Использование perl:
perl -pe 's/([0-9])(?! )/\1 /g' INPUT_FILE
Тест:
[jaypal:~/Temp] cat tmp
1 2 12345
2 4 98231
[jaypal:~/Temp] perl -pe 's/([0-9])(?! )/\1 /g' tmp
1 2 1 2 3 4 5
2 4 9 8 2 3 1
Использование gnu sed:
sed 's/\d/& /3g' INPUT_FILE
Тест:
[jaypal:~/Temp] sed 's/[0-9]/& /3g' tmp
1 2 1 2 3 4 5
2 4 9 8 2 3 1
Использование gnu awk:
gawk '{print $1,$2,gensub(/./,"& ","G", $NF)}' INPUT_FILE
Тест:
[jaypal:~/Temp] gawk '{print $1,$2,gensub(/./,"& ","G", $NF)}' tmp
1 2 1 2 3 4 5
2 4 9 8 2 3 1