A less wordy version of r85947 and r85948

chrome/installer/mac/dirdiffer.sh

 #!/bin/bash -p
 
 # Copyright (c) 2011 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # usage: dirdiffer.sh old_dir new_dir patch_dir
 #
 # dirdiffer creates a patch directory patch_dir that represents the difference
 # between old_dir and new_dir. patch_dir can be used with dirpatcher to
@@ skipping 142 matching lines @@
 XZ="$(dirname "${0}")/xz"
 readonly XZ
 readonly GBS_SUFFIX='$gbs'
 readonly BZ2_SUFFIX='$bz2'
 readonly GZ_SUFFIX='$gz'
 readonly XZ_SUFFIX='$xz'
 readonly PLAIN_SUFFIX='$raw'
 
 # 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}"
 
   echo "${ME}: ${error}" >& 2
 }
 
 declare -a g_cleanup g_verify_exclude
 cleanup() {
   local status=${?}
@@ skipping 361 matching lines @@
   trap - EXIT
 }
 
 if [[ ${#} -ne 3 ]]; then
   usage
   exit 2
 fi
 
 main "${@}"
 exit ${?}