filename(TCL)

filename -- filename conventions supported by Tcl commands

Introduction

All Tcl commands and C procedures that take filenames as arguments expect the filenames to be in one of three forms, depending on the current platform. On each platform, Tcl supports filenames 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 filenames. However, scripts that are intended to be portable should not assume a particular form for filenames. Instead, portable scripts must use the file split and file join commands to manipulate filenames (see the file(TCL) manual page for more details).

Path types

Filenames 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):

mac

On Apple Macintosh systems, Tcl supports two forms of pathnames. The normal Mac style names use colons as path separators. Paths may be relative or absolute, and filenames may contain any character other than colon. A leading colon causes the rest of the path to be interpreted relative to the current directory. If a path contains a colon that is not at the beginning, then the path is interpreted as an absolute path. Sequences of two or more colons anywhere in the path are used to construct relative paths where :: refers to the parent of the current directory, ::: refers to the parent of the parent, and so forth.

In addition to Macintosh style names, Tcl also supports a subset of UNIX-like names. If a path contains no colons, then it is interpreted like a UNIX path. Slash is used as the path separator. The filename . refers to the current directory, and .. refers to the parent of the current directory. However, some names like / or /.. have no mapping, and are interpreted as Macintosh names. In general, commands that generate filenames will return Macintosh style names, but commands that accept filenames will take both Macintosh and UNIX-style names.

The following examples illustrate various forms of pathnames:

:: Relative path to the current folder.
MyFile: Relative path to a file named MyFile in the current folder.
MyDisk:MyFile: Absolute path to a file named MyFile on the device named MyDisk.
:MyDir:MyFile: Relative path to a file named MyFile in a folder named MyDir in the current folder.
::MyFile: Relative path to a file named MyFile in the folder above the current folder.
:::MyFile: Relative path to a file named MyFile in the folder two levels above the current folder.
/MyDisk/MyFile: Absolute path to a file named MyFile on the device named MyDisk.
../MyFile: Relative path to a file named MyFile in the folder above the current folder.

unix

On UNIX platforms, Tcl uses pathnames where the components are separated by slashes. Pathnames may be relative or absolute, and filenames may contain any character other than slash. The filenames . 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. The following examples illustrate various forms of pathnames:

/: 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. In both forms, the filenames . and .. are special and refer to the current directory and the parent of the current directory respectively. The following examples illustrate various forms of pathnames:

\\Host\share/file: Absolute UNC path to a file called file in the root directory of the export point share on the host 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.

Tilde substitution

In addition to the filename rules described above, Tcl also supports csh-style tilde substitution. If a filename starts with a tilde, then the filename 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.

The Macintosh and 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. File names that have a tilde without a user name will be 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 filename. In addition, the character sets allowed on different devices may differ, so scripts should choose filenames that do not contain special characters like: <>:"/\|. The safest approach is to use names consisting of alphanumeric characters only. 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.