Shell Script to validate the date, taking into account leap year rules

Prerequisites: Bash Scripting, Shell Function Library

We will build a bash script to check whether the entered date is valid or not according to the Gregorian Calendar rules. If the date is invalid, the script will tell the reason for its invalidity.

• The script takes three arguments – day, month, and year in numeric form.
• The month specified out of the range [1-12] will be shown as invalid.
• Year 0 will be shown as invalid since there is no such year in Gregorian Calendar.
• The months and their corresponding total number of days will be stored in an array.
• The day specified by the user will be checked against the correct day values stored in the script.
• The script takes leap year into account.

Creating Shell Script

The name of the script will be dateChecker.sh. Make it an executable file.

$ touch dateChecker.sh
$ chmod +x dateChecker.sh

Step 1: Handling Arguments

$ ./dateChecker.sh 12 09 #invalid syntax (only 2 args)

$ ./dateChecker.sh twelve 01 2001 #invalid syntax (date should be in numeric form)

$ ./dateChecker.sh 10 10 2020 #valid syntax

Open the file and add the following script :

#!/bin/bash

# if number of args is not 3, show correct syntax
if [ $# -ne 3 ] ; then
  echo "The Valid Syntax is :"
  echo "\$ ./$(basename $0)"
  echo "\$ ./$(basename $0) [d|dd] [m|mm] yyyy"
  exit 1
else
  day=$1 month=$2 year=$3
fi

The script shows the correct syntax to the user

Step 2: Checking common errors in the Date

MSG=”Invalid Date!”

# if the args contain any character other than digits

if [[ $day =~ [^[:digit:]] ]] || [[ “$month” == [^[:digit:]] ]] || [[ “$year” == [^[:digit:]] ]] ; then

  echo “The day, month and year must be positive numeric only!”

  exit 1

# if the months specified is more than 12

elif [ $month -gt 12 ] ; then

  echo “There are only 12 months”

  echo $MSG

  exit 0

# if day, month or year specified is 0

elif [ $year -eq 0 ] || [ $month -eq 0 ] || [ $day -eq 0 ] ; then

  echo “There is no zero day, month or year in Gregorian Calendar!”

  echo $MSG

  exit 0

fi

• If the entered day, month, and year contains any character other than the digits, the script will exit 1.
• If a Month is out of the range [1-12], then it’s an invalid date.
• If the year is 0, it’s invalid.

Handling common errors in date

Step 3: Storing the values

#if day, month or year have leading zeroes, stripping them away
day=$((10#$day))
month=$((10#$month))
year=$((10#$year))

#array with months as index and the number of days it has as values
array_months=( 31 28 31 30 31 30 31 31 30 31 30 31 )
month_names=( "January" "February" "March" "April" "May" "June" "July"
              "August" "September" "October" "November" "December" )

• Strip away leading zeroes in the day, month, and year variable if any.
• Store the corresponding days of months in array_months.
• Store the names of months in a different array

Step 4: Create the main function

function main ()
{
  # determining whether the year specified is leap
  if (( $year % 400 == 0 )) || (( $year % 4 == 0 && $year % 100 != 0 )) ; then
    #if leap year, February will have 29 days
    array_months[1]=29
  fi

  # zero based indexing in bash arrays
  index=$(( $month -1 ))

  if [ $day -gt ${array_months[index]} ] ; then
    echo "${month_names[index]} has ${array_months[index]} days!"
    echo $MSG
  else
    echo "Valid Date."
  fi
}
main

• If it’s a leap year, then we will update the number of days at index 1 (February) of array_months to 29.
• Total Number of Days in a Month = ${array_months[index]} where index = $month – 1 (zero-based indexing).
• Number of Days specified by user = $day
• If $day is greater than the total number of days in that particular month, the date is invalid.
• Otherwise print the date is valid.

Step 5: Putting everything together

#!/bin/bash

# if number of args is not 3, show correct syntax

if [ $# -ne 3 ] ; then

  echo “The Valid Syntax is :”

  echo “\$ ./$(basename $0)”

  echo “\$ ./$(basename $0) [d|dd] [m|mm] yyyy”

  exit 1

else

  day=$1 month=$2 year=$3

fi

MSG=”Invalid Date!”

# if the args contain any character other than digits

if [[ $day =~ [^[:digit:]] ]] || [[ “$month” == [^[:digit:]] ]] || [[ “$year” == [^[:digit:]] ]] ; then

  echo “The day, month and year must be positive numeric only!”

  exit 1

# if the months specified is more than 12

elif [ $month -gt 12 ] ; then

  echo “There are only 12 months”

  echo $MSG

  exit 0

# if day, month or year specified is 0

elif [ $year -eq 0 ] || [ $month -eq 0 ] || [ $day -eq 0 ] ; then

  echo “There is no zero day, month or year in Gregorian Calendar!”

  echo $MSG

  exit 0

fi

# if day, month or year have leading zeroes, stripping them away

day=$((10#$day))

month=$((10#$month))

year=$((10#$year))

#array with months as index and the number of days it has as values

array_months=( 31 28 31 30 31 30 31 31 30 31 30 31 )

month_names=( “January” “February” “March” “April” “May” “June” “July”

              “August” “September” “October” “November” “December” )

function main ()

{

  #determining whether the year specified is leap

  if (( $year % 400 == 0 )) || (( $year % 4 == 0 && $year % 100 != 0 )) ; then

    #if leap year, February will have 29 days

    array_months[1]=29

  fi

  #zero based indexing in bash arrays

  index=$(( $month -1 ))

  if [ $day -gt ${array_months[index]} ] ; then

    echo “${month_names[index]} has ${array_months[index]} days!”

    echo $MSG

  else

    echo “Valid Date.”

  fi

}

main

Displaying which dates are valid and which dates are invalid and the reason as well.