Computer Science & Perl Programming

Chapter 31. Parsing Command-Line Options

Johan Vromans

Controlling a computer by typing commands into a shell is still the preferred way of working for most programmers. Despite the capabilities of modern window systems, working from a shell is faster and simpler than sequences of mouse movements and button clicks—once you know the names of the commands and how they work.

The expressiveness of a command-line program depends on what options it supports, and how they’re parsed, converted into a form that your program can understand. When you execute ls -l /tmp on Unix, or dir /w c:\windows on MS-DOS, or your_program -height=80, the -l, /w, and -height=80 are options. Sometimes the shell handles the parsing; that’s what happens with the /w in DOS. More often, the program named by the command (ls and your_program) must handle the parsing itself. In this article, I’ll show you how your Perl programs can parse their own options.

Option Parsing Conventions

Under modern command shells, including those on Unix and Windows, options can be either letters or words. Programs that accept single letters might be invoked as program -a -b -c. Or, they might be invoked as program -abc, meaning the same thing. If the options take values, you can bundle them together: -aw80L24x is equivalent to -a -w 80 -L 24 -x. With option words, you sacrifice brevity for clarity: program --all --width=80 --length 24 --extend.

In either case, options precede other program arguments (the /tmp in ls -l /tmp) and the parsing ...