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When using mp3gain in a script the only way to prevent occasional hangs is -c to ignore the clipping warning.
But what if I want to skip tracks that would clip? Or turn the up as far as is safe? I don't want to go to a two step process of first analyzing and parsing the output, and then running mp3gain again with a possibly modified number.
The -k switch is what you want. To re-phrase the help text, that switch means "While applying Track or Album Gain, if the suggested gain would make this mp3 clip, then lower the gain to the maximum non-clipping gain".