How to write a crawler?

You'll be reinventing the wheel, to be sure. But here's the basics:

• A list of unvisited URLs - seed this with one or more starting pages
• A list of visited URLs - so you don't go around in circles
• A set of rules for URLs you're not interested in - so you don't index the whole Internet

Put these in persistent storage, so you can stop and start the crawler without losing state.

Algorithm is:

while(list of unvisited URLs is not empty) {
    take URL from list
    remove it from the unvisited list and add it to the visited list
    fetch content
    record whatever it is you want to about the content
    if content is HTML {
        parse out URLs from links
        foreach URL {
           if it matches your rules
              and it's not already in either the visited or unvisited list
              add it to the unvisited list
        }
    }
}

Crawlers are simple in concept.

You get a root page via a HTTP GET, parse it to find URLs and put them on a queue unless they've been parsed already (so you need a global record of pages you have already parsed).

You can use the Content-type header to find out what the type of content is, and limit your crawler to only parsing the HTML types.

You can strip out the HTML tags to get the plain text, which you can do text analysis on (to get tags, etc, the meat of the page). You could even do that on the alt/title tags for images if you got that advanced.

And in the background you can have a pool of threads eating URLs from the Queue and doing the same. You want to limit the number of threads of course.

If your NPO's sites are relatively big or complex (having dynamic pages that'll effectively create a 'black hole' like a calendar with a 'next day' link) you'd be better using a real web crawler, like Heritrix.

If the sites total a few number of pages you can get away with just using curl or wget or your own. Just remember if they start to get big or you start making your script more complex to just use a real crawler or at least look at its source to see what are they doing and why.

Some issues (there are more):

• Black holes (as described)
• Retries (what if you get a 500?)
• Redirects
• Flow control (else you can be a burden on the sites)
• robots.txt implementation

Wikipedia has a good article about web crawlers, covering many of the algorithms and considerations.

However, I wouldn't bother writing my own crawler. It's a lot of work, and since you only need a "simple crawler", I'm thinking all you really need is an off-the-shelf crawler. There are a lot of free and open-source crawlers that will likely do everything you need, with very little work on your part.

You could make a list of words and make a thread for each word searched at google.
Then each thread will create a new thread for each link it find in the page.
Each thread should write what it finds in a database. When each thread finishes reading the page, it terminates.
And there you have a very big database of links in your database.

Use wget, do a recursive web suck, which will dump all the files onto your harddrive, then write another script to go through all the downloaded files and analyze them.

Edit: or maybe curl instead of wget, but I am not familiar with curl, I do not know if it does recursive downloads like wget.