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Wildcards.md

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Wildcards in Linux

Wildcards are special characters used to represent one or more characters in file and directory names. They are commonly used in commands like ls, cp, mv, and rm to perform operations on multiple files at once. Mastering wildcards can greatly enhance your efficiency when working with the Linux command line.

Summary of Wildcard Characters

  • * : Matches zero or more characters.
  • ? : Matches exactly one character.
  • [] : Matches any one character within the brackets.
  • {} : Used to generate multiple strings or patterns (brace expansion).
  • - : Specifies a range within square brackets.
  • ! : Used to negate a pattern within square brackets.
  • ** : Matches files and directories recursively (with globstar enabled).
  • ~ : Represents the home directory.

Wildcard Characters Explained

1. * (Asterisk)

  • Matches any number of characters (including zero characters).
  • Used to match files or directories with any name or extension.

Examples:

  • *.txt → Matches all files with a .txt extension (e.g., file1.txt, document.txt).
  • file* → Matches all files starting with file (e.g., file1, file_abc, file123.txt).

2. ? (Question Mark)

  • Matches exactly one character.
  • Useful when you want to match files or directories where one character is variable.

Examples:

  • file?.txt → Matches file1.txt, fileA.txt, but not file.txt (expects one character between file and .txt).
  • ?abc → Matches 1abc, aabc, but not abc.

3. [] (Square Brackets)

  • Matches any one of the characters inside the brackets.
  • Can specify a range of characters.

Examples:

  • file[123].txt → Matches file1.txt, file2.txt, file3.txt, but not file4.txt.
  • file[a-d].txt → Matches filea.txt, fileb.txt, filec.txt, filed.txt.

Negation inside square brackets:

  • file[!a-d].txt → Matches files like filee.txt, filez.txt, but not filea.txt, fileb.txt, etc.

4. {} (Curly Braces)

  • Used for brace expansion to generate multiple strings or patterns.
  • Creates combinations of strings, simplifying commands.

Examples:

  • file{1,2,3}.txt → Matches file1.txt, file2.txt, file3.txt.
  • a{a,b,c}d → Matches aad, abd, acd.

Note: Brace expansion is performed before wildcard matching.

5. - (Hyphen)

  • Specifies a range within square brackets [].

Examples:

  • file[a-z].txt → Matches filea.txt to filez.txt (all lowercase letters).
  • file[0-9].txt → Matches file0.txt to file9.txt (digits).

6. ! (Exclamation Mark)

  • Negates a pattern within square brackets [].
  • Matches any character not in the brackets.

Examples:

  • file[!a-c].txt → Matches files not ending with a, b, or c (e.g., filed.txt).
  • file[!1-5].txt → Matches file6.txt, file7.txt, but not file1.txt to file5.txt.

7. ** (Double Asterisk)

  • Matches directories and files recursively.
  • Requires enabling the globstar shell option (Bash 4.0+).

Examples:

  • **/*.txt → Matches all .txt files in the current and all subdirectories.
  • **/file → Matches all files or directories named file at any directory level.

To enable globstar:

shopt -s globstar

8. ~ (Tilde)

  • Represents the home directory of the current user.

Examples:

  • cd ~ → Navigates to the home directory.
  • ls ~/Documents → Lists files in the Documents directory in your home folder.

Practical Examples

  1. *.txt → Matches all .txt files.
  2. file* → Files starting with file.
  3. *file → Files ending with file.
  4. * → All files in the current directory.
  5. *.* → Files with an extension.
  6. file*log* → Files starting with file containing log.
  7. file? → Files like file1, fileA.
  8. file[123] → Matches file1, file2, or file3.
  9. file[a-z] → Files filea to filez.
  10. file[0-9] → Files file0 to file9.
  11. file[a-d].txt → filea.txt to filed.txt.
  12. file[!a-d].txt → Files not ending with a to d.
  13. file{1,2,3}.txt → Specific numbered files.
  14. file{a,b,c}.txt → Files filea.txt, fileb.txt, filec.txt.
  15. *file*.txt → Files containing file ending with .txt.
  16. file[abc]* → Files starting with filea, fileb, or filec.
  17. *file[123].txt → Any files ending with file1.txt, file2.txt, or file3.txt.
  18. **/*.txt → All .txt files recursively.
  19. ~ → Home directory reference.
  20. file[-abc] → Files named file-, filea, fileb, or filec.
  21. file\! → Files starting with file!.

Note: Escape special characters with \ to match them literally.

  1. *~ → Files ending with ~ (backup files).
  2. *.bak → Files with .bak extension.
  3. file{1..5}.txt → file1.txt to file5.txt (sequence expansion).
  4. *.log* → Files containing .log.
  5. *log → Files ending with log.
  6. *test* → Files containing test.
  7. file*txt → Files starting with file ending with txt.
  8. file?.txt → Files like file1.txt, fileA.txt.
  9. [a-c]*.txt → Files starting with a, b, or c, ending with .txt.
  10. [!a-c]*.txt → Files not starting with a, b, or c, ending with .txt.
  11. *[!.txt] → Files not ending with .txt.
  12. *[^a-c]*.txt → Files not containing a to c, ending with .txt.
  13. *.*.* → Files with two dots in the name.
  14. file*.txt → Files starting with file ending with .txt.

Additional Notes

  • Escaping Wildcards: To match wildcard characters (*, ?, etc.) literally, precede them with a backslash (\):

    ls file\*

  • Character Classes: Use character classes for advanced matching:

    • [[:digit:]] → Any digit (same as [0-9]).
    • [[:alpha:]] → Any alphabetic character (same as [a-zA-Z]).
    • [[:alnum:]] → Any alphanumeric character.
    • [[:space:]] → Any whitespace character.
    • [[:upper:]] → Uppercase letters.
    • [[:lower:]] → Lowercase letters.

Examples:

  • file[[:digit:]].txt → Matches file0.txt to file9.txt.
  • *[[:space:]]* → Files with spaces in their names.

Enabling Recursive Matching with **

To use the ** wildcard for recursive searches:

  1. Enable globstar:

    shopt -s globstar

  2. Use ** in your commands:

    ls **/*.txt

    This lists all .txt files in the current directory and all subdirectories.

Caution with Wildcards

  • Using Wildcards with rm:

    Be extremely careful when using wildcards with the rm command to avoid deleting unintended files.

    Safe Practice:

    • Preview files before deleting:

      ls *.txt

    • Use interactive mode:

      rm -i *.txt