Naming files and directories

Most of the non-alphanumeric characters on the keyboard have special uses in Unix, so in general it is a good idea to name file using only

letters (uppercase and lowercase)
digits
the characters ``.'' (period) and ``_'' (underscore)

Using any other characters in the name of a file or directory may have side effects; a Unix command involving such a filename may be interpreted in a wacky way if the Unix shell (command interpreter) sees the special characters as part of the syntax of the command rather than merely part of a file name.

More information: file types/suffixes, Unix syntax, punctuation