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filename: File name conventions supported by Tcl commands

NAME

filename - File name conventions supported by Tcl commands

INTRODUCTION

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).

PATH TYPES

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.

PATH SYNTAX

The rules for native names depend on the value reported in the Tcl array element tcl_platform(platform):

Unix

On Unix and Apple MacOS X platforms, Tcl uses path names where the components are separated by slashes. Path names may be relative or absolute, and file names may contain any character other than slash. The file names . and .. are special and refer to the current directory and the parent of the current directory respectively. Multiple adjacent slash characters are interpreted as a single separator. Any number of trailing slash characters at the end of a path are simply ignored, so the paths foo, foo/ and foo// are all identical, and in particular foo/ does not necessarily mean a directory is being referred.

The following examples illustrate various forms of path names:

/

Absolute path to the root directory.

/etc/passwd

Absolute path to the file named passwd in the directory etc in the root directory.

.

Relative path to the current directory.

foo

Relative path to the file foo in the current directory.

foo/bar

Relative path to the file bar in the directory foo in the current directory.

../foo

Relative path to the file foo in the directory above the current directory.

Windows

On Microsoft Windows platforms, Tcl supports both drive-relative and UNC style names. Both / and \ may be used as directory separators in either type of name. Drive-relative names consist of an optional drive specifier followed by an absolute or relative path. UNC paths follow the general form \\servername\sharename\path\file, but must at the very least contain the server and share components, i.e. \\servername\sharename. In both forms, the file names . and .. are special and refer to the current directory and the parent of the current directory respectively. The following examples illustrate various forms of path names:

\\Host\share/file

Absolute UNC path to a file called file in the root directory of the export point share on the host Host. Note that repeated use of file dirname on this path will give //Host/share, and will never give just //Host.

c:foo

Volume-relative path to a file foo in the current directory on drive c.

c:/foo

Absolute path to a file foo in the root directory of drive c.

foo\bar

Relative path to a file bar in the foo directory in the current directory on the current volume.

\foo

Volume-relative path to a file foo in the root directory of the current volume.

\\foo

Volume-relative path to a file foo in the root directory of the current volume. This is not a valid UNC path, so the assumption is that the extra backslashes are superfluous.

TILDE SUBSTITUTION

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.

PORTABILITY ISSUES

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.

KEYWORDS

current directory, absolute file name, relative file name, volume-relative file name, portability