Shell Scripting

I kind of like shell scripting. It’s quirky, frequently ugly, but it’s damned useful. Like Perl, it annoys me when I see things which could be written in a better manner. One persistent example is abusing the if statement.

  if [ "x$machine" = "x" ]
          echo "not in ~/.machines adding"
          echo $1 >> ~/.machines
          source ~/.bash_aliases

What’s the problem here? That little “x” in the test. It’s not necessary and hasn’t been since the late 80’s. There used to be a parsing bug in shells which meant that an empty argument got removed. So you couldn’t say [ "$machine" = "" ] because you’d end up comparing $machine to ].

But this got fixed, a long time ago. Really. Unless you’re working on an aging System III box, you shouldn’t worry about this, it’s just Cargo cult programming.

So the test can be just [ "$machine" = "" ]. That’s better. But not good enough. Testing for an empty string? The test command (aka [) provides a -z operator for that! So goes down to [ -z "$machine" ].

In this particular case, we’re explicitly targetting bash, though. One of the nice features of modern POSIX shells like bash is that they provide conditional expressions, which have different parsing rules. So you can say [[ -z $machine ]] and not worry about the lack of quotes.

Lastly, I should point out that I missed off the first line of that extract.

  machine=`grep $1 ~/.machines`

With that in place, you can see that we’re not actually using $machine anywhere in the function. It’s purely in place for the if statement. Which means you can simplify this even further. if takes a command as it’s argument. So why not just give it the grep command?

  if grep -q $1 ~/.machines
    echo "not in ~/.machines adding"
    echo $1 >> ~/.machines
    source ~/.bash_aliases

I had to add the -q flag to shut grep up; we’re only interested in the return code, not the output.

Does this matter? Probably not much. But like all programming languages, it’s a lot easier to read when you use it idiomatically.

Comments 3

  1. bob wrote:

    indeed, and in actual use ive found a couple of other bugs which i intend to fix. it was just a quick hack and i certainly dont claim to be a programmer.

    Posted 05 Oct 2006 at 12:36
  2. Dominic Mitchell wrote:

    Not being a programmer is why shell scripts are great. 🙂

    I apologise if it felt like I was picking on you. As I said, it’s a common problem amongst many shell scripts I see, so I just used it as an example. It’s not just you.

    Posted 05 Oct 2006 at 13:53
  3. bob wrote:

    learning is good. 🙂
    i dont feel you were picking on me. i knew it was bad when i did it but couldnt be bothered to spend the time workign out a better way.
    i will incorparate your changes and mention this post so the whole world can learn even more 🙂

    Posted 05 Oct 2006 at 14:11