Windows command line and batch files: potential problems with unescaped special characters in filenames?

Question

Up to this point I assumed that windows disallowed all special characters which have a meaning in the command line. However, it's perfectly possible to have a file named "file%sometext%"

if this filename is called in a batch script or at the command like, the empty variable %sometext% will be expanded to nothing, and the filename parsed as "file".

Are there any other legitimate filenames that could cause problems in a batch script in this way?

Answer 1

Windows NT is an operating system that runs other operating systems. Whatever rules the operating system doing the file operation has is the rules that apply.

These are the Windows' rules. Unix programs will follow Unix's rules. It's possible to create a file in Unix that you can't open in Windows and vice versa.

File Name Conventions

Although each file system can have specific rules about the formation of individual components in a directory or file name, all file systems follow the same general conventions: a base file name and an optional extension, separated by a period. For example, the MS-DOS FAT file system supports 8 characters for the base file name and 3 characters for the extension. This is known as an 8.3 file name. The FAT file system and NTFS support file names that can be up to 255 characters long. This is known as a long file name. To get an MS-DOS file name given a long file name, use the GetShortPathName function. To get the full path of a file, use the GetFullPathName function.

Both file systems use the backslash (\) character to separate directory names and the file name when forming a path.

General rules for applications creating names for directories and files or processing names supplied by the user include the following:

Use any character in the current code page for a name, but do not use a path separator, a character in the range 0 through 31, or any character explicitly disallowed by the file system. A name can contain characters in the extended character set (128–255).

Use the backslash (\), the forward slash (/), or both to separate components in a path. No other character is acceptable as a path separator. Note that UNC names must adhere to the following format: \\server\share. Use a period (.) as a directory component in a path to represent the current directory.

Use two consecutive periods (..) as a directory component in a path to represent the parent of the current directory. Use a period (.) to separate the base file name from the extension in a directory name or file name.

Do not use the following characters in directory names or file names, because they are reserved:

< > : " / \ |

Do not use device names, such as aux, con, lpt1, and prn, as file names or directory names.

Process a path as a null-terminated string. The maximum length for a path, including a trailing backslash, is given by MAX_PATH. The Unicode versions of several functions permit paths that exceed the MAX_PATH length if the path has the "\\?\" prefix. The "\\?\" tells the function to turn off path parsing. However, each component in the path cannot be more than MAX_PATH characters long. Use the "\\?\" prefix with paths for local storage devices and the "\\?\UNC\" prefix with paths having the Universal Naming Convention (UNC) format. The "\\?\" is ignored as part of the path. For example, "\\?\C:\myworld\private" is seen as "C:\myworld\private", and "\\?\UNC\bill_g_1\hotstuff\coolapps" is seen as "\\bill_g_1\hotstuff\coolapps".

Do not assume case sensitivity. Consider names such as OSCAR, Oscar, and oscar to be the same.

The following reserved words cannot be used as the name of a file: CON, PRN, AUX, CLOCK$, NUL, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, and LPT9. Also, reserved words followed by an extension—for example, NUL.tx7—are invalid file names.

By following the rules listed in this section, an application can create valid names for files and directories regardless of the file system in use.

Backslashes (\) are used as element dividers in paths (dividing the file name from the path to it, or directories from one another in a path). You cannot use them in file or directory names. They may be required as part of volume names (for example, "C:\").

Platform SDK Release: August 2001