AIX 环境下取得昨天日期的 Shell

<h1>我需要使用自动shell导出昨天的数据库内容，由于aix的ksh不支持date -d命令,不得不选择其他方式来实现。</h1>

#!/bin/ksh  # Var Declare  #!/bin/sh  # ydate: A Bourne shell script that  # prints yestarday's date  # Output Form: Month Day Year  # change by chenjy 20161026# Set the current month day and year.  month=`date +%m`day=`date +%d`year=`date +%Y`# Add 0 to month. This is a  # trick to make month an unpadded integer.  month=`expr $month + 0`# Subtract one from the current day.  day=`expr $day - 1`# If the day is 0 then determine the last  # day of the previous month.  if [ $day -eq 0 ]; then# Find the preivous month.  month=`expr $month - 1`# If the month is 0 then it is Dec 31 of  # the previous year.  if [ $month -eq 0 ]; then   month=12   day=31   year=`expr $year - 1`# If the month is not zero we need to find  # the last day of the month.  else   case $month in     1|3|5|7|8|10|12) day=31;;     4|6|9|11) day=30;;     2)       if ( [ `expr $year % 4` -eq 0 ] && [ `expr $year % 100` -ne 0 ] || [ `expr $year % 400` -eq 0 ] ) ; then           day=29       else         day=28       fi#       if [ `expr $year % 400` -eq 0 ]; then  #         day=29  #       fi       ;;   esacfificase $day     in 1|2|3|4|5|6|7|8|9) day='0'$dayesaccase $month     in 1|2|3|4|5|6|7|8|9) month='0'$monthesacecho $year$month$day