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Bashの変数展開

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mongod の起動スクリプトに以下の記述があった。

/etc/init.d/mongod
mongod=${MONGOD-/usr/bin/mongod}

空欄か未定義だと - 以降が展開される?
正式な仕様を探したら man bash の "Parameter Expansion" に以下の記載があった。

  Omitting the colon results in a test  only  for  a  parameter  that  is
  unset.

":" コロンが省略された場合、未定義の場合のみ ${parameter:-word} と同じ挙動になるようだ。

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