emptying a file and keeping its ownership

[Jean-Pierre Radley]

In article <1991Jan1.040621.27634 at NCoast.ORG> allbery at ncoast.ORG (Brandon S. Allbery KB8JRR) writes:
>As quoted from <1990Dec31.214030.7816 at athena.mit.edu> by jik at athena.mit.edu (Jonathan I. Kamens):
>+---------------
>|   Several people have suggested using
>|     > $FILE
>| to truncate a file while retaining its old permissions.  This will work under
>| sh or ksh or bash or other shells that allow null commands for redirection. 
>| Unfortunately, csh and tcsh do not allow such null commands :-(.  Therefore,
>+---------------
>
>	% ls -s file
>	4 file
>	% echo > file
>	% ls -s file
>	0 file
>	% _
>
>Not that I like csh's echo behavior in this case, but it does get around the
>"null command" problem.


I find that in my csh, the following works:
	% : > file
That initial colon does the trick. What mechanism is operating here?

 Jean-Pierre Radley	    NYC Public Unix	jpr at jpr.com	CIS: 72160,1341