Perform arithmetic on shell variables.
Syntax let expression [expression]
Each expression is an arithmetic expression to be evaluated. If the last expression evaluates to 0,
returns 1; otherwise 0 is returned.
let is very similar to (( - the only difference being let is a builtin (simple command), and (( is a compound command.
The arguments to let are therefore subject to all the same expansions and substitutions as any other simple command - requiring proper quoting and escaping - whereas the contents of (( aren't subject to word-splitting or pathname expansion (almost never desirable for arithmetic). For this reason, the arithmetic compound command should generally be preferred over let.
let is a bash builtin command.
"Arithmetic is being able to count up to twenty without taking off your shoes" ~ Mickey Mouse
expr - Evaluate expressions.
eval - Evaluate several commands/arguments.
test - Evaluate a conditional expression.