Command Line Parser


Mastery of the command line is prerequisite to effective computer use. Unix allows you to pipe data between small programs that do one job well. However, almost all command line applications need a little direction to be of use. Each program can be thought of as a mini-language that allows you to transform text data in various ways.

If you've written any command line application at all, you've probably used a library like argparse to read in arguments to your program. Let's explore how command line parsers work and try to write one ourselves.

Problem Statement

A command line parser takes a string and interprets it as a series of positional (required) arguments and options. An argument is the input to the program. For example, the sort command takes in as an argument the name of a file and outputs the lines in sorted alphabetical order. Here example_file is the argument

$ cat example_file
$ sort example_file

In addition, the sort command takes a number options also known as flags that modify the operation of the program. For example, if I ran the sort command with the -r flag, the sort ordering is reversed.

Options have two forms, long and short. Short forms are a single letter, like -r. The long forms are indicated by a double dash and then the full word, like --reversed.

$ sort -r example_file

Some options take arguments. This is specified in the short form by immediately following the short form flag with a value (space optional). The long form is specified by a double dash followed by an equal sign.

$ sort -o out_file example_file            # Writes the line sorted contents to out_file
$ sort -oout_file example_file             # Same as above
$ sort --output=out_file example_file      # Same as above

We can combine different options but the required arguments must always be supplied.

$ sort -r -o out_file example_file         # Writes the contents of example_file, sorted in reverse order, to out_file

Write a command line parser that allows users to specify the interface described above. Users should be able to specify:

  • Required arguments
  • Options - with long and/or short aliases and whether or not they take an argument

Then given a string, return the positional parameters, optional parameters and their values if any.


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