When both are specified, the latter, effectively has no effect (ignored). Even though the usage string put them as mutually exclusive options, we can't just usage() here, because other implementations (for instance, GNU one) does not stop at this point, but prints a warning: xargs: warning: options --max-lines and --replace/-I/-i are mutually exclusive, ignoring previous --max-lines value. So, in order to stay backward compatible, print the warning message and continue the execution.