Shell Programming and Scripting

hello, one, two
bye, goal
first, second, third, fourth
hard, difficult.strong, word.line

hello, one
hello, two
bye, goal
first, second
first, third
first, fourth
hard, difficult.strong
hard, word.line

$ nawk -F, '{if(NF>2){printf("%s,%s\n",$1,$2);for(i=3;i<=NF;i++){printf("%s,%s\n",$1,$i)}}else{print}}' input.txt
hello, one
hello, two
bye, goal
first, second
first, third
first, fourth

gawk '{for(i=2;i<=NF;i++){print $1,$i}}' FS="," OFS="," infile.txt > outfile.txt

# awk -F"," '{for(i=2;i<=NF;i++)print $1FS$i}' infile
hello, one
hello, two
bye, goal
first, second
first, third
first, fourth
hard, difficult.strong
hard, word.line