Implement environment variable interpolation in defaults files.

This allows the syntax `${HOME}` to be used, in fields that expect
file paths only.  Any environment variable may be interpolated
in this way. A warning will be raised for undefined variables.
The special variable `USERDATA` is automatically set to the
user data directory in force when the defaults file is parsed.
(Note: it may be different from the eventual user data directory,
if the defaults file or further command line options change that.)

Closes #5982.
Closes #5977.
Closes #6108 (path not taken).

1 files changed, 15 insertions, 0 deletions

diff --git a/MANUAL.txt b/MANUAL.txt
index d97cbcbc9..f06293dd3 100644
--- a/MANUAL.txt
+++ b/MANUAL.txt
@@ -1672,6 +1672,21 @@ one line:
 verbosity: INFO
 ```
 
+In fields that expect a file path (or list of file paths), the
+following syntax may be used to interpolate environment variables:
+
+``` yaml
+csl:  ${HOME}/mycsldir/special.csl
+```
+
+`${USERDATA}` may also be used; this will always resolve to the
+user data directory that is current when the defaults file is
+parsed, regardless of the setting of the environment
+variable `USERDATA`.
+
+This environment variable interpolation syntax *only* works in
+fields that expect file paths.
+
 Default files can be placed in the `defaults` subdirectory of
 the user data directory and used from any directory.  For
 example, one could create a file specifying defaults for writing