Shell Programming and Scripting

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=$(date +"%H:%M:%S" | awk 'BEGIN {  
        # Where time digits wrap.
        split("24 60 60", T);                   ( this commns is splitting string 24 60 60 into T, means t become arrays of 3 value which are 
}
{
        split($1, A, ":")                       ( $1 is the first field in the input line, i.e. the actual time from date +"%H:%M:%S" )
        split(TIME, B, ":")                     ( this command splitting time = 06 30 00 in name of B array)

        # Subtract group by group like longhand arithmetic.
        for(N=3; N>=1; N--)                     ( now loop start and execute for 3 times; 1. loop: seconds;2.: minutes, 3.: hours )
        {
                B[N] -= A[N]                    ( B[3]=00 -= A[N} )                
                M=N

                while(M>0 && (B[M] < 0))        ( handle carry over as explained by Corona688 )
                {
                        B[M] += T[M]
                        B[M-1] --;  
                        M--;
                }
        }

        print B[3]+(B[2]*60)+(B[1]*60*60)       ( print result's seconds + minutes * 60 + hours * 3600 )
}' TIME="06:30:00"); echo $S