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Pseudo threading with BASH

January 27, 2007 at 7:17 pm - Filed under aa, bb - 567 words, reading time ~1 minutes - Permalink - Comments

This night i was in the process of mirroring all the daily pics for fast viewing. Their site has to be hosted on an ADSL link (like, hey this site is on a 200kbs/300kbs link, very unprofessional but no one can raid [stupid wordpress plug-in, this is not RAID in the sense of Redundant Disk Array but raid the verb] our server without our knowledge, think about the autistici/inventati aruba raid for example).

I don't want to explain how to grab something over HTTP, go to and figure out the structure yourself, i want to focus on how to implement pseudo-threading in the bash language. The first task i had to accomplish in order to grab the images was to understand witch daily archives were available. Easy task!

For each year in the interval i had decided, for each month of the year, for each day in the month, make an HTTP query like$year/$month/$day/ and see if the HTTP status is 200 or 404. I thought.

But the problem is that their site is so slow: it has an initial delay of 10, 15 seconds before receiving some data after the request. Also the other problem was that it was really late in the night, like 06:30 AM or so.

What I needed was parallelization of these HTTP requests to minimize the impact of this initial delay. Okay I said to myself, no problem, i'll use C and pthreads.


Holy shit it's 06:30 AM and I'm tired. And drunk. And lazy. Just waited 2 minutes in front of an empty nano screen with just "GNU nano X.X.X File:" and a blinking cursor to realize that i can't use C and pthreads in these conditions.


I'll do it in BASH or PHP I said. The problem is that none of the two have real thread support. What i needed was a hack, a clever solution to a problem. Hack to hack and considering that the solution will be inelegant/unoptimized in both languages i choose BASH Scripting. So this is MY solution:



for j in `seq 2000 2006`; do
 for m in `seq -w 2 1 12`; do
  for d in `seq -w 2 1 31`; do

   while [ TRUE ]; do
    TH_NUM=`ps aux | grep "^ascii" | grep -v "grep" | grep "curl" | wc -l`
    if [ "$TH_NUM" -le "$TH_MAX" ]; then

     echo $(
      RES=`curl -Is "$j/$m/$d/" | head -n1`
      echo "$j/$m/$d/ $RES";
     ) >> results &

     echo -en "."
     echo -en "W"
     sleep 1


Pretty simple uh? It worked 100% as expected. At 07:00 i was sleeping and the program running. An extra for you are these screenshots i taken (cause has almost no images and i know you like images).

The invocation.

Fixed number of "child" processes.


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