Shell Expansions and Wildcards

Select groups of files predictably before performing an action.

Use brace expansion, globbing, command substitution, arithmetic expansion, and defaults.

Outcome: Patterns like *.txt become helpful time savers rather than confusing symbols.

Mental model

Understand what is happening

The shell expands patterns before commands run.

Globs match filenames; regex is different.

Always echo a pattern first if you are unsure.

Guided lab

Work through the mission

Brace expansion

Safe

Creates multiple practice files quickly.

Terminal

echo file_{1..5}.txt
touch file_{1..5}.txt
ls file_*.txt

Globbing lab

Safe

Shows common filename patterns.

Terminal

mkdir -p glob-lab
cd glob-lab
touch a.txt b.txt c.txt 1.txt 22.txt abc.txt
echo *.txt
echo ?.txt
echo [ab].txt
cd ..

Safe cleanup

Caution

Removes only the glob-lab folder created in the sandbox.

Terminal

rm -rf glob-lab

Checkpoint

Verify the mission

The expanded names match the intended files.

Try it yourself

One step without a recipe

Create red_file.txt, green_file.txt, blue_file.txt and match them with *file.txt.

Need a hint?

Before you run `rm` with a wildcard, run `echo` with that same pattern. If the file list looks wrong, your pattern is wrong. Two seconds of checking saves two hours of recovery.

Explain it back

Before you move on

What does ?.txt match?
Why should you echo a risky glob first?