Provided by: tcl8.6-doc_8.6.10+dfsg-1_all 
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.
SEE ALSO
file(3tcl), glob(3tcl)
KEYWORDS
current directory, absolute file name, relative file name, volume-relative file name,
portability