#!/bin/bash# days-between.sh:    Number of days between two dates.# Usage: ./days-between.sh [M]M/[D]D/YYYY [M]M/[D]D/YYYY## Note: Script modified to account for changes in Bash, v. 2.05b +,#+      that closed the loophole permitting large negative#+      integer return values.ARGS=2                # Two command line parameters expected.E_PARAM_ERR=65        # Param error.REFYR=1600            # Reference year.CENTURY=100DIY=365ADJ_DIY=367           # Adjusted for leap year + fraction.MIY=12DIM=31LEAPCYCLE=4MAXRETVAL=255         #  Largest permissible                      #+ positive return value from a function.diff=                 # Declare global variable for date difference.value=                # Declare global variable for absolute value.day=                  # Declare globals for day, month, year.month=year=Param_Error ()        # Command line parameters wrong.{  echo "Usage: `basename $0` [M]M/[D]D/YYYY [M]M/[D]D/YYYY"  echo "       (date must be after 1/3/1600)"  exit $E_PARAM_ERR}  Parse_Date ()                 # Parse date from command line params.{  month=${1%%/**}  dm=${1%/**}                 # Day and month.  day=${dm#*/}  let "year = `basename $1`"  # Not a filename, but works just the same.}  check_date ()                 # Checks for invalid date(s) passed.{  [ "$day" -gt "$DIM" ] || [ "$month" -gt "$MIY" ] ||  [ "$year" -lt "$REFYR" ] && Param_Error  # Exit script on bad value(s).  # Uses or-list / and-list.  #  # Exercise: Implement more rigorous date checking.}strip_leading_zero () #  Better to strip possible leading zero(s){                     #+ from day and/or month  return ${1#0}       #+ since otherwise Bash will interpret them}                     #+ as octal values (POSIX.2, sect 2.9.2.1).day_index ()          # Gauss' Formula:{                     # Days from March 1, 1600 to date passed as param.                      #           ^^^^^^^^^^^^^  day=$1  month=$2  year=$3  let "month = $month - 2"  if [ "$month" -le 0 ]  then    let "month += 12"    let "year -= 1"  fi    let "year -= $REFYR"  let "indexyr = $year / $CENTURY"  let "Days = $DIY*$year + $year/$LEAPCYCLE - $indexyr \              + $indexyr/$LEAPCYCLE + $ADJ_DIY*$month/$MIY + $day - $DIM"  #  For an in-depth explanation of this algorithm, see  #+   http://weblogs.asp.net/pgreborio/archive/2005/01/06/347968.aspx  echo $Days}  calculate_difference ()            # Difference between two day indices.{  let "diff = $1 - $2"             # Global variable.}  abs ()                             #  Absolute value{                                  #  Uses global "value" variable.  if [ "$1" -lt 0 ]                #  If negative  then                             #+ then    let "value = 0 - $1"           #+ change sign,  else                             #+ else    let "value = $1"               #+ leave it alone.  fi}if [ $# -ne "$ARGS" ]              # Require two command line params.then  Param_Errorfi  Parse_Date $1check_date $day $month $year       #  See if valid date.strip_leading_zero $day            #  Remove any leading zeroesday=$?                             #+ on day and/or month.strip_leading_zero $monthmonth=$?let "date1 = `day_index $day $month $year`"Parse_Date $2check_date $day $month $yearstrip_leading_zero $dayday=$?strip_leading_zero $monthmonth=$?date2=$(day_index $day $month $year) # Command substitution.calculate_difference $date1 $date2abs $diff                            # Make sure it's positive.diff=$valueecho $diffexit 0#  Compare this script with#+ the implementation of Gauss' Formula in a C program at:#+    http://buschencrew.hypermart.net/software/datedif