teaching machines

CS 330 Lecture 4 – Shell Scripting, Part 3

January 30, 2017 by . Filed under cs330, lectures, spring 2017.

Dear students,

Okay, let’s really get around to writing these shell scripts this time:

  1. Send files to limbo/recycling bin, rather than hell/non-existence.
  2. Send a mass email.
  3. Upload a local file to a web server and let the world see it.

Let’s consider the shell as a programming language by discussing the following questions:

What’s the purpose of the shell? What’s something you might want to do with a shell script that isn’t as easy in another language?
I’d argue that the strengths of the shell are that it lives outside the purpose of any particular program. It lets you program interaction with the operating system, the programs you have installed, and the file system. It is also the programming language that is probably used most often interactively, usings its read-eval-print loop (REPL).
Does the shell support operators and expressions? What can you do with its data?
Shells do support arithmetic, but that’s not its strong suit. Doug McIlroy said this:
This is the Unix philosophy: Write programs that do one thing and do it well. Write programs to work together. Write programs to handle text streams, because that is a universal interface.
Where it shines is in string manipulation, file name manipulation in particular. In zshell, appending :r after a variable strips off the extension, giving the base name. :t strips off the parent directory. :h strips off the file name from the parent directory’s path. I can write a script that converts a bunch of images with a certain extension to another with a script like this:
for i in *.$src; do
  convert $i ${i:r}.$dst
Does the shell have a mechanism for abstracting code?
Yes, one can either put general, reusable code in its own file, or inside a function. We saw last lecture, however, that shells also have a notion of scope. When we ran our script that tried to return us to a remembered directory, it didn’t have the effect we wanted. That’s because the working directory was changed for the process that the shell spawned to run our script. The shell’s process and its working directory weren’t affected. Sometimes we don’t want such strict walls between scopes. In the shell we have a few options: source or aliases.
alias here='pwd > ~/.where'
alias there='cd $(cat ~/.where)'
Does the shell have a mechanism for passing data around?
Yes, through positional parameters. Unlike Java and C, however, nobody outside the program checks to see if all the parameters were properly passed. For return values, we can only return integers through a process’s exit status.

However, the data that we output from a shell script can often be parsed by the caller. It is here where the distinction between printing and returning that we stressed in CS1 turns to rubbish.

How are errors handled?
Normally, the command generating the error fails, but execution picks up at the next command. This is different from our Java experience. In C, things generally didn’t grind to a halt except when we encountered a segfault or ran out of memory. Execution continued on. It’d be nice to force a shell script to stop at the first error, wouldn’t it? We can set an option to achieve that:
# set -e     # uncomment this to bail at the first error
cp ~/missing.file found
date >> found

If we have time, we’ll have a peek at some Ruby scripts to do the following:

Here’s your TODO list for next time: