KodeKloud Notes

In this guide, we’ll dive into the core shell expansions available in POSIX-compliant shells (like Bash and Zsh). You’ll learn how to use:

Brace expansions
Parameter expansions
Command substitutions
Filename generation (globs)

These mechanisms let you generate sequences, extract substrings, capture command output, and match multiple filenames—streamlining your shell scripts and command-line workflows.

Table of Expansion Types

Expansion Type	Description	Example
Brace	Generate comma- or range-separated strings	`echo {A,B,C}` → A B C
Parameter	Manipulate variable values (substrings, defaults)	`${var##/}` → strips longest `/` prefix
Command Substitution	Insert command output into another command	`echo "Date: $(date +%F)"`
Filename Generation	Match files with wildcards (globs)	`ls *.txt` → lists all `.txt` files

For an in-depth reference, see the Bash manual on Shell Expansions.

1. Brace Expansion

Brace expansion quickly generates arbitrary strings. It’s purely a string generation mechanism—no variables or globs involved.

# Generates A, B, C
echo {A,B,C}

# Generates numbers 1 through 5
echo {1..5}

Note

Brace expansions must not be quoted for the shell to recognize them. For example, echo "{A,B}" will literally output {A,B}.

2. Parameter Expansion

Parameter expansion lets you inspect or transform variable values without invoking external commands.

Basic Variable Expansion

USER_HOME=$HOME
echo "Your home directory is: $USER_HOME"

Removing Directory Components

To strip the longest matching prefix (e.g., remove everything up to the last slash), use ##*/:

#!/usr/bin/env bash

some_script="/usr/bin/my_script.sh"
# Remove the longest match of '*/' from the front
echo "${some_script##*/}"

Output:

$ ./expansions.sh
my_script.sh

Warning

Always quote your expansions (e.g., "${var}") to prevent word splitting and globbing in unexpected ways.

3. Command Substitution

Command substitution captures the stdout of a command and embeds it in another command’s arguments:

current_date=$(date +%Y-%m-%d)
echo "Today is $current_date"

4. Filename Generation (Globbing)

Globbing uses wildcard patterns to match filenames:

# List all .log files
ls *.log

# Recursive match in subdirectories (with Bash extglob)
shopt -s globstar
echo **/*.md

Next Steps

Now that you’ve seen the major shell expansions, try combining them to simplify your scripts—generate file lists, parse log entries, or batch-rename files with a single command.

For more examples and edge cases, consult the GNU Bash Reference Manual.

[object Object],