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filename(n)                  Tcl Built-In Commands                 filename(n)

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NAME
filename - File name conventions supported by Tcl commands
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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
platform element of the tcl_platform array:

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.