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# Bash Multiplication Assignment
The `*=` operator is an arithmetic compound assignment operator in Bash that performs in-place multiplication. It evaluates the arithmetic expression on its right-hand side, multiplies the result by the current integer value of the variable on its left-hand side, and assigns the computed product back to that variable.
## Syntax
Because Bash variables are treated as strings by default, the `*=` operator must be executed within an arithmetic evaluation context. This is typically achieved using double parentheses `(( ))` or the `let` builtin.
```bash theme={"dark"}
(( variable *= expression ))
```
```bash theme={"dark"}
let "variable *= expression"
```
## Mechanics and Evaluation Rules
**1. Single Evaluation of the Left-Hand Side**
While `(( x *= y ))` is functionally similar to `(( x = x * y ))`, the `*=` operator evaluates the left-hand side exactly once. This distinction is critical when the left-hand side contains an expression with side effects, such as a post-increment operator.
```bash theme={"dark"}
i=0
a=(2 2 2)
(( a[i++] *= 5 ))
# 'i++' is evaluated only once. a[0] becomes 10, and i becomes 1.
# If written as (( a[i++] = a[i++] * 5 )), 'i++' would evaluate twice.
```
**2. Right-Hand Side Precedence**
The right-hand side `expression` is fully evaluated before the multiplication occurs. The operator implicitly wraps the right-hand side in parentheses, ensuring its operations complete before the product is calculated.
```bash theme={"dark"}
x=10
(( x *= 2 + 3 ))
# Evaluates as: x = 10 * (2 + 3)
# Result: x equals 50 (not 23)
```
**3. Unset or Null Variables**
If the target variable on the left-hand side is unset or contains a null string, Bash evaluates its initial value as `0` within the arithmetic context. Consequently, multiplying an unset variable will always initialize it to `0`.
```bash theme={"dark"}
unset y
(( y *= 5 ))
# Result: y equals 0
```
**4. Integer Restriction**
Bash arithmetic strictly supports integers. Attempting to use floating-point numbers on either side of the `*=` operator will result in a syntax error (`syntax error: invalid arithmetic operator`).
**5. Recursive String Evaluation**
In Bash arithmetic contexts, strings are not simply coerced to `0`. Instead, they are evaluated recursively as variable names or arithmetic expressions. If the left-hand variable contains a string, Bash attempts to resolve that string as a variable name. If the resolved name is unset, it evaluates to `0`. If the string contains invalid arithmetic syntax, Bash throws a syntax error.
```bash theme={"dark"}
text=10
z="text"
(( z *= 5 ))
# 'z' resolves to 'text', which evaluates to 10. 10 * 5 = 50.
# Result: z equals 50
w="foo bar"
(( w *= 5 ))
# Result: syntax error in expression (error token is "bar")
```
## Invalid Usage
Using `*=` outside of an arithmetic context is syntactically invalid for variable assignment. Bash assignment syntax strictly requires a valid identifier (consisting only of alphanumeric characters and underscores, and not starting with a number) immediately preceding the equals sign.
Because an asterisk (`*`) is an invalid identifier character, Bash does not parse a sequence like `x*=3` as an assignment. Instead, it parses the entire token as a command, which results in a `command not found` error.
```bash theme={"dark"}
# INCORRECT: Outside arithmetic context
x=5
x*=3 # Bash attempts to execute a command literally named 'x*=3'
```
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