在shell中我要怎么取得其一天的时间？（在线等待）

在shell中，用date可以得到当前的时间，请问我怎么得到前一天的时间呢？
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• To print the date of the day before yesterday:

date --date='2 days ago'

• To print the date of the day three months and one day hence:
  date --date='3 months 1 day'

• To print the day of year of Christmas in the current year:
  date --date='25 Dec' +%j

• To print the current full month name and the day of the month:
  date '+%B %d'