3.4 URLCHK: Look for Changed Web Pages

Most people who make heavy use of Internet resources have a large bookmark file with pointers to interesting web sites. It is impossible to regularly check by hand if any of these sites have changed. A program is needed to automatically look at the headers of web pages and tell which ones have changed. URLCHK does the comparison after using GETURL with the HEAD method to retrieve the header.

Like GETURL, this program first checks that it is called with exactly one command-line parameter. URLCHK also takes the same command-line variables Proxy and ProxyPort as GETURL, because these variables are handed over to GETURL for each URL that gets checked. The one and only parameter is the name of a file that contains one line for each URL. In the first column, we find the URL, and the second and third columns hold the length of the URL's body when checked for the two last times. Now, we follow this plan:

1. Read the URLs from the file and remember their most recent lengths
2. Delete the contents of the file
3. For each URL, check its new length and write it into the file
4. If the most recent and the new length differ, tell the user

It may seem a bit peculiar to read the URLs from a file together with their two most recent lengths, but this approach has several advantages. You can call the program again and again with the same file. After running the program, you can regenerate the changed URLs by extracting those lines that differ in their second and third columns:

     
     BEGIN {
       if (ARGC != 2) {
         print "URLCHK - check if URLs have changed"
         print "IN:\n    the file with URLs as a command-line parameter"
         print "    file contains URL, old length, new length"
         print "PARAMS:\n    -v Proxy=MyProxy -v ProxyPort=8080"
         print "OUT:\n    same as file with URLs"
         print "JK 02.03.1998"
         exit
       }
       URLfile = ARGV[1]; ARGV[1] = ""
       if (Proxy     != "") Proxy     = " -v Proxy="     Proxy
       if (ProxyPort != "") ProxyPort = " -v ProxyPort=" ProxyPort
       while ((getline < URLfile) > 0)
          Length[$1] = $3 + 0
       close(URLfile)      # now, URLfile is read in and can be updated
       GetHeader = "gawk " Proxy ProxyPort " -v Method=\"HEAD\" -f geturl.awk "
       for (i in Length) {
         GetThisHeader = GetHeader i " 2>&1"
         while ((GetThisHeader | getline) > 0)
           if (toupper($0) ~ /CONTENT-LENGTH/) NewLength = $2 + 0
         close(GetThisHeader)
         print i, Length[i], NewLength > URLfile
         if (Length[i] != NewLength)  # report only changed URLs
           print i, Length[i], NewLength
       }
       close(URLfile)
     }
     

Another thing that may look strange is the way GETURL is called. Before calling GETURL, we have to check if the proxy variables need to be passed on. If so, we prepare strings that will become part of the command line later. In GetHeader, we store these strings together with the longest part of the command line. Later, in the loop over the URLs, GetHeader is appended with the URL and a redirection operator to form the command that reads the URL's header over the Internet. GETURL always produces the headers over /dev/stderr. That is the reason why we need the redirection operator to have the header piped in.

This program is not perfect because it assumes that changing URLs results in changed lengths, which is not necessarily true. A more advanced approach is to look at some other header line that holds time information. But, as always when things get a bit more complicated, this is left as an exercise to the reader.