basics:

uwsm start [options] -- ${compositor} [arguments]

Always use -- to disambiguate command line if any dashed arguments are intended for launched compositor.

${compositor} can be an executable or a valid desktop entry ID (optionally with an action ID appended via ':'), or one of special values: select|default

If ${compositor} is given as a path, or -F option is given, "hardcode" mode is engaged: the resulting command line will always be written to unit drop-ins and contain full path to executable as seen by uwsm start. Path to executable will also be written if encountered in a desktop entry's Exec.

Optional parameters to provide more metadata:

• -[a|e]D DesktopName1[:DesktopMame2:...]: append (-a) or exclusively set (-e) ${XDG_CURRENT_DESKTOP}
• -N Name
• -C "Compositor description"

Arguments and metadata are stored in specifier unit drop-ins if needed.

uwsm start ... command will wait until graphical session ends, also holding open the login session it resides in. Graphical session will also deactivate if process that started it ends.

Some details

uwsm start [-[a|e]D DesktopName1[:DesktopMame2:...]] [-N Name] [-C "Compositor description"] [-F] -- ${compositor} [with "any complex" --arguments]

If ${compositor} is a desktop entry ID, uwsm will find it in wayland-sessions data hierarchy. Exec will be used for command line, and DesktopNames will fill $XDG_CURRENT_DESKTOP, Name and Comment will go to units' description.

Arguments provided on command line are appended to the command line from session's desktop entry (unlike application entries), no argument processing is done (Please file a bug report if you encounter any wayland-sessions desktop entry with %-fields which would require this behavior to be altered).

If you want to customize compositor execution provided with a desktop entry, copy it to ~/.local/share/wayland-sessions/ and change to your liking, including adding actions

If ${compositor} is select or default, uwsm invokes a menu to select desktop entries available in wayland-sessions data hierarchy (including their actions). Selection is saved, previous selection is highlighted (or launched right away in case of default). Selected entry is used as instance ID.

There is also a separate select action (uwsm select) that only selects and saves default ${compositor} and does nothing else, which is handy for seamless shell profile integration.

Things uwsm start ... will do:

• Prepare unit structure in runtime directory.
• Fork a process protected from TERM and HUP signals that will find future compositor unit's MainPID and wait for it to end, ensuring login session is kept open until graphical session ends.
• Start wayland-session-bindpid@.service unit pointing to uwsm's own PID to rig graphical session shutdown in case uwsm (or login session) ends.
• Finally, replace itself with systemctl command which will actually start the compositor unit and wait while wayland session is running.

Where to launch from

Shell profile integration

To launch automatically after login on virtual console 1, if systemd is at graphical.target, add this to shell profile:

if uwsm check may-start && uwsm select; then
	exec systemd-cat -t uwsm_start uwsm start default
fi

uwsm check may-start checker subcommand, among other things, screens for being in interactive login shell, which is essential, since profile sourcing can otherwise lead to nasty loops.

uwsm start select shows whiptail menu to select default desktop entry from wayland-sessions directories. At this point one can cancel and continue with the normal login shell.

exec in shell profile causes uwsm (via systemd-cat) to replace login shell, binding it to user's login session.

systemd-cat -t uwsm_start part is optional, it executes the command given to it (uwsm) with its stdout and stderr connected to the systemd journal, tagged with identifier uwsm_start. Otherwise it might be hard to see the output.

uwsm start default launches the previously selected default compositor.

From a display manager

To launch uwsm from a display/login manager, uwsm can be used inside desktop entries. Example /usr/local/share/wayland-sessions/my-compositor.desktop:

[Desktop Entry]
Name=My compositor (with UWSM)
Comment=My cool compositor
Exec=uwsm start -N "My compositor" -D mycompositor -C "My cool compositor" mywm
DesktopNames=mycompositor
Type=Application

Things to keep in mind:

• For consistency, command line arguments should mirror the keys of the entry
• Command in Exec= should start with uwsm start
• It should not point to itself (as a combination of Desktop Entry ID and Action ID)
• It should not point to a Desktop Entry ID and Action ID that also uses uwsm

Potentially such entries may be found and used by uwsm itself, i.e. in shell profile integration situation, or when launched manually. Following the principles above ensures uwsm will properly recognize itself and parse requested arguments inside the entry without any side effects.

Some display managers may fail to handle quoting correctly. Workaround in this case is to use single-word arguments.

Aternatively, if a display manager supports wrapper commands/scripts, uwsm can be inserted there to receive either Entry and Action IDs, or a parsed command line.

Testing and feedback is needed.

How to stop

Either of:

• loginctl terminate-user "" (this ends all login sessions and units of current user, good for resetting everything, including runtime units, environments, etc.)
• loginctl terminate-sesion "$XDG_SESSION_ID" (this ends login session that uwsm was launched in, special unit wayland-session-bindpid@.service waiting for the former login shell process will exit and bring down graphical session units. Empty argument will only work if loginctl is called from within login session scope itself, so variable should be used when calling from graphical session units)
• uwsm stop (brings down graphical session units. Login session will end if uwsm start replaced login shell)
• systemctl --user stop wayland-wm@*.service (effectively the same as previous one)

Longer story, tour under the hood

Some extended examples and partial recreation of some behaviors via excessive shell code, just for deeper explanation.

Dive

Start and bind

(At least for now) units are generated by the script.

Run uwsm start -o ${compositor} to populate ${XDG_RUNTIME_DIR}/systemd/user/ with them and do nothing else (-o).

Any remainder arguments are appended to compositor argument list (even when ${compositor} is a desktop entry). Use -- to disambigue:

uwsm start -o -- ${compositor} with "any complex" --arguments

Desktop entries can be overridden or added in ${XDG_DATA_HOME}/wayland-sessions/.

Basic set of generated units:

• templated targets boud to stock systemd user-level targets
  • wayland-session-pre@.target
  • wayland-session@.target
  • wayland-session-xdg-autostart@.target
• templated services
  • wayland-wm-env@.service - environment preloader service
  • wayland-wm@.service - main compositor service
  • wayland-wm-app-daemon.service - fast app command generator
• slices for apps nested in stock systemd user-level slices
  • app-graphical.slice
  • background-graphical.slice
  • session-graphical.slice
• tweaks
  • wayland-wm-env@${compositor}.service.d/50_custom.conf, wayland-wm@${compositor}.service.d/50_custom.conf - if arguments and/or various names, path to executable were given on command line, they go here.
  • app-@autostart.service.d/slice-tweak.conf - assigns XDG autostart apps to app-graphical.slice
• shutdown and cleanup units
  • wayland-session-bindpid@.service - starts waitpid utility for a given PID. Invokes wayland-session-shutdown.target when deactivated. uwsm start starts this unit pointing to itself just before replacing itself with systemctl unit startup command.
  • wayland-session-shutdown.target - conflicts with operational units. Triggered by deactivation of wayland-wm*@*.service and wayland-session-bindpid@*.service units, both successful or failed. But can also be called manually for shutdown.

After units are generated, compositor can be started by: systemctl --user start wayland-wm@${compositor}.service

But this would run it completely disconnected from a login session or any process that started it. To fix that use wayland-session-bindpid@.service to track PID of login shell ($$) and stop graphical session when it exits:

systemctl --user start wayland-session-bindpid@$$.service

Add --wait to hold the terminal until session ends, exec it to replace login shell with systemctl invocation reusing its PID:

exec systemctl --user start --wait wayland-wm@${compositor}.service

This makes the end of login shell also be the end of wayland session and vice versa.

When wayland-wm-env@.service is started during graphical-session-pre.target startup, uwsm aux prepare-env ${compositor} is launched (with shared set of custom arguments).

It runs shell code to prepare environment, that sources shell profile, uwsm/env* files, anything that plugins dictate. Environment state at the end of shell code is given back to the main process. uwsm is also smart enough to find login session associated with current TTY and set $XDG_SESSION_ID, $XDG_VTNR.

The difference between initial env (that is the state of activation environment) and after all the sourcing and setting is done, plus Varnames.always_export, minus Varnames.never_export, is added to activation environment of systemd user manager and dbus.

Those variable names, plus Varnames.always_cleanup minus Varnames.never_cleanup are written to a cleanup list file in runtime dir.

Startup finalization

This step is not needed if compositor at least puts WAYLAND_DISPLAY into systemd activation environment: uwsm will autodetect this and handle the rest. If something goes wrong, startup can be fixed by using combination of uwsm finalize command and configuration variables UWSM_FINALIZE_VARNAMES, UWSM_WAIT_VARNAMES, UWSM_WAIT_VARNAMES_SETTLETIME

wayland-wm@.service uses Type=notify and waits for compositor to signal started state. Activation environments will also need to receive essential variables like WAYLAND_DISPLAY to launch graphical applications successfully.

A forked process inside wayland-wm@.service waits for WAYLAND_DISPLAY and all vars mentioned in UWSM_WAIT_VARNAMES, then signals unit readiness. It also appends variable cleanup list with any delta it has seen since unit startup.

Separate wayland-session-waitenv.service does the same waiting thing and either exits successfully allowing graphical-session.target to proceed, or times out, bringing everything down.

uwsm finalize [VAR [VAR2...]] can be run by compositor, essentially it performs actions analogous to:

dbus-update-activation-environment WAYLAND_DISPLAY DISPLAY [VAR [VAR3...]]
systemctl --user import-environment WAYLAND_DISPLAY DISPLAY [VAR [VAR3...]]
systemd-notify --ready

(dbus-update-activation-environment action equivalent is redundant for dbus-broker and is skipped automatically)

Additional variable names are taken from UWSM_FINALIZE_VARNAMES var.

Only defined variables are used. Variables that are not blacklisted by Varnames.never_cleanup set are also added to cleanup list in the runtime dir.

Stop

Just stop the main service: systemctl --user stop "wayland-wm@${compositor}.service", everything else will stopped by systemd.

Wildcard systemctl --user stop "wayland-wm@*.service" will also work, as does stopping wayland-session@*.target

Or activate shutdown target: systemctl --user start wayland-session-shutdown.target

If an instance of wayland-session-bindpid@.service is active and pointing to a PID in login session, any of the above stop commands also doubles as a logout command.

When wayland-wm-env@${compositor}.service is stopped, uwsm aux cleanup-env is launched. It looks for any cleanup files (uwsm/env_cleanup_*.list) in runtime dir. Listed variables, plus Varnames.always_cleanup minus Varnames.never_cleanup are emptied in dbus activation environment and unset from systemd user manager environment.

When no compositor is running, units can be removed (-r) by uwsm stop -r.

Add compositor to -r to remove only customization drop-ins: uwsm stop -r ${compositor}.

Profile integration

This example does the same thing as check may-start + start subcommand combination described earlier: starts wayland session automatically upon login on tty1 if system is in graphical.target

Screening for being in interactive login shell here is essential ([ "${0}" != "${0#-}" ]). wayland-wm-env@${compositor}.service sources profile, which has a potential for nasty loops if run unconditionally. Other conditions are a recommendation:

MY_COMPOSITOR=sway
if [ "${0}" != "${0#-}" ] &&
   ! systemctl --user is-active -q wayland-wm@*.service &&
   [ "$XDG_VTNR" = "1" ] &&
   {
       # wait while graphical.target is in startup queue
       while case "$(systemctl list-jobs --plain --no-legend --full graphical.target)" in
       *start*) true ;; *) false ;; esac; do
         sleep 1
       done
       systemctl is-active -q graphical.target
   }
then
    # generate units
    uwsm start -o ${MY_COMPOSITOR}
    # bind wayland session to login shell PID $$
    echo Starting ${MY_COMPOSITOR} compositor
    systemctl --user start wayland-session-bindpid@$$.service &&
    exec systemctl --user start --wait wayland-wm@${MY_COMPOSITOR}.service
fi

uwsm start also has a mechanism that holds the login session open until the compositor unit is deactivated. It works by forking a process immune to TERM and HUP signals inside login session. This process finds compositor unit's MainPID and waits until it ends. This mechanism would be too complicated to replicate in shell for purposes of this demonstration.

Compositor-specific actions

Shell plugins provide compositor-specific functions during environment preparation.

Named ${__WM_BIN_ID__}.sh, they should only contain specifically named functions.

${__WM_BIN_ID__} is derived from the item 0 of compositor command line by applying s/(^[^a-zA-Z]|[^a-zA-Z0-9_])+/_/ and converting to lower case.

It is used as plugin id and suffix in function names.

Variables available to plugins:

• __WM_ID__ - compositor ID, effective first argument of start.
• __WM_ID_UNIT_STRING__ - compositor ID escaped for systemd unit name.
• __WM_BIN_ID__ - processed first item of compositor argv.
• __WM_DESKTOP_NAMES__ - :-separated desktop names from DesktopNames= of entry and -D cli argument.
• __WM_FIRST_DESKTOP_NAME__ - first of the above.
• __WM_DESKTOP_NAMES_LOWERCASE__ - same as the above, but in lower case.
• __WM_FIRST_DESKTOP_NAME_LOWERCASE__ - first of the above.
• __WM_DESKTOP_NAMES_EXCLUSIVE__ - (true|false) indicates that __WM_DESKTOP_NAMES__ came from cli argument and are marked as exclusive.
• __OIFS__ - contains shell default field separator (space, tab, newline) for convenient restoring.

Standard functions:

• load_wm_env - standard function for loading env files
• process_config_dirs - called by load_wm_env, iterates over whole XDG Config and system XDG Data hierarchies (decreasing priority)
• in_each_config_dir - called by process_config_dirs for each config dir, does nothing ATM
• process_config_dirs_reversed - called by load_wm_env, same as process_config_dirs, but in reverse (increasing priority)
• in_each_config_dir_reversed - called by process_config_dirs_reversed for each config dir, loads uwsm/env, uwsm/env-${desktop} files
• source_file - sources $1 file, providing messages for log.

See code inside uwsm/main.py for more auxillary funcions.

Functions that can be added by plugins, replacing standard funcions:

• quirks__${__WM_BIN_ID__} - called before env loading.
• load_wm_env__${__WM_BIN_ID__}
• process_config_dirs_reversed__${__WM_BIN_ID__}
• in_each_config_dir_reversed__${__WM_BIN_ID__}
• process_config_dirs__${__WM_BIN_ID__}
• in_each_config_dir__${__WM_BIN_ID__}

Original functions are still available for calling explicitly if combined effect is needed.

Example:

#!/bin/false

# function to make arbitrary actions before loading environment
quirks__my_cool_wm() {
  # here additional vars can be set or unset
  export I_WANT_THIS_IN_SESSION=yes
  unset I_DO_NOT_WANT_THAT
  # or prepare a config for compositor
  # or set a var to modify what sourcing uwsm/env, uwsm/env-${__WM_ID__}
  # in the next stage will do
  ...
  # add a var to be exported by uwsm finalize:
  UWSM_FINALIZE_VARNAMES="${UWSM_FINALIZE_VARNAMES}${UWSM_FINALIZE_VARNAMES:+ }ANOTHER_VAR1 ANOTHER_VAR2"
  # add a var to wait and depend on before graphical session:
  UWSM_WAIT_VARNAMES="${UWSM_WAIT_VARNAMES}${UWSM_WAIT_VARNAMES:+ }ANOTHER_VAR1 ANOTHER_VAR2"
}

in_each_config_dir_reversed__my_cool_wm() {
  # custom mechanism for loading of env files (or a stub)
  # replaces standard function, but we want it also
  # so call it explicitly
  in_each_config_dir_reversed "$1"
  # and additionally source our file
  source_file "${1}/${__WM_ID__}/env"
}

Compliments

Inspired by and adapted some techniques from:

• sway-services
• sway-systemd
• sway
• Presentation by Martin Pitt

Special thanks to @skewballfox for help with python and pointing me to useful tools.