A less wordy version of r85947 and r85948

chrome/installer/mac/dirdiffer.sh

Index: chrome/installer/mac/dirdiffer.sh
===================================================================
--- chrome/installer/mac/dirdiffer.sh	(revision 85948)
+++ chrome/installer/mac/dirdiffer.sh	(working copy)
@@ -160,8 +160,8 @@
 
 # Workaround for http://code.google.com/p/chromium/issues/detail?id=83180#c3
 # In bash 4.0, "declare VAR" no longer initializes VAR if not already set.
-DIRDIFFER_EXCLUDE="${DIRDIFFER_EXCLUDE:-}"
-DIRDIFFER_NO_DIFF="${DIRDIFFER_NO_DIFF:-}"
+: ${DIRDIFFER_EXCLUDE:=}
+: ${DIRDIFFER_NO_DIFF:=}

[TVL, 2011/05/20 02:14:01]

i'll trust you on this syntax, it isn't one I've seen before.

[Mark Mentovai, 2011/05/20 02:26:35]

TVL wrote:
:= does an assignment within the expansion if the variable is unset or empty.
The : up front just tells the shell to throw away the result of the expansion.
So the shell does the expansion, which does the assignment, and then chucks it
out. If the variable started off as unset (or empty), it’ll wind up set (or
reset) to empty. Exactly what we want.

mark@rj bash$ declare -p VAR && echo "${VAR}"
-bash: declare: VAR: not found
mark@rj bash$ : ${VAR:=}
mark@rj bash$ declare -p VAR && echo "${VAR}"
declare -- VAR=""

mark@rj bash$ VAR=yes
mark@rj bash$ : ${VAR:=}
mark@rj bash$ declare -p VAR && echo "${VAR}"
declare -- VAR="yes"
yes

 
 err() {
   local error="${1}"