The A-Z of Programming Languages: Bourne shell, or sh
- 05 March, 2009 08:45
Unix Specialist Steve Parker has posted 'Steve's Bourne / Bash scripting tutorial' in which he writes: Shell script programming has a bit of a bad press amongst some Unix systems administrators. This is normally because of one of two things: a) The speed at which an interpreted program will run as compared to a C program, or even an interpreted Perl program; b) Since it is easy to write a simple batch-job type shell script, there are a lot of poor quality shell scripts around. Do you agree?
It would be hard to disagree because he probably knows more about it than I do. The truth of the matter is you can write bad code in any language, or most languages anyway, and so the shell is no exception to that. Just as you can write obfuscated C you can write obfuscated shell. It may be that it is easier to write obfuscated shell than it is to write obfuscated C. I don’t know. But that’s the first point.
The second point is that the shell is a string processing language and the string processing is fairly simple. So there is no fundamental reason why it shouldn’t run fairly efficiently for those tasks. I am not familiar with the performance of Bash and how that is implemented. Perhaps some of the people that he is talking about are running Bash versus the shell but again I don’t have any performance comparisons for them. But that is where I would go and look. I know when I wrote the original implementation of the shell I spent a lot of time making sure that it was efficient. And in particular with respect to the string processing but also just the reading of the command file. In the original implementation that I wrote, the command file was pre-loaded and pre-digested so when you executed it you didn’t have to do any processing except the string substitutions and any of the other semantics that would change values. So that was about as efficient as you could get in an interpretive language without generating code.
I will say, and it is funny because Maurice Wilkes asked me this question when I told him what I was doing, and he said ‘how can you afford to do that?’ Meaning, how can you afford to write programs when the primitives are commands that you are executing and the costs of executing commands is so high relative to executing a function in a C program, for example. As I have said earlier, the primary performance limitation is that you have to do a Unix fork and exec whenever you execute a command. These are much more expensive than a C function call. And because commands are the abstraction mechanism, that made it inefficient if you are executing many commands that don't do much.
Where do you envisage the Bourne shell's future lying?
I don’t know; it’s a hard question. I imagine it will be around as long as Unix is around. It appears to be the most ubiquitous of the Unix shells …What people tell me is if they want one that is going to work on all the Unix systems out there in the world, they write it in the Bourne shell (or Bash). So, that’s one reason. I don’t know if it is true but that is what they tell me. And I don’t see Unix going away any time soon. It seems to have had a revival with the open source movement, in particular the GNU Project and the Linux kernel.
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