The Difference Between let, expr and $[] in Bash Scripts

Bash scripting is a powerful tool for automating tasks in Unix-like operating systems. When it comes to arithmetic operations, Bash provides several ways to perform them, including the use of let, expr, and $[]. In this article, we will delve into the differences between these three methods.

Arithmetic Expansion: $[] and $(())

Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result. The format for arithmetic expansion is $[expression] or $((expression)).

The $[] Method

The $[] method is an older syntax used for arithmetic expansion. Here is an example:

a=$[1+1]
echo $a

In this example, 1+1 is the arithmetic expression, and $a will output 2. However, this method is deprecated and not recommended for use in modern scripts due to its lack of portability. It is specific to the bash shell and may not work in other shells.

The $(( )) Method

The $(( )) method is the recommended way of performing arithmetic expansion in Bash. It is more portable and works in both bash and ksh shells. Here’s an example:

a=$((1+1))
echo $a

Like the previous example, 1+1 is the arithmetic expression, and $a will output 2. This method only supports integer arithmetic and does not support floating-point calculations.

The let Keyword

let is a built-in command in bash and ksh shells that allows for arithmetic operations. Here’s an example:

let a=1+1
echo $a

In this example, 1+1 is the arithmetic expression, and $a will output 2. The let command supports both integer and floating-point calculations. However, it cannot be used in conditional statements.

The expr Command

expr is a command-line utility that performs arithmetic operations. It is not a built-in shell command, so it is slower than the other methods. Here’s an example:

a=$(expr 1 + 1)
echo $a

In this example, 1 + 1 is the arithmetic expression, and $a will output 2. The expr command is shell-neutral and can be used in any shell. It supports integer arithmetic, true/false evaluations, and regular expressions.

Conclusion

While let, expr, and $[] can all be used for arithmetic operations in Bash scripts, there are differences in their syntax and behavior. The $[] method is deprecated and should be avoided. The let command and $(( )) method are built-in and faster but have limitations. The expr command is the most flexible but slower because it is not a built-in command. For most purposes, the $(( )) method is recommended due to its balance of speed, flexibility, and portability.