Lesson 19 · Globbing with * and ?

Globbing with * and ?

When you type `*.txt`, the shell - not the program - turns it into the list of matching filenames. Today you implement that filename expansion for the `*` and `?` wildcards.

The goal

Expand a word containing `*` or `?` into the sorted list of matching filenames in the current directory.

Start here - the target

TO DO

Scenario: Expanding wildcard patterns

Giventhe current directory contains the files "a.txt", "ab.txt", and "b.md"

Whenthe word "*.txt" is expanded

Thenit becomes ["a.txt", "ab.txt"] in sorted order

Andthe word "?.txt" becomes ["a.txt"] (? matches exactly one character)

Background

Here is a surprise for most people: when you run rm *.txt, the rm program never sees the *. The shell expands *.txt into the actual list of matching filenames first, then hands rm that list. This is globbing, and it means every program gets wildcard support for free without knowing wildcards exist.

Two wildcards today. * matches any run of characters (including none), and ? matches exactly one character. To expand a pattern, scan the current directory and keep every filename the pattern matches, then sort the results - shells return glob matches in sorted order so output is predictable. One conventional rule: files whose names begin with . are hidden and are not matched by a leading * or ?, which is why * does not sweep up .git. A small recursive matcher handles both wildcards cleanly.

Make it work

// match(pattern, name): '*' matches any run, '?' matches one char
// then scan the directory and keep names that match, sorted
for each entry in readdir(".")
    if (entry[0] != '.' && match(pattern, entry))   // skip dotfiles
        add(results, entry);
sort(results);

CheckpointDONE

`echo *.txt` lists your text files. Commit and stop here.