Filename expansion (globbing)

What

• Globbing is the process in which the shell (like Bash) automatically expands wildcard patterns into matching filenames before executing a command.
• It’s basically a way to use shorthand symbols (wildcards) to match multiple files instead of listing them manually.

Globbing with *

ls *.txt

• * (asterisk) is a wildcard that matches any characters.
  • 0 to unlimited number of characters
• This command expands to list all files ending in .txt in the current directory. If you had notes.txt and report.txt, Bash would rewrite it as:

ls notes.txt report.txt

• ls folder/* will list everything in the folder
• if nothing matches then the entire term with the globbing will be treated as it’s one file
• escaping the wildcard
  • ls '*.txt'

Additional globbing wildcards

• ?
  • matches any single character
• [0-9]
  • a range, allowing us to specify a character range (here: all numbers)
  • echo ./images/IMG?[0-9][0-9][0-9][0-9].[a-z][a-z][a-z]
• **
  • Normally, * matches files and directories at the current level.
    • ** makes it recursive, meaning it searches through all subdirectories.
    • matches 0 up to arbitrary many characters (including /)
  • only supported in bash 4.0 or up,
  • u might have to do this: shopt -s globstar
  • usage
    • echo * → current files/folders (x subdirectories)
    • echo ** → current files + directories + and all subdirectories & files
    • echo **/* → current files + subdirectory & files
    • echo */** → subdirectory + files (and not current files)

Be careful with globbing

• If we use this in a wrong way it can lead to permanent damage
• bash doesn’t differentiate between a folder and a parameter
  • name of a file can be interpreted as a parameter
• example
  • we have a folder named -rf lol
    • if we execute rm *, then * will expand so -rf will appear in the command → rm -rf
    • then rm will think -rf is a parameter
  • touch -rf
    • we wanted to create a file named -rf, but touch processed it as multiple arguments
    • -r (unknown option, so ignored) + f (as filename, but it doesn’t exist)
    • use touch ./rf
  • if we don’t want folder -rf to be interpreted as a command option, use ./ in front
    • rm ./*