Wildcards

Note: Work in progress.

Description

Wildcards or file globing is a shell feature that, using special characters, allows us to rapidly specify groups of filenames. Because we work with files all the time, it is useful to be able to work with multiple files at the same time.

Why and when should I use a wildcard?

Using wildcards can save you a lot of time. Imagine you need to move all the text files in a folder. You could move them one by one—if you know each file’s name. But what if there are 300 files? Moving them one at a time would take forever! Wildcards help you by allowing you to match all files that meet a certain pattern or criteria, so you can move them all at once with a single command. Here are some examples:

  • Copy all photos from a drive to a flash drive.
  • Delete certain types of files from a folder.
  • List all files that have a date in their name.

There are only 3 wildcards. Here is how they work:

Wildcard definition example
* matches 0 to any number of characters ls ~/Downloads/*.png
? matches 1 character ls ~/Downloads/f?ll.sh
[] matches 1 character from a set ls ~/Downloads/f[0-9]ll.sh

What does “matches x number of character” really mean?

This refers to the number of characters in the file name that can be matched everytime the wildcard is used in a command. For example, here is an illustration using the * wildcard:

Notice that because the * wildcard matches 0 characters and also any number of characters, this command: ls *SC*.png matches the files Script_remover.sh and BackupScript.sh.

To practice the wildcards, run this command:

curl https://cis106.com/assets/extras/wildcards.sh | bash

This will create a directory in your home directory called wildcard_extra_practice. This directory contains a bunch of files and directories (created at random). All the examples here will use the files in this directory.

The * (star/asterisk) wildcard

Examples:

  • list all of the directories inside a given directory without listing their content
    • ls -1d wildcard_extra_practice/*/
  • move all the .sh (shell scripts) files
    • mv wildcard_extra_practice/*.sh wildcard_extra_practice/scripts/
  • list and then move all the image files in the wildcard_extra_practice directory. You will need to create a directory first.
    • mkdir wildcard_extra_practice/images/
    • ls wildcard_extra_practice/img-*.*
    • mv -v wildcard_extra_practice/img-*.*images/

The ? (question mark) wildcard

Examples:

  • List all the files that contain a 4 letter file extension.
    • ls -1X wildcard_extra_practice/*.????
  • List all the files that contain a 4 letter file extension and start with letter i:
    • ls -1X wildcard_extra_practice/i*.????
  • List all th files Microsoft Office 365 files.
    • ls -1X wildcard_extra_practice/*.???x

The [] (square brackets) wildcard

POSIX class Represents Means Example using ls
[:upper:] [A-Z] Uppercase letters ls *[[:upper:]]* - List files with uppercase letters
[:lower:] [a-z] Lowercase letters ls *[[:lower:]]* - List files with lowercase letters
[:digit:] [0-9] Digits ls *[[:digit:]]* - List files containing digits
[:alpha:] [A-Za-z] Alphabetic characters ls *[[:alpha:]]* - List files with alphabetic characters
[:alnum:] [A-Za-z0-9] Alphanumeric characters ls *[[:alnum:]]* - List files with alphanumeric characters
[:space:] [ \t\n\r\f\v] Whitespace characters ls *[[:space:]]* - List files with spaces in names
[:punct:] Punctuation Punctuation characters ls *[[:punct:]]* - List files with punctuation characters
[:blank:] [ \t] Space and tab ls *[[:blank:]]* - List files with spaces or tabs
[:xdigit:] [0-9A-Fa-f] Hexadecimal digits ls *[[:xdigit:]]* - List files with hexadecimal characters
[:cntrl:] Control characters Control characters ls *[[:cntrl:]]* - List files with control characters
[:print:] Printable Printable characters ls *[[:print:]]* - List files with printable characters
[:graph:] Graphical Visible characters (not spaces) ls *[[:graph:]]* - List files with graphical characters
[:word:] [A-Za-z0-9_] Word characters (alphanumeric + underscore) ls *[[:word:]]* - List files containing word characters
[:ascii:] ASCII characters All ASCII characters (0-127) ls *[[:ascii:]]* - List files containing ASCII characters

Examples:

  • List all the files that start with a capital letter
    • ls wildcard_extra_practice/[A-Z]*
  • List all the files that contain a number in their name
    • `ls wildcard_extra_practice/[0-9]

Note:

  • Wildcards also have their limitations.
  • For instance, “List all the files that do not contain a digit in the file name” You may assume that the approach would be ls *[!0-9]*. However, this does not work. This wildcard translates to:
    “all files and directories in the current directory whose names contain at least one character that is not a digit (0-9)” However, if you ever need this, here are two approaches:
    • Use ls + grep:
      • ls *[!0-9]* | grep -v '[0-9]'
    • Use find:
      • find ./ -not -name '*[0-9]*'
    • Find is preferred for scripts because ls is not suited for shell scripts. ls is intended to be a human friendly tool and the output is difficult to parse properly.