Shell Programming and Scripting

chr1 a1 a2 a3 a4 a5 a6,a7,a8,a9
chr1 b1 b2 b3 b4 b5 b6,b7
chr2 c1 c2 c3 c4 c5 c6,c7,c8,c9,c10
...

chr1 a6
chr1 a7
chr1 a8
chr1 a9
chr1 b6
chr1 b7
chr2 c6
chr2 c7
chr2 c8
chr2 c9
chr2 10
...

chr1 a1 a2 a3 a4 a5 a6,a7,a8,a9
chr1 b1 b2 b3 b4 b5 b6,b7
chr2 c1 c2 c3 c4 c5 c6,c7,c8,c9,c10

chr1 a6
chr1 a7
chr1 a8
chr1 a9
chr1 b6
chr1 b7
chr2 c6
chr2 c7
chr2 c8
chr2 c9
chr2 10
...

awk '{ n=split($NF,A,","); while (++i<=n) { print $1, A[i] } i=0 }' file

awk '{gsub(/,/,RS $1 FS,$NF); print $1,$NF}' file

awk '
        {
                n = split ($NF, A, ",")         # Split last field using field separator comma "," Get number of elements created in variable: n
                while ( ++i <=n )               # while ++i <= n
                {
                        print $1, A[i]          # Print first field and element in array: A indexed by variable: i
                }
                i = 0                           # Reset variable: i value to 0
        }
' file