TIP #99 Version 1.15: Add 'file link' to Tcl

This is not necessarily the current version of this TIP.

Abstract

Tcl can read links, but cannot create them. This TIP proposes adding a file link subcommand to allow cross-platform creation of links.

Proposal

Add a new subcommand with the following syntax:

      file link ?-linktype? source ?target?

If only one argument is given, that argument is assumed to be source, and this command returns the value of the link given by source (i.e. the name of the file it points to). If source isn't a link or its value cannot be read, then an error is returned.

If 2 arguments are given, then these are assumed to be source and target. If source already exists, or if target doesn't exist, an error will be returned. Otherwise, Tcl creates a new link called source which points to the existing filesystem object at target, where the type of the link is platform-specific (on Unix a symbolic link will be the default). This is useful for the case where the user wishes to create a link in a cross-platform way, and doesn't care what type of link is created.

If the user wishes to make a link of a specific type only, (and signal an error if for some reason that is not possible), then the optional linktype argument should be given. Accepted values for linktype are "-symbolic" and "-hard".

When creating links on filesystems that either do not support any links, or do not support the specific type requested, an error message will be returned (in particular Windows 95, 98 and ME do not support any symbolic links at present, but Unix, MacOS and Windows NT/2000/XP (on NTFS drives) do).

The TIP proposes implementing:

           Unix,MacOSX       Win-NTFS             MacOS
symbolic:      yes        directories-only         yes
hard:          yes          files-only              no

This also leaves the avenue open, in the future, for the addition of other link types (e.g. Windows shortcuts) through additions to list of acceptable 'linktype's. This TIP only proposes adding the above options.

This means that a general 'file link $source $target' should always succeed on the above platforms (for both files and directories), but uses of '-hard' or '-symbolic' could fail, depending on the current platform.

Rationale

There are many requests on comp.lang.tcl for this functionality (see http://groups.google.com/groups?dq=&hl=en&lr=&ie=UTF8&oe=UTF8&threadm=4dd3bea3.0206100250.95eeb4e%40posting.google.com&rnum=1&prev=/&frame=on for a recent thread), and if Tcl can read links (file readlink, file lstat), it really ought to be able to write them.

Discussion has shown that both symbolic and hard links are desirable, and that for cross-platform use a general-purpose 'file link' which creates something is useful.

Alternatives

There is no cross-platform alternative available. TclX provides a link command for Unix only, and Unix platforms can also use exec ln ?-s? command to achieve the same effect.

Reference Implementation

See the 'testfilelink' command in tclTest.c, which is a partial implementation (used by the test suite), also see: http://sourceforge.net/tracker/index.php?func=detail&aid=562970&group_id=10894&atid=310894

These implementations need to be updated with the '-linktype' option, but are otherwise fully functional.

Copyright

This document has been placed in the public domain.

This is not necessarily the current version of this TIP.