What characters are forbidden in Windows and Linux directory names?

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)