From b82f58bfe396b395bce3452bc0ba2f972fb01ab8 Mon Sep 17 00:00:00 2001
From: Ray Strode <rstrode@redhat.com>
Date: Tue, 9 Aug 2016 10:20:22 -0400
Subject: basic: support default and alternate values for env expansion

Sometimes it's useful to provide a default value during an environment
expansion, if the environment variable isn't already set.

For instance $XDG_DATA_DIRS is suppose to default to:

/usr/local/share/:/usr/share/

if it's not yet set. That means callers wishing to augment
XDG_DATA_DIRS need to manually add those two values.

This commit changes replace_env to support the following shell
compatible default value syntax:

XDG_DATA_DIRS=/foo:${XDG_DATA_DIRS:-/usr/local/share/:/usr/share}

Likewise, it's useful to provide an alternate value during an
environment expansion, if the environment variable isn't already set.

For instance, $LD_LIBRARY_PATH will inadvertently search the current
working directory if it starts or ends with a colon, so the following
is usually wrong:

LD_LIBRARY_PATH=/foo/lib:${LD_LIBRARY_PATH}

To address that, this changes replace_env to support the following
shell compatible alternate value syntax:

LD_LIBRARY_PATH=/foo/lib${LD_LIBRARY_PATH:+:${LD_LIBRARY_PATH}}

[zj: gate the new syntax under REPLACE_ENV_ALLOW_EXTENDED switch, so
existing callers are not modified.]
---
 man/environment.d.xml | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

(limited to 'man/environment.d.xml')

diff --git a/man/environment.d.xml b/man/environment.d.xml
index 4022c25c36..be7758a2f9 100644
--- a/man/environment.d.xml
+++ b/man/environment.d.xml
@@ -78,8 +78,16 @@
     <literal><replaceable>KEY</replaceable>=<replaceable>VALUE</replaceable></literal> environment
     variable assignments, separated by newlines. The right hand side of these assignments may
     reference previously defined environment variables, using the <literal>${OTHER_KEY}</literal>
-    and <literal>$OTHER_KEY</literal> format. No other elements of shell syntax are supported.
-    </para>
+    and <literal>$OTHER_KEY</literal> format. It is also possible to use
+
+    <literal>${<replaceable>FOO</replaceable>:-<replaceable>DEFAULT_VALUE</replaceable>}</literal>
+    to expand in the same way as <literal>${<replaceable>FOO</replaceable>}</literal> unless the
+    expansion would be empty, in which case it expands to <replaceable>DEFAULT_VALUE</replaceable>,
+    and use
+    <literal>${<replaceable>FOO</replaceable>:+<replaceable>ALTERNATE_VALUE</replaceable>}</literal>
+    to expand to <replaceable>ALTERNATE_VALUE</replaceable> as long as
+    <literal>${<replaceable>FOO</replaceable>}</literal> would have expanded to a non-empty value.
+    No other elements of shell syntax are supported.</para>
 
     <para>Each<replaceable>KEY</replaceable> must be a valid variable name. Empty lines
     and lines beginning with the comment character <literal>#</literal> are ignored.</para>
@@ -95,8 +103,8 @@
         <programlisting>
         FOO_DEBUG=force-software-gl,log-verbose
         PATH=/opt/foo/bin:$PATH
-        LD_LIBRARY_PATH=/opt/foo/lib
-        XDG_DATA_DIRS=/opt/foo/share:${XDG_DATA_DIRS}
+        LD_LIBRARY_PATH=${LD_LIBRARY_PATH:+:$LD_LIBRARY_PATH}/opt/foo/lib
+        XDG_DATA_DIRS=/opt/foo/share:${XDG_DATA_DIRS:-/usr/local/share/:/usr/share/}
         </programlisting>
       </example>
     </refsect2>
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