Trim a file in csh

In bash, if you want to clear out a file, the easiest command may be "> file":

# cat test.txt
Hello world!
# > test.txt
# cat test.txt
#

But in csh, "> file" will cause "Invalid null command." error:

# cat test.txt
Hello world!
# > test.txt
Invalid null command.

The solution is using ": > file" command:

# cat test.txt
Hello world!
# : > test.txt
# cat test.txt
#

Or employ "cat /dev/null > file".

Reference:
What does a leading colon (:) mean in a script?.