> -val=`echo $arg|sed 's/^[^=]*=\(.*\)/\1/'` || exit 71
> +val=`echo "$arg"|sed 's/^[^=]*=\(.*\)/\1/'` || exit 71

This seems correct, but should also be applied to the line before for consistency:

-  opt=`echo $arg|sed 's/^\([^=]*\).*/\1/'`  || exit 71
+  opt=`echo "$arg"|sed 's/^\([^=]*\).*/\1/'`  || exit 71

> - for p in $PRUNEPATHS; do
> + for p in "$PRUNEPATHS"; do

This defeats the purpose of the for-loop, as this tries to check if any of
the paths in PRUNEPATHS ends on '/', while the patch would reduce that check
only to the last element (due to quoting into a single string).

I'd suggest to change this to:

-for p in $PRUNEPATHS; do
-    case "$p" in
-       /*/)   echo "$0: $p: pruned paths should not contain trailing slashes" >&2
-              exit 1
-    esac
-done
+nl='
+'
+if echo "$PRUNEPATHS" | tr ' ' "$nl" | grep '[^/]/$' >/dev/null; then
+  echo "$0: $p: pruned paths should not contain trailing slashes" >&2
+  exit 1
+fi

> test -z "$PRUNEREGEX" &&
> -  PRUNEREGEX=`echo $PRUNEPATHS|sed -e 's,^,\\\(^,' -e 's, ,$\\\)\\\|\\\(^,g' -e 's,$,$\\\),'`
> +  PRUNEREGEX=`echo "$PRUNEPATHS"|sed -e 's,^,\\\(^,' -e 's, ,$\\\)\\\|\\\(^,g' -e 's,$,$\\\),'`

I see several problems in this area.

updatedb allows to define PRUNEREGEX as environment variable from outside,
and only uses the value of --prunepath if the former is unset.
Usually, options should override environment variables, not the other way round.

Furthermore, PRUNEPATHS seems to be defined to allow already-expanded items only.
If one wants to use regular expressions, then why not directly define PRUNEREGEX from outside?

In that regard, it would maybe be better to introduce a --pruneregex option
which takes the final pruning expression for find(1).  It could still override
any of PRUNEPATHS and --prunepaths value, but shouldn't do that silently.

Here is a patch:

--- updatedb.org        2009-01-27 13:29:28.575086300 +0100
+++ updatedb    2009-02-09 10:19:41.132505500 +0100
@@ -163,7 +163,7 @@
 # Trailing slashes result in regex items that are never matched, which
 # is not what the user will expect.   Therefore we now reject such
 # constructs.
-for p in $PRUNEPATHS; do
+for p in "$PRUNEPATHS"; do
     case "$p" in
        /*/)   echo "$0: $p: pruned paths should not contain trailing slashes" >&2
               exit 1
@@ -172,7 +172,7 @@
 
 # The same, in the form of a regex that find can use.
 test -z "$PRUNEREGEX" &&
-  PRUNEREGEX=`echo $PRUNEPATHS|sed -e 's,^,\\\(^,' -e 's, ,$\\\)\\\|\\\(^,g' -e 's,$,$\\\),'`
+  PRUNEREGEX=`echo "$PRUNEPATHS"|sed -e 's,^,\\\(^,' -e 's, ,$\\\)\\\|\\\(^,g' -e 's,$,$\\\),'`
 
 # The database file to build.
 : ${LOCATE_DB=/var/locatedb}

This is http://bugs.debian.org/415567

István Váradi writes: -----------------------
The updatedb script uses the PRUNEPATHS environment variable without quoting it. This causes some problems when putting regular expressions into the value of this variable. For example, if I use an asterisk () in a path, it will be expanded by the shell before using it as a regular expression. Thus, ./lost+found becomes ../lost+found (and possibly others, depending on how many files or directories, whose names start with a dot I have in the working directory).
---------------------------------------------

Afaict the fix should be trivial:

- for p in $PRUNEPATHS; do
+ for p in "$PRUNEPATHS"; do
[...]
-  PRUNEREGEX=`echo $PRUNEPATHS|sed -e 's,^,\\\(^,' -e 's, ,$\\\)\\\|\\\(^,g' -e 's,$,$\\\),'`
+   PRUNEREGEX=`echo "$PRUNEPATHS"|sed -e 's,^,\\\(^,' -e 's, ,$\\\)\\\|\\\(^,g' -e 's,$,$\\\),'`

thanks, cu andreas