What is the purpose of the question mark in a file name?

Answer 1

You cannot have a file name containing a question mark; it is an invalid character. However, you can use the question mark as a wildcard. E.g., to list every .txt file that has exactly two characters in its file name you would use the following command:

dir ??.txt

The '?' wildcard simply means any one character. If you wish to specify any group of characters, use the asterisk wildcard instead:

dir *.txt

For more information, look up "wildcard" in the command line documentation.

Most systems limit a file name to alphabet letters A-Z (and a-z) and numbers 0-9 and some special characters such as "$", "_", "#" with a single period separating the name part from an extension part (i.e. document.txt spreadsheet.xls). However, the actual directory structure in modern systems uses 8-bit characters for the file name so it is not impossible to have "invalid" characters in a file name. On the Microsoft platforms, a system or hardware misadventure can resulted in a corrupted file name entry with punctuation or non-printable characters in the file name. Some utilities deal with this by displaying question mark characters as printable substitutes for them.

Unix/linux implementations, being less restrictive, allow file names to have non-printable characters by using the backslash ("\") escape code.