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# Bash Remove Matching Prefix Pattern
The `#` (hash or pound) character in Bash is an overloaded lexical token that serves multiple distinct syntactic functions depending on its execution context. It operates as a comment initiator, a parameter expansion modifier (for prefix truncation, length calculation, and pattern substitution anchoring), a special parameter for argument counting, a radix indicator in arithmetic evaluation, and an event designator in history expansion.
## 1. Lexical Comments
When `#` appears as the first character of a word (unquoted and unescaped), the Bash parser treats it and all subsequent characters on that line as a comment. The parser discards the comment during the tokenization phase before command execution.
```bash theme={"dark"}
# This is a full-line comment
command argument # This is an inline comment
```
## 2. Parameter Expansion: Prefix Truncation
Within parameter expansion, `#` and `##` strip a matching prefix pattern from the value of a parameter. The pattern undergoes tilde expansion, parameter expansion, command substitution, and arithmetic expansion.
* **Shortest Match (`#`):** Deletes the shortest matching prefix.
* **Longest Match (`##`):** Deletes the longest matching prefix.
```bash theme={"dark"}
${parameter#pattern}
${parameter##pattern}
```
If the pattern matches the beginning of the expanded value, the result is the expanded value with the matching prefix deleted. If the parameter is an array subscripted by `@` or `*`, or if it is the positional parameters (`@` or `*`), the prefix truncation operation is applied to each element individually, expanding to a list of the modified elements.
```bash theme={"dark"}
${array[@]#pattern}
${@##pattern}
```
## 3. Parameter Expansion: Pattern Substitution Anchor
In pattern substitution, a `#` immediately following the first slash acts as a prefix anchor. It forces the pattern match to occur only at the beginning of the expanded value of the parameter.
```bash theme={"dark"}
${parameter/#pattern/replacement}
```
Similar to prefix truncation, if this expansion is applied to an array subscripted by `@` or `*`, or to the positional parameters, the anchored substitution is applied to each element individually.
## 4. Parameter Expansion: Length Evaluation
When `#` is placed immediately before a parameter name within parameter expansion braces, it evaluates the length of the parameter's contents rather than returning the contents themselves.
* **String Length:** Evaluates to the number of characters in the parameter. If the parameter is an array referenced without a subscript, it defaults to index `0` and evaluates the string length of the first element (`${parameter[0]}`).
```bash theme={"dark"}
${#parameter}
```
* **Array Element Count:** If the parameter is an array subscripted by `@` or `*`, it evaluates to the total number of populated elements in the array.
```bash theme={"dark"}
${#array_name[@]}
${#array_name[*]}
```
* **Positional Parameter Count:** If the parameter is `@` or `*`, it evaluates to the number of positional parameters currently set.
```bash theme={"dark"}
${#@}
${#*}
```
## 5. Special Parameter: Argument Count
In strict Bash terminology, `#` is a special parameter maintained by the shell, not a variable (variables are a specific subset of parameters requiring valid alphanumeric names). The parameter expansion `$#` evaluates to a decimal integer representing the total number of positional parameters passed to the current script or the current function scope.
```bash theme={"dark"}
$#
```
## 6. Arithmetic Evaluation: Base (Radix) Indicator
Within arithmetic expansion contexts (such as `$(( ))`, `(( ))`, or the `let` builtin), `#` acts as a delimiter separating an arithmetic base from its corresponding integer value.
```bash theme={"dark"}
$((base#number))
```
The `base` must be a decimal integer between 2 and 64. The `number` is evaluated according to the specified `base`. If the base is omitted, base 10 is assumed (unless prefixed by `0` for octal or `0x` for hexadecimal).
## 7. Interactive History Expansion
In interactive shells with history expansion enabled, `#` functions as an event designator when prefixed with `!`. The `!#` sequence evaluates to the entire command line typed so far before the current cursor position.
```bash theme={"dark"}
!#
```
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