原文：http://bash.cyberciti.biz/time-and-date/validation-shell-script/
日期要求以mm/dd/yyyy的格式用键盘输入，用shell脚本检查是否是有效的日期。
下面来看下该shell脚本的源代码：代码比较长，请耐心看。
#!/bin/bash
# Shell program to find the validity of a given date
# -----------------------------------------------
# Copyright (c) 2005 nixCraft project <http://cyberciti.biz/fb/>
# This script is licensed under GNU GPL version 2.0 or above
# -------------------------------------------------------------------------
# This script is part of nixCraft shell script collection (NSSC)
# Visit http://bash.cyberciti.biz/ for more information.
# -------------------------------------------------------------------------
# store day, month and year
dd=0
mm=0
yy=0
# store number of days in a month
days=0
# get day, month and year
echo -n "Enter day (dd) : "
read dd
echo -n "Enter month (mm) : "
read mm
echo -n "Enter year (yyyy) : "
read yy
# if month is negative (<0) or greater than 12
# then it is invalid month
if [ $mm -le 0 -o $mm -gt 12 ];
then
echo "$mm is invalid month."
exit 1
fi
# Find out number of days in given month
case $mm in
1) days=31;;
2) days=28 ;;
3) days=31 ;;
4) days=30 ;;
5) days=31 ;;
6) days=30 ;;
7) days=31 ;;
8) days=31 ;;
9) days=30 ;;
10) days=31 ;;
11) days=30 ;;
12) days=31 ;;
*) days=-1;;
esac
# find out if it is a leap year or not
if [ $mm -eq 2 ]; # if it is feb month then only check of leap year
then
if [ $((yy % 4)) -ne 0 ] ; then
: # not a leap year : means do nothing and use old value of days
elif [ $((yy % 400)) -eq 0 ] ; then
# yes, it's a leap year
days=29
elif [ $((yy % 100)) -eq 0 ] ; then
: # not a leap year do nothing and use old value of days
else
# it is a leap year
days=29
fi
fi
# if day is negative (<0) and if day is more than
# that months days then day is invaild
if [ $dd -le 0 -o $dd -gt $days ];
then
echo "$dd day is invalid"
exit 3
fi
# if no error that means date dd/mm/yyyy is valid one
echo "$dd/$mm/$yy is a vaild date" |