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, let 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

Related linux commands:

expr – Evaluate expressions 
eval
 – Evaluate several commands/arguments
test – Evaluate a conditional expression

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