Shell Programming and Scripting

# Waiting until 06:30
until [ `date +%H` -le 6 ]          # Wait until after midnight and before 06:00
do
   sleep 3600                       # Sleep for an hour
done

until [ `date +%H` -eq 6 ]          # Wait until it's after 06:00
do
   sleep 1800                       # Sleep for 30 minutes
done

until [ `date +%M` -ge 24 ]         # Wait until it's after 06:24
do
   sleep 60                         # Sleep for 10 minutes
done

until [ `date +%M` -eq 30 ]         # Wait until we hit 06:30
do
   sleep 1                          # Just sleep one second now for shortest interval
done

# Run main code here

S=$(date +"%H:%M:%S" | BEGIN {
        # Where time digits wrap.
        split("24 60 60", T);
}
{
        split($1, A, ":")
        split(TIME, B, ":")

        # Subtract group by group like longhand arithmetic.
        for(N=3; N>=1; N--)
        {
                B[N] -= A[N]
                M=N

                while(M>0 && (B[M] < 0))
                {
                        B[M] += T[M]
                        B[M-1] --;
                        M--;
                }
        }

        print B[3]+(B[2]*60)+(B[3]*60*60)
}' )

sleep $SECONDS

S=$(date +"%H:%M:%S" | awk 'BEGIN {
        # Where time digits wrap.
        split("24 60 60", T);
}
{
        split($1, A, ":")
        split(TIME, B, ":")

        # Subtract group by group like longhand arithmetic.
        for(N=3; N>=1; N--)
        {
                B[N] -= A[N]
                M=N

                while(M>0 && (B[M] < 0))
                {
                        B[M] += T[M]
                        B[M-1] --;
                        M--;
                }
        }

        print B[3]+(B[2]*60)+(B[1]*60*60)
}' TIME="06:30:00"); echo $S
35316

echo $(($(date +%s -d"6:30 tomorrow") - $(date +%s)))
35316

=$(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 24 60 60)
}
{
        split($1, A, ":")               ( not getting this command)
        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)
        {
                B[N] -= A[N]     ( B[3]=00 -= A[N}
                M=N

                while(M>0 && (B[M] < 0))
                {
                        B[M] += T[M]
                        B[M-1] --;
                        M--;
                }
        }

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