> ## Documentation Index
> Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Overview
> This guide explores core shell expansions in POSIX-compliant shells to enhance shell scripts and command-line workflows.
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: $(date +%F)"` |
| Filename Generation | Match files with wildcards (globs) | `ls *.txt` → lists all `.txt` files |
For an in-depth reference, see the [Bash manual on Shell Expansions][bash-expansion].
[bash-expansion]: https://www.gnu.org/software/bash/manual/html_node/Shell-Expansions.html
***
## 1. Brace Expansion
Brace expansion quickly generates arbitrary strings. It’s purely a string generation mechanism—no variables or globs involved.
```bash theme={null}
# Generates A, B, C
echo {A,B,C}
# Generates numbers 1 through 5
echo {1..5}
```
Brace expansions must not be quoted for the shell to recognize them. For example, `echo "{A,B}"` will literally output `{A,B}`.
***
## 2. Parameter Expansion
Parameter expansion lets you inspect or transform variable values without invoking external commands.
### Basic Variable Expansion
```bash theme={null}
USER_HOME=$HOME
echo "Your home directory is: $USER_HOME"
```
### Removing Directory Components
To strip the longest matching prefix (e.g., remove everything up to the last slash), use `##*/`:
```bash theme={null}
#!/usr/bin/env bash
some_script="/usr/bin/my_script.sh"
# Remove the longest match of '*/' from the front
echo "${some_script##*/}"
```
Output:
```bash theme={null}
$ ./expansions.sh
my_script.sh
```
Always quote your expansions (e.g., `"${var}"`) to prevent word splitting and globbing in unexpected ways.
***
## 3. Command Substitution
Command substitution captures the stdout of a command and embeds it in another command’s arguments:
```bash theme={null}
current_date=$(date +%Y-%m-%d)
echo "Today is $current_date"
```
***
## 4. Filename Generation (Globbing)
Globbing uses wildcard patterns to match filenames:
```bash theme={null}
# 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][bash-expansion].
```markdown theme={null}
```