As Chapter 2 explained, both files and directories are identified by their names. A directory is really just a special kind of file, so the rules for naming directories are the same as the rules for naming files.

Filenames may contain any character except /, which is reserved as the separator between files and directories in a pathname. Filenames are usually made of upper- and lowercase letters, numbers, “.” (dots), and “_” (underscores). Other characters (including spaces) are legal in a filename, but they can be hard to use because the shell gives them special meanings. However, spaces are a standard part of Macintosh file and folder names, so while we recommend using only letters, numbers, dots, and underscore characters for filenames, the reality is that you will have to work with spaces in file and directory names. The Finder, by contrast, dislikes colons (which older versions of Mac OS used as a directory separator, just as Unix uses the slash). If you display a file called test:me in the Finder, the name is shown as test/me instead. (The reverse is also true: if you create a file in the Finder whose name contains a slash, it will appear as a colon in the Terminal.)

If you have a file with spaces in its name, the shell will be confused if you type its name on the command line. That’s because the shell breaks command lines into separate arguments at the spaces. To tell the shell not to break an argument at spaces, either put quotation marks (") around the argument or preface each space with a backslash ().

For example, the rm program, covered later in this chapter, removes Unix files. To remove a file named a confusing name, the first rm command in the following snippet doesn’t work, but the second one does. Also note that you can escape spaces (that is, avoid having the shell interpret them inappropriately) by using a backslash character, as shown in the third example:

% ls -l
total 2
-rw-r--r--   1 taylor  staff   324 Feb  4 23:07 a confusing name
-rw-r--r--   1 taylor  staff    64 Feb  4 23:07 another odd name
% rm a confusing name
rm: a: no such file or directory
rm: confusing: no such file or directory
rm: name: no such file or directory
% rm "a confusing name"
% rm another odd name
%

You should also use a backslash () before any of the following special characters, which have meaning to the shell: * # ` " ' $ | & ? ; ~ ( ) < > ! ^.

A filename must be unique inside its directory, but other directories may have files with the same names. For example, you may have the files called chap1 and chap2 in the directory /Users/carol/work and also have different files with the same names in /Users/carol/play.