The counter, written both ways

Answer

i=1 i=`expr $i + 1` echo $i i=$((i + 1)) echo $i

Both print an incremented value — 2 then 3. The second line starts a process to do it; the fourth does not.

S&K 3e ch13 §13.2; arithmetic expansion from POSIX.1-2024 §2.6.4