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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.

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Table of Expansion Types

Expansion TypeDescriptionExample
BraceGenerate comma- or range-separated stringsecho {A,B,C} → A B C
ParameterManipulate variable values (substrings, defaults)${var##*/} → strips longest */ prefix
Command SubstitutionInsert command output into another commandecho "Date: $(date +%F)"
Filename GenerationMatch files with wildcards (globs)ls *.txt → lists all .txt files
For an in-depth reference, see the Bash manual on Shell Expansions.

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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}.

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2. Parameter Expansion

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

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Basic Variable Expansion

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

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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.

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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"

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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

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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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