Overview

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}

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

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.

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Introduction

Conventions

Refresher

Streams

Expansions - Part One

Globs

Expansions - Part Two

Special Shell Variables

Arrays

Good Practices applied

awk

sed

Table of Expansion Types

1. Brace Expansion

2. Parameter Expansion

Basic Variable Expansion

Removing Directory Components

3. Command Substitution

4. Filename Generation (Globbing)

Next Steps

​Table of Expansion Types

​1. Brace Expansion

​2. Parameter Expansion

​Basic Variable Expansion

​Removing Directory Components

​3. Command Substitution

​4. Filename Generation (Globbing)

​Next Steps

Table of Expansion Types

1. Brace Expansion

2. Parameter Expansion

Basic Variable Expansion

Removing Directory Components

3. Command Substitution

4. Filename Generation (Globbing)

Next Steps