2 Aug 2009
5:15 PM
CGI scripting in awk?!
Awk is a frequently underestimated scripting language.
Although it's been around since the 1970s, it never really caught on except
as an occasional adjunct to bourne shell scripts. This is a shame because
it has what I consider to be the right attributes for a good scripting
language: code requires very little boilerplate; there are just a few concepts
to learn. The original
awk paper
describes all of its concepts in just 7 page. Of course, new features
have been added to awk since then, but even with those changes included,
the POSIX awk definition
is still refereshingly short.
When I recently had a small CGI script to write, I decided to try awk. I was
inspired by some
ambitious
wikis written in awk. It turned out to be pleasantly simple. The resulting code
demonstrates some of awk's strengths, like its string manipulation and associative
arrays. For example, here's code for parsing of the CGI query string.
function cgi_parse(query, defaults, out, params, i, j, count, param, val) {
count = 0
for (i in defaults) {
out[i] = defaults[i]
}
split(query, params, "&")
for (i in params) {
j = index(params[i], "=")
if (j <= 0) continue
param = url_decode(substr(params[i], 1, j - 1))
val = url_decode(substr(params[i], j + 1))
if (param in defaults) {
out[param] = val
count++
}
}
return count
}
Awk's limitations are also apparent here. For example, the declaration
of local variables as parameters to the function is ugly. Also, arrays aren't
first class values - they can't be returned in a function, and we can't copy
an entire array with a single assignment. Instead, we copy the defaults array
into the out array one entry at a time.
Attached in cgi.awk are a couple of small routines that can be useful
in CGI scripts. cgi_parse() is the above function that parses the CGI
query string into an array. html_encode() escapes strings so that they
can be safely included inside HTML. An example CGI script using these
routines is in the attachment cgi_example.awk. You'll need a tiny shell
wrapper to run them from a web server.
#! /bin/sh -
awk -f cgi.awk -f cgi_example.awk