Understanding Expansion in Bash

Bash expansions allow the shell to replace specific characters and patterns with actual values before command execution. Users can handle files, variables, arithmetic, and command output dynamically, while quoting controls how text and symbols are interpreted. This guide explores these powerful features step by step.

Pathname Expansion (*, ?, [])

echo *

Displays all files in the current directory.

echo D*

Lists all files starting with 'D'.

Handling Hidden Files

echo .[!.]*

Matches hidden files excluding . and ... Alternatively:

ls -A

Tilde Expansion (~)

echo ~

Expands to the current user’s home directory.

echo ~username

Expands to another user’s home directory.

Arithmetic Expansion

echo $((2 + 2))

Performs basic calculations.

echo $(((5**2) * 3))

Exponentiation and multiplication result: 75

Brace Expansion

echo Front-{A,B,C}-Back

Output: Front-A-Back Front-B-Back Front-C-Back

Sequences and Bulk Creation

echo Number_{01..05}

mkdir {2007..2009}-{01..12}

Creates year-month folders.

Parameter Expansion ($VAR)

echo $USER

Displays the username.

printenv | less

Lists environment variables.

Command Substitution

echo $(ls)

Injects command output into another command.

ls -l $(which cp)

Displays details about cp.

file $(ls -d /usr/bin/* | grep zip)

Quoting Techniques

Without Quotes

echo text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER

Double Quotes

echo "text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER"

Suppresses pathname and brace expansion; preserves others.

Single Quotes

echo 'text ~/*.txt {a,b} $(echo foo) $((2+2)) $USER'

Suppresses all expansions.

Escaping Characters

echo "The balance is: \$5.00"

Prevents incorrect variable expansion like $5.

Escaping Special Characters

mv bad\&filename good_filename

echo "Backslash: \\"

Control Characters with Escape Sequences

Sequence	Meaning
\a	Bell sound (beep)
\n	Newline
\t	Tab
\r	Carriage return
\b	Backspace

Usage in echo

echo -e "Time's up\a"

echo $'\a'

Conclusion

Bash expansion and quoting provide foundational tools for efficient scripting and command execution. By understanding how variables, commands, patterns, and special characters are interpreted, users can write smarter scripts, debug outputs effectively, and control shell behavior with precision.