expansion.md

Here's a concise cheatsheet on Expansion and Pattern Matching in Linux, complete with a markdown table of metacharacters and their matches. This can serve as a quick reference for understanding how to use these powerful features in the shell effectively.

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Expansion and Pattern Matching Cheatsheet

Linux shells, like Bash, offer various ways to expand variables and match patterns using special characters called metacharacters. These are used in command-line operations, scripting, and file manipulation, making tasks more efficient and versatile.

Expansion Types

1. Brace Expansion

Brace expansion is used to generate arbitrary strings by specifying a pattern within braces {}.

• Syntax:

  echo file{1,2,3}.txt

• Example:

  touch file{A,B,C}.txt
  # Creates files: fileA.txt, fileB.txt, fileC.txt

2. Tilde Expansion

Tilde expansion replaces the tilde ~ with the home directory path of the current user or specified user.

• Syntax:

  cd ~

• Example:

  cd ~/Documents
  # Changes directory to the Documents folder in the home directory

3. Parameter Expansion

Parameter expansion is used to manipulate variables and retrieve their values in different ways.

• Syntax:

  ${parameter}
  ${parameter:-word}  # Use default value if unset

• Example:

  NAME="Linux"
  echo ${NAME}          # Outputs: Linux
  echo ${UNSET_VAR:-Default}  # Outputs: Default

4. Command Substitution

Command substitution allows the output of a command to replace the command itself in the command line.

• Syntax:

  $(command)
  `command`

• Example:

  TODAY=$(date)
  echo "Today is $TODAY"
  # Outputs: Today is Fri Aug 02 14:21:30 UTC 2024

5. Arithmetic Expansion

Arithmetic expansion allows you to perform arithmetic operations using the shell.

• Syntax:

  $((expression))

• Example:

  echo $((5 + 3))
  # Outputs: 8

6. Pathname Expansion (Globbing)

Pathname expansion uses wildcards to match filenames and paths.

• Syntax:

  ls *.txt

• Example:

  echo *.txt
  # Matches all files ending with .txt in the current directory

Pattern Matching with Metacharacters

Linux shells use metacharacters to match patterns, providing flexibility in file manipulation and command execution. Here is a table of common metacharacters and their functions:

Metacharacter	Description	Example	Matches
*	Matches zero or more characters	file*	file, file1, file123, fileA
?	Matches exactly one character	file?.txt	file1.txt, fileA.txt
[...]	Matches any one of the enclosed characters	file[abc].txt	filea.txt, fileb.txt, filec.txt
[^...]	Matches any one character not enclosed	file[^abc].txt	filex.txt, filey.txt
[a-z]	Matches any one character in the specified range	file[a-c].txt	filea.txt, fileb.txt
[!a-z]	Matches any character not in the specified range	file[!a-c].txt	filed.txt, filex.txt
{}	Matches any of the patterns separated by commas	file{1,2,3}.txt	file1.txt, file2.txt
{a..z}	Matches characters in a specified range (brace expansion)	file{a..c}.txt	filea.txt, fileb.txt, filec.txt
\	Escapes special characters to treat them literally	file\*.txt	file*.txt

Pattern Matching Examples

Here's a deeper look into pattern matching using the metacharacters from the table above:

1. Asterisk (*)

• Example:

  ls *.jpg

• Description:

  • Matches all .jpg files, such as image.jpg, photo1.jpg, and cat.jpg.

2. Question Mark (?)

• Example:

  ls file?.txt

• Description:

  • Matches file1.txt, file2.txt, etc., but not file123.txt.

3. Square Brackets ([...])

• Example:

  ls file[123].txt

• Description:

  • Matches file1.txt, file2.txt, or file3.txt.

4. Negated Square Brackets ([^...])

• Example:

  ls file[^123].txt

• Description:

  • Matches any file like filex.txt, filea.txt, but not file1.txt, file2.txt, or file3.txt.

5. Brace Expansion ({})

• Example:

  touch {fileA,fileB,fileC}.txt

• Description:

  • Creates fileA.txt, fileB.txt, fileC.txt.

6. Range within Braces ({a..z})

• Example:

  echo {a..d}

• Description:

  • Outputs a b c d.

Advanced Pattern Matching Techniques

The shell also provides advanced pattern matching operators like #, %, ##, and %%, used primarily in parameter expansion:

1. Single Character Match (#)

• Syntax:

  ${variable#pattern}

• Example:

  FILENAME="path/to/file.txt"
  echo ${FILENAME#*/}
  # Outputs: "to/file.txt" (removes shortest match from the beginning)

2. Greedy Character Match (##)

• Syntax:

  ${variable##pattern}

• Example:

  FILENAME="path/to/file.txt"
  echo ${FILENAME##*/}
  # Outputs: "file.txt" (removes longest match from the beginning)

3. Single Character Match from End (%)

• Syntax:

  ${variable%pattern}

• Example:

  FILENAME="path/to/file.txt"
  echo ${FILENAME%/*}
  # Outputs: "path/to" (removes shortest match from the end)

4. Greedy Character Match from End (%%)

• Syntax:

  ${variable%%pattern}

• Example:

  FILENAME="path/to/file.txt"
  echo ${FILENAME%%/*}
  # Outputs: "path" (removes longest match from the end)

Example: Removing a File Extension

FILENAME="document.txt"
BASENAME=${FILENAME%.txt}
echo $BASENAME
# Outputs: "document"

Example Commands and Use Cases

Here's a brief walkthrough of some practical examples to demonstrate how expansion and pattern matching can be applied:

1. Renaming Multiple Files

You can use brace expansion to rename multiple files quickly.

mv file{1,2}.txt newfile{1,2}.txt
# Renames file1.txt to newfile1.txt and file2.txt to newfile2.txt

2. Creating Multiple Directories

Use brace expansion to create multiple directories with a single command.

mkdir project/{src,bin,lib,doc}
# Creates directories: project/src, project/bin, project/lib, project/doc

3. Finding and Removing Files

Combining pathname expansion and commands like find makes it easy to locate and remove files.

find . -name "*.log" -type f -delete
# Finds and deletes all .log files in the current directory and subdirectories

4. Batch Processing Files

Using * and ? can help process a batch of files simultaneously.

cp backup_2024-??.tar.gz /backups
# Copies files like backup_2024-01.tar.gz, backup_202