Add "cache.sh", a script that caches a programs output

bsp/default/root_file_system/etc/cache.sh

@@ -0,0 +1,38 @@
+#!/bin/ash
+
+[ $# -ge 2 ] || exit 1
+
+CACHEDIR='/tmp/cache'
+
+MAXAGE="$1"
+shift 1
+COMMAND="$@"
+
+# generate an ID based on the command to be executed
+ID=$(echo "$COMMAND" | md5sum | cut -d" " -f1) || exit 1
+
+CACHED=false
+
+# create directory for cached output
+[ -d "$CACHEDIR" ] || mkdir -p "$CACHEDIR"
+[ -d "$CACHEDIR" ] || exit 1
+
+# if there is an entry for the command to be executed...
+if [ -f "$CACHEDIR/$ID/timestamp" ]; then
+	TIMESTAMP=$(cat "$CACHEDIR/$ID/timestamp")
+	CURRENTTIME=$(date +%s)
+	# ...check the timestamp and determine if it is sufficiently recent
+	if [ -n "$TIMESTAMP" ] && [ $(($CURRENTTIME-$TIMESTAMP)) -lt $MAXAGE ] && [ -f "$CACHEDIR/$ID/output" ]; then
+		CACHED=true
+	fi
+fi
+
+# if there is cached output data just put it out...
+if $CACHED; then
+	cat "$CACHEDIR/$ID/output"
+else
+# ...if not execute the command and save the output and a timestamp
+	[ -d "$CACHEDIR/$ID" ] || mkdir -p "$CACHEDIR/$ID"
+	$COMMAND | tee "$CACHEDIR/$ID/output"
+	date +%s > "$CACHEDIR/$ID/timestamp"
+fi