One one line (semi-colons necessary):
for i in 0 1 2 3 4 5 6 7 8 9; do for j in 0 1 2 3 4 5 6 7 8 9; do echo "$i$j"; done; done
Formatted for legibility (no semi-colons needed):
for i in 0 1 2 3 4 5 6 7 8 9
do
for j in 0 1 2 3 4 5 6 7 8 9
do
echo "$i$j"
done
done
There are different views on how the shell code should be laid out over multiple lines; that's about what I normally use, unless I put the next operation on the same line as the do (saving two lines here).