All Tcl commands and C procedures that take file names as arguments expect the file names to be in one of three forms, depending on the current platform. On each platform, Tcl supports file names in the standard forms(s) for that platform. In addition, on all platforms, Tcl supports a Unix-like syntax intended to provide a convenient way of constructing simple file names. However, scripts that are intended to be portable should not assume a particular form for file names. Instead, portable scripts must use the file split and file join commands to manipulate file names (see the file manual entry for more details).
File names are grouped into three general types based on the starting point for the path used to specify the file: absolute, relative, and volume-relative. Absolute names are completely qualified, giving a path to the file relative to a particular volume and the root directory on that volume. Relative names are unqualified, giving a path to the file relative to the current working directory. Volume-relative names are partially qualified, either giving the path relative to the root directory on the current volume, or relative to the current directory of the specified volume. The file pathtype command can be used to determine the type of a given path.
The rules for native names depend on the value reported in the Tcl platform element of the tcl_platform array:
The following examples illustrate various forms of path names:
In addition to the file name rules described above, Tcl also supports csh-style tilde substitution. If a file name starts with a tilde, then the file name will be interpreted as if the first element is replaced with the location of the home directory for the given user. If the tilde is followed immediately by a separator, then the $HOME environment variable is substituted. Otherwise the characters between the tilde and the next separator are taken as a user name, which is used to retrieve the user's home directory for substitution. This works on Unix, MacOS X and Windows (except very old releases).
Old Windows platforms do not support tilde substitution when a user name follows the tilde. On these platforms, attempts to use a tilde followed by a user name will generate an error that the user does not exist when Tcl attempts to interpret that part of the path or otherwise access the file. The behaviour of these paths when not trying to interpret them is the same as on Unix. File names that have a tilde without a user name will be correctly substituted using the $HOME environment variable, just like for Unix.
Not all file systems are case sensitive, so scripts should avoid code that depends on the case of characters in a file name. In addition, the character sets allowed on different devices may differ, so scripts should choose file names that do not contain special characters like: <>:?"/\|. The safest approach is to use names consisting of alphanumeric characters only. Care should be taken with filenames which contain spaces (common on Windows systems) and filenames where the backslash is the directory separator (Windows native path names). Also Windows 3.1 only supports file names with a root of no more than 8 characters and an extension of no more than 3 characters.
On Windows platforms there are file and path length restrictions. Complete paths or filenames longer than about 260 characters will lead to errors in most file operations.
Another Windows peculiarity is that any number of trailing dots ``.'' in filenames are totally ignored, so, for example, attempts to create a file or directory with a name ``foo.'' will result in the creation of a file/directory with name ``foo''. This fact is reflected in the results of file normalize. Furthermore, a file name consisting only of dots ``.........'' or dots with trailing characters ``.....abc'' is illegal.