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NN: khler
E-mail: khler@163.com
QQ: 23381103
MSN: pragmac@hotmail.com
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自动备份都喜欢在夜里0点，备份当天的日志，但是当系统到了夜里0时0分时，日期已经是第二天了，所以必须减1，当然了，日期的减1可没这么简单，什么月初第一天减1不一定是31，还可能是30，或者29，甚至28，前一个月当然也不能简单的减1完事，还得考虑年份...
Linux里面就有个相当牛x的命令："1 days ago"，嘿嘿
ubuntu:
hyh@hyh-Ubuntu:~$ date
2011年 02月 17日 星期四 10:40:40 CST
hyh@hyh-Ubuntu:~$ date --date "1 days ago"
2011年 02月 16日 星期三 10:40:59 CST
hyh@hyh-Ubuntu:~$
CenOS:
[root@localhost ~]# date --date "1 days ago"
2011年 02月 16日 星期三 10:43:29 CST
[root@localhost ~]# dd=$(date --date "1 days ago" +%Y-%m-%d)
[root@localhost ~]# echo $dd
2011-02-16
[root@localhost ~]#
看看下面怎么实现就知道有多复杂，上面的命令有多爽了：
#!/bin/sh
# ydate: A Bourne shell script that
# prints yestarday's date
# Output form: Month Day Year
# From Focus on Unix: http://unix.about.com
# 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 ]; then
if [ `expr $year % 400` -eq 0 ]; then
day=29
elif [ `expr $year % 100` -eq 0 ]; then
day=28
else
day=29
fi
else
day=28
fi
;;
esac
fi
fi
# Print the month day and year.
echo $month $day $year
exit 0
# ydate: A Bourne shell script that
# prints yestarday's date
# Output form: Month Day Year
# From Focus on Unix: http://unix.about.com
# 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 ]; then
if [ `expr $year % 400` -eq 0 ]; then
day=29
elif [ `expr $year % 100` -eq 0 ]; then
day=28
else
day=29
fi
else
day=28
fi
;;
esac
fi
fi
# Print the month day and year.
echo $month $day $year
exit 0
当然还有减一个时区的，也挺有意思：
$#看当前时区
$echo $TZ
CST-8
$#显示当前时间
$date
Mon Apr 2 15:48:36 CST 2002
$#改变当前时区，
TZ=CST+16;export TZ
$#显示当前时间（中间未改变系统时间，但date命令的显示已为昨天）
Mon Apr 1 15:48:33 CST 2002
