Look into environment tweaks for systemd units and desktop files

1 files changed, 58 insertions, 18 deletions

diff --git a/guides/setups/operating-systems/apps.org b/guides/setups/operating-systems/apps.org
index 6bba368..efede03 100644
--- a/guides/setups/operating-systems/apps.org
+++ b/guides/setups/operating-systems/apps.org
@@ -113,34 +113,74 @@ infopath-no-defaults=On
 EOF
 #+end_src
 
-* TODO ~bash~ completions
+* TODO systemd services
+=systemd.unit(5)= § "User Unit Search Path" mentions a couple of
+variables that we could append:
 
-* TODO Desktop entry files
-Prepend =${HOME}/apps/${program}/share= to =XDG_DATA_DIRS=?
+- =$XDG_CONFIG_DIRS/systemd/user/*=
+- =$XDG_DATA_DIRS/systemd/user/*=
 
-* TODO systemd services
-See systemd.unit(5)
+And:
 
-* Putting it all together
-Presenting =~/apps/activate=, to be sourced from =~/.profile=:
+#+begin_quote
+When the variable =$SYSTEMD_UNIT_PATH= is set, the contents of this
+variable overrides the unit load path.  If =$SYSTEMD_UNIT_PATH= ends
+with an empty component (=:=), the usual unit load path will be
+appended to the contents of the variable.
+#+end_quote
 
-#+begin_src bash
-# Hey Emacs; this is a -*- shell-script -*-.
-# Hopefully invoked by bash 🤞
+So we could either:
+
+1. find the directory 𝒟 where an app tucks =systemd/user= in its
+   installation tree, and append 𝒟 to one of the XDG variables,
+2. find the directory where an app tucks its =.service= files, and
+   append that directory to =SYSTEMD_UNIT_PATH=, keeping a final
+   colon.
+
+(1) could feed multiple birds with one scone: in particular,
+=XDG_DATA_DIRS= is used for other purposes (see § Desktop entry
+files).  Serendipitously, [[https://github.com/systemd/systemd/blob/1d87a00a951dca801c8ccd79c1460fa91efa7dce/src/basic/path-lookup.c#L147][the systemd sources]] suggest =DATA= over
+=CONFIG= for our purposes:
+
+#+begin_src c
+        /* Implement the mechanisms defined in
+         *
+         * https://standards.freedesktop.org/basedir-spec/basedir-spec-0.6.html
+         *
+         * We look in both the config and the data dirs because we
+         * want to encourage that distributors ship their unit files
+         * as data, and allow overriding as configuration.
+         */
+#+end_src
 
-_apps_dir=$(dirname ${BASH_SOURCE})
+* TODO Desktop entry files
+The Freedesktop [[https://specifications.freedesktop.org/menu-spec/menu-spec-latest.html][Desktop Menu]] and [[https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html][Desktop Entry]] specs spell out how
+=XDG_DATA_DIRS= will be searched for =applications/*.desktop= files.
 
-_apps_PATH=$(
-    shopt -s nullglob
-    bins=( "${_apps_dir}"/*/bin )
-    IFS=: eval 'echo "${bins[*]}"'
-)
+* TODO ~bash~ completions
 
-export PATH=${_apps_PATH}:${PATH}
-#+end_src
+* Putting it all together
+Presenting [[./apps-demo/activate][=~/apps/activate=]], to be sourced from =~/.profile=:
+
+#+INCLUDE: "apps-demo/activate" src sh
 
 * Resources
 ** [[https://nullprogram.com/blog/2017/06/19/][nullprogram.com]] — Building and Installing Software in $HOME
 Explains how to set things up so that one can use =~/.local= as a
 first-class citizen for applications, libraries and development
 resources (headers, pkg-config, manpages).
+
+** [[https://specifications.freedesktop.org][specifications.freedesktop.org]] — Freedesktop.org Specifications
+Hosts the specifications for the behavior and environment variables
+most GNU/Linux distros (that I have experience with) adhere to, e.g.
+
+- the [[https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html][Base Directory spec]] :: defines the various
+  =XDG_*_@(HOME|DIR|DIRS)= variables referenced here;
+- the [[https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html][Desktop Entry spec]] :: explains the significance of a desktop
+  file's filename with respect to its position within =XDG_DATA_DIRS=;
+- the [[https://specifications.freedesktop.org/menu-spec/menu-spec-latest.html][Desktop Menu spec]] :: defines the expected file locations for
+  desktop files (and more) with respect to XDG base directories.
+
+** [[https://wiki.archlinux.org/title/Environment_variables#Per_user][wiki.archlinux.org]] — Environment variables § Per user - ArchWiki
+A thorough overview of the myriad of ways users can amend environment
+variables, as examined in § PATH.