Clean way to temporarily replace a config file?

Question

I use a drawing program called Inkscape, which has both a GUI and a command line interface. When used on the command line, it has a large number of options that can only be controlled through a user-specific config file, which is hardcoded to be:

$HOME/.config/inkscape/preferences.xml

This config file always contains the options that were most recently used in the GUI, which may be the wrong ones when I'm scripting.

To work around this, I have my script save a copy of the config file, replace it with a standard config file, run the program, and then copy the saved config file back.

This works OK but is not really clean. For example, it won't work properly if two instances of the script are being run concurrently.

On Unix, is there a cleaner way to carry out this task of faking out a program so it takes its config file from someplace that I want, rather than from the pathname hardcoded in the program? Maybe something involving links, or something like BSD jails?

Answer 1

Inkscape has a feature for this as of 0.47:

$ INKSCAPE_PORTABLE_PROFILE_DIR=/some/other/path inkscape --args

Put your script's custom preferences.xml file in /some/other/path. It should be a dedicated directory, because Inkscape will populate it with all the other files it normally puts in ~/.config/Inkscape when you run it like this.

Answer 2

Because Inkscape is FOSS software, we can just add an option to the program which will let you pass the name of another config file, like so:

=== modified file 'src/inkscape.cpp'
--- src/inkscape.cpp    2013-09-28 19:20:27 +0000
+++ src/inkscape.cpp    2013-11-02 04:07:45 +0000
@@ -1443,6 +1443,12 @@
             prefdir = g_strdup(val);
         }

+        // Also accept an override via the command line
+        extern gchar* sp_preferences;
+        if (sp_preferences) {
+            prefdir = sp_preferences;
+        }
+
 #ifdef HAS_SHGetSpecialFolderLocation
         // prefer c:\Documents and Settings\UserName\Application Data\ to
         // c:\Documents and Settings\userName\;

=== modified file 'src/main.cpp'
--- src/main.cpp    2013-09-24 18:31:44 +0000
+++ src/main.cpp    2013-11-02 04:05:30 +0000
@@ -179,6 +179,7 @@
     SP_ARG_VERB_LIST,
     SP_ARG_VERB,
     SP_ARG_SELECT,
+    SP_ARG_PREFERENCES,
     SP_ARG_LAST
 };

@@ -228,6 +229,7 @@
 static gboolean sp_query_all = FALSE;
 static gchar *sp_query_id = NULL;
 static gboolean sp_shell = FALSE;
+gchar *sp_preferences = NULL;
 static gboolean sp_vacuum_defs = FALSE;
 #ifdef WITH_DBUS
 static gboolean sp_dbus_listen = FALSE;
@@ -520,6 +522,11 @@
      N_("Start Inkscape in interactive shell mode."),
      NULL},

+    {"preferences", 0,
+     POPT_ARG_STRING, &sp_preferences, SP_ARG_PREFERENCES,
+     N_("Specify a different preferences.xml file."),
+     NULL},
+
     POPT_AUTOHELP POPT_TABLEEND
 };

I wouldn't expect the Inkscape developers to accept this patch, for two reasons. First, they've got an alternative feature with the same effect already. But second, I wouldn't expect them to like the way I made sp_preferences program-global instead of module-global. That sort of code is fine for a personal feature that you do not intend to be part of the mainline software, however.

The above patch may look fairly ugly to a non-programmer or one not familiar with C++ and patch files, but trust me, this is about as simple as changes to software get. It's only 10 lines of new code.

(If you do your own count and come up with 13 new lines, three of those are blank or have only a curly brace, so you don't include those in the SLOC count.)

Answer 3

You could use symbolic links. Put the config file you use with your GUI somewhere, lets call it GUI_CONFIG, and the one with scripts to SCRIPT_CONFIG. At the beginning of your script put the line:

ln -sf SCRIPT_CONFIG $HOME/.config/inkscape/preferences.xml

and at the end:

ln -sf GUI_CONFIG $HOME/.config/inkscape/preferences.xml

When the script executes it will make preferences.xml a symbolic link that points to the config it needs. When it finishes, it points it back at the one the GUI uses. Running multiple scripts concurrently won't destroy your config, as it does when copying temp files, though the first script completing will break the config file for still running scripts. It maybe best to put the calls to ln around the individual calls to Inkscape, rather than the scripts as a whole, to try to prevent these race conditions.

Another option would be to run Inkscape as a different user from scripts, so you can set it up with a different config file. However, you'll then have to deal with permissions, possibly by copying files back and forth to /tmp.

Answer 4

Looking through the documentation for the preferences subsystem in Inkscape it isn't possbile.

• http://wiki.inkscape.org/wiki/index.php/Preferences_subsystem

Your options:

1. do what your doing
2. play games via linking the file
3. user a different user ID
4. modify the source (see @WarrenYoung's answer for this!)