What characters are forbidden in Windows and Linux directory names?

Question

I know that / is illegal in Linux, and the following are illegal in Windows (I think) * . " / \ [ ] : ; | ,

What else am I missing?

I need a comprehensive guide, however, and one that takes into account double-byte characters. Linking to outside resources is fine with me.

I need to first create a directory on the filesystem using a name that may contain forbidden characters, so I plan to replace those characters with underscores. I then need to write this directory and its contents to a zip file (using Java), so any additional advice concerning the names of zip directories would be appreciated.

Answer 1

Let's keep it simple and answer the question, first.

1. The forbidden printable ASCII characters are:

   • Linux/Unix:

       / (forward slash) 

   • Windows:

       < (less than)   > (greater than)   : (colon - sometimes works, but is actually NTFS Alternate Data Streams)   " (double quote)   / (forward slash)   \ (backslash)   | (vertical bar or pipe)   ? (question mark)   * (asterisk) 

2. Non-printable characters

   If your data comes from a source that would permit non-printable characters then there is more to check for.

   • Linux/Unix:

       0 (NULL byte) 

   • Windows:

       0-31 (ASCII control characters) 

   ^{Note: While it is legal under Linux/Unix file systems to create files with control characters in the filename, it might be a nightmare for the users to deal with such files.}

3. Reserved file names

   The following filenames are reserved:

   • Windows:

       CON, PRN, AUX, NUL    COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9   LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, LPT9 

     (both on their own and with arbitrary file extensions, e.g. LPT1.txt).

4. Other rules

   • Windows:

     Filenames cannot end in a space or dot.

   • macOS:

     You didn't ask for it, but just in case: Colon : and forward slash / depending on context are not permitted (e.g. Finder supports slashes, terminal supports colons). (More details)

Answer 2

A “comprehensive guide” of forbidden filename characters is not going to work on Windows because it reserves filenames as well as characters. Yes, characters like * " ? and others are forbidden, but there are a infinite number of names composed only of valid characters that are forbidden. For example, spaces and dots are valid filename characters, but names composed only of those characters are forbidden.

Windows does not distinguish between upper-case and lower-case characters, so you cannot create a folder named A if one named a already exists. Worse, seemingly-allowed names like PRN and CON, and many others, are reserved and not allowed. Windows also has several length restrictions; a filename valid in one folder may become invalid if moved to another folder. The rules for naming files and folders are on the Microsoft docs.

You cannot, in general, use user-generated text to create Windows directory names. If you want to allow users to name anything they want, you have to create safe names like A, AB, A2 et al., store user-generated names and their path equivalents in an application data file, and perform path mapping in your application.

If you absolutely must allow user-generated folder names, the only way to tell if they are invalid is to catch exceptions and assume the name is invalid. Even that is fraught with peril, as the exceptions thrown for denied access, offline drives, and out of drive space overlap with those that can be thrown for invalid names. You are opening up one huge can of hurt.