man: add systemd.environment-generator(7) with two examples

v2:
  - add example files to EXTRA_DIST
v3:
  - rework for the new scheme where nothing is written to disk
v4:
  - use separate dirs for system and user env generators

4 files changed, 213 insertions, 1 deletions

diff --git a/man/50-xdg-data-dirs.sh b/man/50-xdg-data-dirs.sh
new file mode 100755
index 0000000000..073174cb40
--- /dev/null
+++ b/man/50-xdg-data-dirs.sh
@@ -0,0 +1,12 @@
+#!/bin/bash
+
+# set the default value
+XDG_DATA_DIRS="${XDG_DATA_DIRS:-/usr/local/share/:/usr/share}"
+
+# add a directory if it exists
+if [[ -d /opt/foo/share ]]; then
+   XDG_DATA_DIRS=/opt/foo/share:${XDG_DATA_DIRS}
+fi
+
+# write our output
+echo XDG_DATA_DIRS=$XDG_DATA_DIRS
diff --git a/man/90-rearrange-path.py b/man/90-rearrange-path.py
new file mode 100755
index 0000000000..c6ff32210f
--- /dev/null
+++ b/man/90-rearrange-path.py
@@ -0,0 +1,40 @@
+#!/usr/bin/python3
+
+"""
+
+Proof-of-concept systemd environment generator that makes sure that bin dirs
+are always after matching sbin dirs in the path.
+(Changes /sbin:/bin:/foo/bar to /bin:/sbin:/foo/bar.)
+
+This generator shows how to override the configuration possibly created by
+earlier generators. It would be easier to write in bash, but let's have it
+in Python just to prove that we can, and to serve as a template for more
+interesting generators.
+
+"""
+
+import os
+import pathlib
+
+def rearrange_bin_sbin(path):
+    """Make sure any pair of …/bin, …/sbin directories is in this order
+
+    >>> rearrange_bin_sbin('/bin:/sbin:/usr/sbin:/usr/bin')
+    '/bin:/sbin:/usr/bin:/usr/sbin'
+    """
+    items = [pathlib.Path(p) for p in path.split(':')]
+    for i in range(len(items)):
+        if 'sbin' in items[i].parts:
+            ind = items[i].parts.index('sbin')
+            bin = pathlib.Path(*items[i].parts[:ind], 'bin', *items[i].parts[ind+1:])
+            if bin in items[i+1:]:
+                j = i + 1 + items[i+1:].index(bin)
+                items[i], items[j] = items[j], items[i]
+    return ':'.join(p.as_posix() for p in items)
+
+if __name__ == '__main__':
+    path = os.environ['PATH'] # This should be always set.
+                              # If it's not, we'll just crash, we is OK too.
+    new = rearrange_bin_sbin(path)
+    if new != path:
+        print('PATH={}'.format(new))
diff --git a/man/systemd.environment-generator.xml b/man/systemd.environment-generator.xml
new file mode 100644
index 0000000000..e162dfcbae
--- /dev/null
+++ b/man/systemd.environment-generator.xml
@@ -0,0 +1,159 @@
+<?xml version='1.0'?> <!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [
+<!ENTITY % entities SYSTEM "custom-entities.ent" >
+%entities;
+]>
+
+<!--
+  This file is part of systemd.
+
+  Copyright 2017 Zbigniew Jędrzejewski-Szmek
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+-->
+
+<refentry id="systemd.environment-generator" xmlns:xi="http://www.w3.org/2001/XInclude">
+  <refentryinfo>
+    <title>systemd.environment-generator</title>
+    <productname>systemd</productname>
+
+    <authorgroup>
+      <author>
+        <contrib>Developer</contrib>
+        <firstname>Zbigniew</firstname>
+        <surname>Jędrzejewski-Szmek</surname>
+        <email>zbyszek@in.waw.pl</email>
+      </author>
+    </authorgroup>
+  </refentryinfo>
+
+  <refmeta>
+    <refentrytitle>systemd.environment-generator</refentrytitle>
+    <manvolnum>7</manvolnum>
+  </refmeta>
+
+  <refnamediv>
+    <refname>systemd.environment-generator</refname>
+    <refpurpose>Systemd environment file generators</refpurpose>
+  </refnamediv>
+
+  <refsynopsisdiv>
+    <cmdsynopsis>
+      <command>&systemenvgeneratordir;/some-generator</command>
+    </cmdsynopsis>
+    <cmdsynopsis>
+      <command>&userenvgeneratordir;/some-generator</command>
+    </cmdsynopsis>
+
+    <para>
+      <literallayout><filename>/run/systemd/system-environment-generators/*</filename>
+<filename>/etc/systemd/system-environment-generators/*</filename>
+<filename>/usr/local/lib/systemd/system-environment-generators/*</filename>
+<filename>&systemenvgeneratordir;/*</filename></literallayout>
+    </para>
+
+    <para>
+      <literallayout><filename>/run/systemd/user-environment-generators/*</filename>
+<filename>/etc/systemd/user-environment-generators/*</filename>
+<filename>/usr/local/lib/systemd/user-environment-generators/*</filename>
+<filename>&userenvgeneratordir;/*</filename></literallayout>
+    </para>
+  </refsynopsisdiv>
+
+  <refsect1>
+    <title>Description</title>
+    <para>Generators are small executables that live in
+    <filename>&systemenvgeneratordir;/</filename> and other directories listed above.
+    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> will
+    execute those binaries very early at the startup of each manager and at configuration
+    reload time, before running the generators described in
+    <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+    and before starting any units. Environment generators can override the environment that the
+    manager exports to services and other processes.</para>
+
+    <para>Generators are loaded from a set of paths determined during compilation, as listed
+    above. System and user environment generators are loaded from directories with names ending in
+    <filename>system-environment-generators/</filename> and
+    <filename>user-environment-generators/</filename>, respectively. Generators found in directories
+    listed earlier override the ones with the same name in directories lower in the list. A symlink
+    to <filename>/dev/null</filename> or an empty file can be used to mask a generator, thereby
+    preventing it from running. Please note that the order of the two directories with the highest
+    priority is reversed with respect to the unit load path, and generators in
+    <filename>/run</filename> overwrite those in <filename>/etc</filename>.</para>
+
+    <para>After installing new generators or updating the configuration, <command>systemctl
+    daemon-reload</command> may be executed. This will re-run all generators, updating environment
+    configuration. It will be used for any services that are started subsequently.</para>
+
+    <para>Environment file generators are executed similarly to unit file generators described
+    in
+    <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
+    with the following differences:</para>
+
+    <itemizedlist>
+      <listitem>
+        <para>Generators are executed sequentially in the alphanumerical order of the final
+        component of their name. The output of each generator output is immediately parsed and used
+        to update the environment for generators that run after that. Thus, later generators can use
+        and/or modify the output of earlier generators.</para>
+      </listitem>
+
+      <listitem>
+        <para>Generators are run by every manager instance, their output can be different for each
+        user.</para>
+      </listitem>
+    </itemizedlist>
+
+    <para>It is recommended to use numerical prefixes for generator names to simplify ordering.</para>
+  </refsect1>
+
+  <refsect1>
+    <title>Examples</title>
+
+    <example>
+      <title>A simple generator that extends an environment variable if a directory exists in the file system</title>
+
+      <programlisting># 50-xdg-data-dirs.sh
+
+<xi:include href="50-xdg-data-dirs.sh" parse="text" /></programlisting>
+    </example>
+
+    <example>
+      <title>A more complicated generator which reads existing configuration and mutates one variable</title>
+
+      <programlisting># 90-rearrange-path.py
+
+<xi:include href="90-rearrange-path.py" parse="text" /></programlisting>
+    </example>
+
+    <example>
+      <title>Debugging a generator</title>
+
+      <programlisting>SYSTEMD_LOG_LEVEL=debug VAR_A=something VAR_B="something else" \
+&systemenvgeneratordir;/path-to-generator
+</programlisting>
+    </example>
+  </refsect1>
+
+  <refsect1>
+    <title>See also</title>
+
+    <para>
+      <citerefentry><refentrytitle>systemd.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+    </para>
+  </refsect1>
+</refentry>
diff --git a/man/systemd.generator.xml b/man/systemd.generator.xml
index b268104c9d..fb0f0c4da8 100644
--- a/man/systemd.generator.xml
+++ b/man/systemd.generator.xml
@@ -342,7 +342,8 @@ find $dir</programlisting>
       <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>systemd-sysv-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
       <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
-      <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+      <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd.environment-generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>
     </para>
   </refsect1>
 </refentry>