Wildcards and 'globbing'

Globs are wildcard patterns that let the command-line match multiple files at once.

Using *

The most useful wildcard is *, which means 'match anything'. For example this command:

% ls *.txt

will list all files ending in '.txt' in the current directory.

A glob can have multiple '*' characters in it - e.g. to get a list of files that have 'file' in the name and end in '.txt':

% ls *file*.txt

This feature can be combined with any command that uses a file, and is quite powerful.

To illustrate this, here is a command sequence that creates a new temporary folder, copies all text files into it, removes listing.txt, concatenates all the others into one big text file, and then removes the temporary files:

% mkdir tmp_folder
% cp *.txt tmp_folder/
% rm tmp_folder/listing.txt
% cat tmp_folder/*.txt > concatenated.txt
% rm tmp_folder/*.txt
% rmdir tmp_folder

Note

Depending on your shell, you might find that pressing <tab> before <enter> has the effect of expanding the wildcard, right there on your terminal. This is what is happening under the hood: *.txt is shorthand for typing out all the files that would match *.txt, one by one, on the command line.

Apart from *, which is the most useful glob special character, there are a few others as well. But '*' is the most useful.

How globbing is processed

Importantly you should realise that the shell processes globbing (and any other filename expansions) before it passes the arguments to the command. Suppose your folder looks like this:

file1.txt
file2.txt
image.jpg

and you type:

% ls *

Then, as far as ls is concerned, this is equivalent to typing:

ls file1.txt file2.txt image.jpg

That is - the ls command does not know you are using a glob, because the shell expands it beforehand.

Avoiding globbing

What if you don't want a glob, but want the command to see the argument you typed instead? The answer is to wrap the argument in single quotes:

% ls '*'
ls: *: No such file or directory

Here ls has correctly told me that I don't have a file called * in my directory.

Single quotes '<something>' and double quotes "<something>" behave differently in this regard - compare to:

% ls "*"

If you want the shell not to expand what's in the quotes, use single quotes.

Question

Another way to see what's going on is to use echo (which you'll recall just prints out its arguments). Try these three commands:

% echo *
% echo "*"
% echo '*'

Make sure you understand the results!