Calculates differences between months, also across years.

monthdiff.sh

#!/bin/bash 
# Script for calculating month differences 
# Copyright (C) 2016 Christoph "criztovyl" Schulz 
# 
# This program is free software: you can redistribute it and/or modify 
# it under the terms of the GNU General Public License as published by 
# the Free Software Foundation, either version 3 of the License, or 
# (at your option) any later version. 
# 
# This program is distributed in the hope that it will be useful, 
# but WITHOUT ANY WARRANTY; without even the implied warranty of 
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 
# GNU General Public License for more details. 
# 
# You should have received a copy of the GNU General Public License 
# along with this program. If not, see <http://www.gnu.org/licenses/>. 

if [[ "$1" =~ ^-{0,2}h(elp)?$ ]]; then
    echo -e "Usage: $0 YYYY-MM YYYY-MM\n first one is start, second one end."
    echo "  Both dates can be \"-\" when should be read from stdin."
    echo "  When both are \"-\", start date is read first."
    echo "  Use \"\" for use default value, e.g. when using default start but custom end."
    echo "Defaults:"
    echo "  MONTHDIFF_X_DEFAULT: Default string value, X must be START or END."
    echo "  MONTHDIFF_X_DEFAULT_EXEC: Command that returns default value, X must be START or END."
    echo "  Will look for string value first and then for command."
    echo "  Examples:"
    echo "   MONTHDIFF_START_DEFAULT=\"2015-09\""
    echo "   MONTHDIFF_END_DEFAULT_EXEC=\"date +%Y-%m\""
    exit
fi

EXIT_DATE_INVALID=5
EXIT_MISSING_DATE=6

# Determine defaults
for name in start end; do

    var=MONTHDIFF_${name^^}_DEFAULT

    if [ "${!var}" ]; then
        eval $name"_default"=${!var}
    else 
        var=$var"_EXEC"
        [ "${!var}" ] && eval $name"_default"=`${!var}`
    fi
done

start=$1
end=$2

for name in start end; do 
    [ "${!name}" == "-" ] && read $name;
    [ -z "${!name}" ] && eval $name=\$$name"_default"
done

[ -z "$start" ] || [ -z "$end" ] && { echo "Missing start and/or end date."; exit $EXIT_MISSING_DATE; }


yearNmonth(){
    # Args: varName
    # Returns: String that would create an array with index 0 year and index 1 month. (Use it with eval)

    varName=$1

    RE_yearNmonth='([[:digit:]]{4})-0?([[:digit:]]{1,2})'; [[ "${!varName}" =~ $RE_yearNmonth ]] || { echo "exit $EXIT_DATE_INVALID"; exit; }

    echo "declare $varName=(${BASH_REMATCH[1]} ${BASH_REMATCH[2]})"
}

echo "From $start to $end" >&2
eval $(yearNmonth "start")
eval $(yearNmonth "end")
# start   to end    
# yyyy-aa to zzzz-bb: x=( a-b + 12*(y-z) )*(-1)
echo $(( ( ${start[1]}-${end[1]} + 12*(${start[0]}-${end[0]}) )*(-1) ))