What do >! and >>! do in tcsh

In normal bash redirection > redirecting standard output to a file, overwriting when it exists and >> redirecting standard output to a file, appending when it exists.

In a tcsh (c shell) script I found the operators >! >>! being used. What do this operators do? tcsh does also have the > and >> operators, so what is the difference?

Solution

In tcsh redirection the ! symbol means overwrite the existing file even if noclobber is set.

In other words, if noclobber is set then:

cmd > file will write stdout to file if file does not exist
cmd > file will fail if file exists
cmd >> file will append stdout to file if file exists
cmd >> file will fail if file does not exist
cmd >! file will write stdout to file, overwriting any existing file
cmd >>! file will append stdout to file, creating the file if it does not already exist

If noclobber is not set then the ! has no effect:

cmd > file will write stdout to file, overwriting any existing file
cmd >> file will append stdout to file
cmd >! file will write stdout to file, overwriting any existing file
cmd >>! file will append stdout to file