fails:Kernel#` is a private method
fails:Kernel#` returns the standard output of the executed sub-process
fails:Kernel#` tries to convert the given argument to String using #to_str
fails:Kernel#` produces a String in the default external encoding
fails:Kernel#` sets $? to the exit status of the executed sub-process
fails:Kernel.` needs to be reviewed for spec completeness
fails:Kernel#` raises an Errno::ENOENT if the command is not executable
fails:Kernel.` tries to convert the given argument to String using #to_str
