redirect - What do >! and >>! do in tcsh

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Top 3 Answer for redirect - What do >! and >>! do in tcsh

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95

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
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89

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.

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