LShift Ltd.

Audible Ping

Does your internet go away occasionally? Do you have a computer hooked up to a moderately loud stereo system? Want to know as soon as the link comes back? Here’s a script (put it in a file, e.g. ~/bin/audible-ping) that pings some outside host, playing a short .wav file when ping finally starts to get some responses:

#!/bin/sh
ping "$@" | \
  grep --line-buffered 'bytes from' 2>&1 | \
  perl -e 'while (<>) { `play /path/to/the/ping/sound.wav`; }'

It will play the .wav file once per successful ping packet reply. (Obviously you’ll need to come up with your own short beep-like sound file, and replace the path given above appropriately; use play’s --volume=n parameter to adjust the volume to taste.)

We use it here when our DSL connection goes away, running

$ audible-ping 123.123.123.123

on our jukebox computer. (IP address changed to protect the innocent.) We use a literal IP address rather than a DNS name because if your ‘net is AWOL, the DNS query will time out and ping will exit rather than continuing to retry. The script lets us know the very moment our DSL connection comes back.

2 comments August 12th, 2005 tonyg

From Stateful Parsing to Transactional Parsing

Thinking further on the problem of stateful parsing (from yesterday’s article), the way we’ve done it in the past (essentially by using Scheme’s parameter mechanism) is a special case of a transactional system, with rollback on error and commit after each toplevel expression parsed.

This suggests that if Scheme had an implementation of a Software Transactional Memory, then it would be a good fit for a stateful packrat parser. The symbol-table (or whatever state needs to be speculatively propagated along a line of parsing) would simply be placed under transactional control. Each time an attempt is made to match against a particular nonterminal a nested transaction would be entered. If the nonterminal matched the input successfully, the nested transaction would be committed; otherwise it would be rolled back, and any alternative options would be tried in their own nested transactions.

Add comment August 12th, 2005 tonyg