Limit file clobbering during update

Don't clobber files in-place unless there's actual changes.

update.sh

@@ -36,6 +36,34 @@ cleanup() {
 }
 trap 'cleanup' EXIT
 
+copy_perms() {
+    local -r FROM=$1 TO=$2
+    chmod --reference="$FROM" -- "$TO"
+    if [[ $UID -eq 0 ]]; then
+        chown --reference="$FROM" -- "$TO"
+    fi
+}
+
+replace_with() {
+    local -r ORIG=$1 NEW=$2
+    if ! diff -q -- "$ORIG" "$NEW" &>/dev/null; then
+        # There's a change
+        if [[ -f $ORIG ]]; then
+            copy_perms "$ORIG" "$NEW"
+            if [[ -n $BACKUPSUFFIX ]]; then
+                mv -- "$ORIG" "$ORIG$BACKUPSUFFIX"
+            fi
+        else
+            mkdir -p -- "${ORIG%/*}"
+        fi
+        mv -- "$NEW" "$ORIG"
+    else
+        printf 'File "%s" not modified\n' \
+            "$ORIG" \
+            >&2
+    fi
+}
+
 (cd "$WORKDIR" && git submodule update --init --remote)
 
 for (( I = 0 ; I < ${#OUT[@]} ; ++I )); do
@@ -52,44 +80,16 @@ for (( I = 0 ; I < ${#OUT[@]} ; ++I )); do
 
     TEMP_OUT=$(mktemp -p "$TEMP_DIR")
 
-    if [[ -f $MY_OUT ]]; then
-        cp -a -- "$MY_OUT" "$TEMP_OUT"
-    fi
-
     wget \
         -O "$TEMP_OUT" \
         "$MY_URL"
 
-    if [[ -f $MY_OUT ]]; then
+    replace_with "$MY_OUT" "$TEMP_OUT"
-        chmod --reference="$MY_OUT" "$TEMP_OUT"
-        if [[ -n $BACKUPSUFFIX ]]; then
-            mv -- "$MY_OUT" "$MY_OUT$BACKUPSUFFIX"
-        fi
-    fi
-
-    mkdir -p -- "${MY_OUT%/*}"
-    mv -- "$TEMP_OUT" "$MY_OUT"
-
-    # If the old one is the same, don't keep it around
-    if [[ -n $BACKUPSUFFIX && -f $MY_OUT$BACKUPSUFFIX ]]; then
-        if diff -q "$MY_OUT" "$MY_OUT$BACKUPSUFFIX"; then
-            rm -f -- "$MY_OUT$BACKUPSUFFIX"
-        fi
-    fi
-
 done
 
-if [[ -n $BACKUPSUFFIX && -f $BLOCKLIST ]]; then
+TEMP_BLOCKLIST=$(mktemp -p "$TEMP_DIR")
-    mv -- "$BLOCKLIST" "$BLOCKLIST$BACKUPSUFFIX"
+"$WORKDIR/make-block.pl" --out="$TEMP_BLOCKLIST"
-fi
+replace_with "$BLOCKLIST" "$TEMP_BLOCKLIST"
-"$WORKDIR/make-block.pl" --out="$BLOCKLIST"
-
-# If the old one is the same the same, don't keep it around
-if [[ -n $BACKUPSUFFIX && -f $BLOCKLIST$BACKUPSUFFIX ]]; then
-    if diff -q "$BLOCKLIST" "$BLOCKLIST$BACKUPSUFFIX"; then
-        rm -f -- "$BLOCKLIST$BACKUPSUFFIX"
-    fi
-fi
 
 if [[ ${#DNSMASQ_RESTART_COMMAND[@]} -gt 0 ]]; then
     "${DNSMASQ_RESTART_COMMAND[@]}"