> ## Documentation Index
> Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Bash Multiplication
The `*` (asterisk) token in Bash is a context-dependent, overloaded operator whose evaluation behavior is strictly determined by the shell's parsing phase, the syntactic construct enclosing it, and the presence of quoting mechanisms.
**Quoting and Escaping**
The special parsing properties of the `*` token in pathname expansion, string pattern matching, and regular expressions are entirely neutralized when quoted or escaped. Enclosing the asterisk in single quotes (`'*'`), double quotes (`"*"`), or escaping it with an unquoted backslash (`\*`) forces the shell to evaluate it as a literal asterisk character rather than a quantifier or wildcard. Because `*` has no special meaning inside double quotes, a backslash preceding it (`"\*"`) is not stripped and evaluates to a literal backslash followed by an asterisk.
```bash theme={"dark"}
echo "*"
echo '*'
echo \*
[[ $string == "*" ]]
```
**Pathname Expansion**
During the pathname expansion phase, an unquoted `*` functions as a wildcard character matching any string of zero or more characters. By default, it does not match a leading period (`.`) or a directory separator (`/`). Its parsing behavior is mutable via shell options (`shopt`), specifically `dotglob` (allows matching hidden files) and `nocaseglob` (disables case sensitivity).
If the `*` wildcard matches no files, Bash's default behavior is to leave the pattern unexpanded, evaluating it as the literal string containing the asterisk (e.g., `*.txt` remains `*.txt`). This behavior can be modified using the `nullglob` shell option, which causes unmatched patterns to expand to a null string, or the `failglob` shell option, which triggers an expansion error upon encountering an unmatched pattern.
When the `globstar` option is enabled (`shopt -s globstar`), a double asterisk (`**`) evaluates to a recursive directory traversal, but *only* when it is a standalone path component. If `**` is adjacent to other characters within the same path component (e.g., `prefix**`), it degrades and functions as a standard single `*`.
```bash theme={"dark"}
echo prefix*suffix
echo dir/**/*.txt
```
**String Pattern Matching**
Outside of filesystem interaction, an unquoted `*` functions as a standard globbing wildcard for string pattern matching within specific syntactic constructs. When evaluated on the right-hand side of the `==` or `!=` operators within the `[[ ]]` conditional command, or within the pattern clauses of a `case` statement, an unquoted `*` matches zero or more characters of the target string. If the `*` is quoted, it loses its globbing properties and matches a literal asterisk.
```bash theme={"dark"}
[[ $string == *pattern* ]]
[[ $string != prefix*"literal_asterisk_here*" ]]
case $string in
*pattern*) echo "Match found" ;;
esac
```
**Arithmetic Multiplication and Exponentiation**
Within arithmetic evaluation contexts (`$(( ))`, `(( ))`, `let`), the asterisk functions as a mathematical operator for integer arithmetic. A single asterisk (`*`) operates as a binary multiplication operator. A double asterisk (`**`) operates as an exponentiation operator, evaluating the left operand to the power of the right operand.
```bash theme={"dark"}
result=$(( operand1 * operand2 ))
result=$(( base ** exponent ))
(( variable = operand1 * operand2 ))
```
**Special Parameters and Array Subscripts**
When utilized as a Special Parameter (`$*`) or an array subscript (`[*]`), the operator expands to all positional parameters or all initialized elements of an array, respectively. The evaluation mechanics change fundamentally based on double-quoting:
* **Unquoted** (`$*` or `${array[*]}`): Expands to separate words, immediately subjecting the output to word splitting and pathname expansion.
* **Quoted** (`"$*"` or `"${array[*]}"`): Expands to a single contiguous string, concatenating each element separated by the first character of the Internal Field Separator (`$IFS`) variable.
```bash theme={"dark"}
echo $*
echo "$*"
echo "${array_name[*]}"
```
**Variable Name Expansion**
Within parameter expansion, using the `!` prefix followed by a string and the `*` operator expands to the names of all currently defined variables that begin with the specified prefix.
```bash theme={"dark"}
echo "${!prefix*}"
```
**Parameter Expansion (Substring Processing)**
Within brace-enclosed parameter expansion, `*` acts as a globbing wildcard to define patterns for substring removal or replacement. It matches zero or more characters to identify the exact prefix or suffix boundary to be stripped or substituted.
```bash theme={"dark"}
echo "${variable#*pattern}" # Non-greedy prefix removal
echo "${variable##*pattern}" # Greedy prefix removal
echo "${variable%pattern*}" # Non-greedy suffix removal
echo "${variable%%pattern*}" # Greedy suffix removal
echo "${variable/pattern*/replacement}" # Pattern replacement
```
**Extended Globbing**
When the `extglob` shell option is enabled (`shopt -s extglob`), `*` acts as a pattern quantifier. The syntactic construct `*(pattern-list)` matches zero or more occurrences of the provided patterns, where multiple patterns are separated by a pipe (`|`). Because `extglob` alters the shell's parsing phase, the option must be explicitly enabled before the construct is parsed to avoid syntax errors.
```bash theme={"dark"}
shopt -s extglob
echo *(pattern1|pattern2)
```
**Extended Regular Expressions (ERE)**
When evaluated on the right-hand side of the `=~` binary operator within the `[[ ]]` conditional construct, `*` ceases to be a globbing wildcard. Instead, it functions as a POSIX Extended Regular Expression quantifier, dictating that the preceding character, character class, or subexpression must occur zero or more times. The right-hand side must remain unquoted for `*` to function as a regex quantifier; if quoted, Bash treats it as a literal asterisk character.
```bash theme={"dark"}
[[ $string =~ preceding_element* ]]
```
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