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Filename Expansion

Filename Expansion, also known as globbing, is a feature in Linux shells that enables users to use wildcards to match multiple filenames or directory names. It simplifies file management by allowing patterns to represent a group of files rather than specifying each file individually.

How Filename Expansion Works

When a shell encounters a command with wildcards, it scans the current directory (or the specified path) for files and directories matching the pattern. The matching filenames are substituted in place of the pattern before the command is executed.

For example:

ls *.txt

  • This command lists all files in the current directory with a .txt extension.

Wildcards in Filename Expansion

1. Asterisk (*)

  • Matches zero or more characters.
  • Example:

ls *.log    # Matches all files ending with .log

ls file*    # Matches files like file, file1, file123, file_name

2. Question Mark (?)

  • Matches exactly one character.
  • Example:

ls file?.txt    # Matches files like file1.txt, file2.txt but not file12.txt

3. Square Brackets ([])

  • Matches any one character within the brackets.
  • You can specify:
    • A set of characters: [abc]
    • A range of characters: [a-z] or [0-9]
    • Negation with ! or ^: [^abc] matches anything except a, b, or c.
  • Example:

ls file[123].txt    # Matches file1.txt, file2.txt, file3.txt

ls file[a-d].txt    # Matches filea.txt, fileb.txt, filec.txt, filed.txt

4. Curly Braces ({})

  • Matches a comma-separated list of strings.
  • Example:

mv file{1,2,3}.txt backup/    # Moves file1.txt, file2.txt, file3.txt to backup/

cp {file1,file2}.txt /tmp/    # Copies file1.txt and file2.txt to /tmp/

5. Tilde (~)

  • Expands to the home directory of the current user.
  • Example:

cd ~    # Changes directory to the user’s home

ls ~/Documents/*.pdf    # Lists all PDF files in the Documents directory

Combining Wildcards

Wildcards can be combined for more advanced patterns:

ls file[0-9]*.txt    # Matches files starting with ‘file’ followed by a number and ending in .txt

ls *.{jpg,png,gif}   # Matches files ending with .jpg, .png, or .gif

Special Characters

1. Escaping Characters

To match a literal wildcard character (*, ?, etc.), escape it with a backslash (\) or quote the pattern:

ls file\*    # Matches file* exactly

ls “file*”   # Matches file* exactly

2. Hidden Files

Files beginning with a dot (.) are hidden. To include them in filename expansion, use:

ls -a *      # Lists all files, including hidden files

ls .*        # Lists only hidden files

Practical Examples

1. Deleting Specific Files

Delete all .log files:

rm *.log

2. Copying Files with a Pattern

Copy all .txt and .csv files to a backup directory:

cp *.{txt,csv} backup/

3. Finding Specific Files

List all image files with specific extensions:

ls *.{jpg,png,gif}

4. Archiving Files

Create a tarball of all files starting with report:

tar -cvf reports.tar report*

How Shells Process Filename Expansion

  1. Pattern Matching:
    1. The shell matches the pattern against the names of files and directories in the specified path.
  2. Expansion:
    1. The matched filenames replace the pattern in the command.
  3. Command Execution:
    1. The shell executes the command with the expanded list of filenames.

Advanced Use with shopt in Bash

The shopt command in Bash allows you to control shell behavior related to filename expansion:

  1. Nullglob: Ensures that a wildcard that matches no files results in an empty list instead of the pattern itself.

shopt -s nullglob

ls *.nonexistent    # Produces no output

  • Dotglob: Includes hidden files (those starting with .) in matches.

shopt -s dotglob

ls *                # Lists all files, including hidden ones

  • Extglob: Enables extended globbing for more complex patterns.

shopt -s extglob

ls !(file1.txt)     # Lists all files except file1.txt

Limitations of Filename Expansion

  1. No Recursive Matching:
    1. Wildcards do not match files in subdirectories unless combined with tools like find or ** with shopt -s globstar.

shopt -s globstar

ls **/*.txt    # Matches .txt files in the current directory and subdirectories

  • Limited Pattern Matching:
    • For more complex matching (e.g., regular expressions), use tools like find, grep, or sed.

Conclusion

Filename expansion is a powerful feature of Linux shells, allowing users to efficiently work with groups of files using wildcards. Understanding and using the different types of patterns can significantly enhance productivity and make command-line tasks easier. By mastering these tools, you can handle complex file operations with minimal effort.