i-am.ws

Shell Hell

Unix shell programming appears to be an art, not a science. That's probably also why I'm not so much of a Perl fan. Take note, I do like both scripting and art a lot, but when it comes to Computer Science (mind that last word :) I think computer languages should be very unambiguous and easy to read. If that makes a compiler's life a little harder or means a bit more typing for the programmer, so be it....

But this story is not about bashing Perl, it's about unexpected problems with simple shell scripting. I was installing W3C's libwww package to get going with some XML-RPC development. It was the typical process: "./configure" followed by "gmake", nothing fancy. The configure step went fine, but while gmaking, it hicked up severely. I traced it back to some "test" commands in the libtool script, when the condition of 'test' had an empty variable.

Something like:

#!/bin/sh
something=""
if test $something = "yes"
then
    echo "something is yes"
fi

Or when the something="" statement had been absent at all. In both cases, the result is an error at the "if test" line.

The cause of all this trouble is that for undefined variables there is a difference between

if test $something = "yes"; then

and

if test "$something" = "yes"; then

Mind the additional quotes. The libwww package is missing those a couple of times, which appears to be fine when the variable has a value, but not when it is undefined.

Back to art vs science. That things like this are illegal syntax I'm OK with. But it should be made clear up front. Preferably as a compile error, although I realize that for scripting languages that is not too applicable. I had hoped for an error like "line 13: illegal syntax" or something similar. But that's not what I got:

./test.sh: test: argument expected

So it took me 10 mins and many more echo statements to find out which of all those test commands in the 5000+ lines of code script was the culprit. After finding it, of course the problem repeated itself, quite a couple of times. It took a while to get this script fixed, but don't worry, I did ... :-)

Later, when redoing the install on RedHat, I discovered RH8 is having the same issues with the test command, but it states:

./test.sh: line 13: test: =: unary operator expected

The message is even more cryptic, but it has the big advantage of giving you a line number. It's the small things that matter :-). Which would have saved me many echo statements and a couple of hours.

With all these problems out of the way, I could dig into XMLRPC-C to build an interface between a Windows client and a Solaris backend. So far, I find XML-RPC to be an elegant protocol, which results in quick implementations and simple solutions.

Posted at 08:58PM Apr 16, 2006 by WWWillem in Software  |