What do >! and >>! do in tcsh

问题:

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?

回答1:

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

回答2:

The exclamation mark suppresses the check for the type of file being written to in certain cases.

To quote the tcsh man page:

If the shell variable noclobber is set, then the file must not exist or be a character special file (e.g., a terminal or ‘/dev/null’) or an error results. This helps prevent accidental destruction of files. In this case the ‘!’ forms can be used to suppress this check.