mirror of https://github.com/tLDP/LDP
155 lines
3.8 KiB
Bash
155 lines
3.8 KiB
Bash
#!/bin/bash
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# days-between.sh: Number of days between two dates.
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# Usage: ./days-between.sh [M]M/[D]D/YYYY [M]M/[D]D/YYYY
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ARGS=2 # Two command line parameters expected.
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E_PARAM_ERR=65 # Param error.
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REFYR=1600 # Reference year.
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CENTURY=100
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DIY=365
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ADJ_DIY=367 # Adjusted for leap year + fraction.
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MIY=12
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DIM=31
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LEAPCYCLE=4
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MAXRETVAL=256 # Largest permissable
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# positive return value from a function.
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diff= # Declare global variable for date difference.
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value= # Declare global variable for absolute value.
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day= # Declare globals for day, month, year.
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month=
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year=
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Param_Error () # Command line parameters wrong.
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{
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echo "Usage: `basename $0` [M]M/[D]D/YYYY [M]M/[D]D/YYYY"
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echo " (date must be after 1/3/1600)"
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exit $E_PARAM_ERR
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}
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Parse_Date () # Parse date from command line params.
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{
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month=${1%%/**}
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dm=${1%/**} # Day and month.
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day=${dm#*/}
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let "year = `basename $1`" # Not a filename, but works just the same.
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}
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check_date () # Checks for invalid date(s) passed.
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{
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[ "$day" -gt "$DIM" ] || [ "$month" -gt "$MIY" ] || [ "$year" -lt "$REFYR" ] && Param_Error
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# Exit script on bad value(s).
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# Uses "or-list / and-list".
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#
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# Exercise: Implement more rigorous date checking.
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}
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strip_leading_zero () # Better to strip possible leading zero(s)
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{ # from day and/or month
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val=${1#0} # since otherwise Bash will interpret them
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return $val # as octal values (POSIX.2, sect 2.9.2.1).
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}
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day_index () # Gauss' Formula:
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{ # Days from Jan. 3, 1600 to date passed as param.
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day=$1
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month=$2
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year=$3
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let "month = $month - 2"
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if [ "$month" -le 0 ]
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then
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let "month += 12"
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let "year -= 1"
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fi
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let "year -= $REFYR"
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let "indexyr = $year / $CENTURY"
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let "Days = $DIY*$year + $year/$LEAPCYCLE - $indexyr + $indexyr/$LEAPCYCLE + $ADJ_DIY*$month/$MIY + $day - $DIM"
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# For an in-depth explanation of this algorithm, see
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# http://home.t-online.de/home/berndt.schwerdtfeger/cal.htm
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if [ "$Days" -gt "$MAXRETVAL" ] # If greater than 256,
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then # then change to negative value
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let "dindex = 0 - $Days" # which can be returned from function.
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else let "dindex = $Days"
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fi
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return $dindex
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}
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calculate_difference () # Difference between to day indices.
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{
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let "diff = $1 - $2" # Global variable.
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}
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abs () # Absolute value
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{ # Uses global "value" variable.
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if [ "$1" -lt 0 ] # If negative
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then # then
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let "value = 0 - $1" # change sign,
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else # else
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let "value = $1" # leave it alone.
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fi
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}
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if [ $# -ne "$ARGS" ] # Require two command line params.
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then
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Param_Error
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fi
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Parse_Date $1
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check_date $day $month $year # See if valid date.
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strip_leading_zero $day # Remove any leading zeroes
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day=$? # on day and/or month.
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strip_leading_zero $month
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month=$?
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day_index $day $month $year
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date1=$?
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abs $date1 # Make sure it's positive
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date1=$value # by getting absolute value.
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Parse_Date $2
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check_date $day $month $year
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strip_leading_zero $day
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day=$?
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strip_leading_zero $month
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month=$?
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day_index $day $month $year
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date2=$?
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abs $date2 # Make sure it's positive.
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date2=$value
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calculate_difference $date1 $date2
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abs $diff # Make sure it's positive.
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diff=$value
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echo $diff
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exit 0
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# Compare this script with the implementation of Gauss' Formula in C at
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# http://buschencrew.hypermart.net/software/datedif
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