6 - Shell Expansion

Think of Shell Expansion as the "translator" that sits between what you type and what the computer actually executes.
Before a command like ls or touch even starts to run, the Shell (the program reading your typing) scans your line for special characters and "expands" them into their full, literal meaning.

Step 1 { }

Step 2 *

Matches zero or more characters in filenames.

Step 3 [ ]

Matches one character from a specific set/range

Step 4 ?

Matches exactly one character in filenames.

Step 5 $()

Command inside $() will be execute first

The shell runs date +%Y-%m-%d first, receives the text (e.g., 2026-03-16), and then hands that text to touch
- touch report_$(date +%Y-%m-%d).txt
Display created file
- ls

student@localhost:~$ touch log_{2024..2025}_{Jan,Feb,Mar}.txt

student@localhost:~$ ls

Desktop Downloads log_2024_Jan.txt log_2025_Feb.txt log_2025_Mar.txt Pictures Templates

Documents log_2024_Feb.txt log_2024_Mar.txt log_2025_Jan.txt Music Public Videos

student@localhost:~$ ls log_2024_*

log_2024_Feb.txt log_2024_Jan.txt log_2024_Mar.txt

student@localhost:~$ ls log_202[45]_Mar.txt

log_2024_Mar.txt log_2025_Mar.txt

student@localhost:~$ ls log_202[45]_Mar.txt

log_2024_Mar.txt log_2025_Mar.txt

student@localhost:~$ touch report_$(date +%Y-%m-%d).txt

student@localhost:~$ ls

Desktop Downloads log_2024_Jan.txt log_2025_Feb.txt log_2025_Mar.txt Pictures report_2026-03-16.txt Videos

Documents log_2024_Feb.txt log_2024_Mar.txt log_2025_Jan.txt Music Public Templates

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