Bash: let statement vs assignment

Question

What is the difference between assigning to a variable like var=foo and using let like let var=foo? Or cases like var=${var}bar and

Answer 1

let does exactly what (( )) do, it is for arithmetic expressions. There is almost no difference between let and (( )).

Your examples are invalid. var=${var}bar is going to add word bar to the var variable (which is a string operation), let var+=bar is not going to work, because it is not an arithmetic expression:

$ var='5'; let var+=bar; echo "$var"
5

Actually, it IS an arithmetic expression, if only variable bar was set, otherwise bar is treated as zero.

$ var='5'; bar=2; let var+=bar; echo "$var"
7