> ## 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 Addition
The `+` operator in Bash is a context-dependent token whose behavior is strictly dictated by the syntactic construct in which it is evaluated. It functions as an arithmetic operator, an assignment/append operator, a parameter expansion modifier, an attribute toggler, a directory stack index indicator, and a pattern-matching quantifier.
## Arithmetic Evaluation
Within arithmetic expansion, the `+` token functions as a standard binary addition operator or a unary positive operator. Bash arithmetic is restricted to integers; floating-point operations are not natively supported.
**Syntax:**
```bash theme={"dark"}
$(( operand1 + operand2 ))
let "result = operand1 + operand2"
$(( +operand ))
```
## Append and Assignment Operator (`+=`)
The `+=` token is a compound assignment operator that performs context-dependent appending or arithmetic addition.
**Syntax:**
```bash theme={"dark"}
variable+="string"
array+=(element1 element2)
(( variable += integer ))
```
* **String Concatenation:** When applied to a standard scalar variable, it appends the right-hand string to the existing value.
* **Array Appending:** When applied to an indexed or associative array using compound assignment syntax `()`, it appends new elements to the array.
* **Arithmetic Assignment:** Within an arithmetic evaluation context `(( ))` or `let`, it performs integer addition and assigns the computed sum back to the variable.
* **Integer Attribute Assignment:** If a scalar variable has the integer attribute set (via `declare -i`), using `+=` on it performs arithmetic addition rather than string concatenation, even outside of an explicit arithmetic context like `(( ))` or `let`.
## Parameter Expansion (Alternate Value)
In brace-enclosed parameter expansion, the `+` operator tests the state of a variable and substitutes an alternate string based on whether the variable is set or null. It does not modify the original variable.
**Syntax:**
```bash theme={"dark"}
${parameter+word}
${parameter:+word}
```
* **`${parameter+word}`**: If `parameter` is declared/set (even if its value is null), the expansion evaluates to `word`. If unset, it evaluates to nothing.
* **`${parameter:+word}`**: The inclusion of the colon (`:`) enforces a strict non-null test. If `parameter` is set *and not null*, it evaluates to `word`. If unset or null, it evaluates to nothing.
## Shell Option and Attribute Toggling
When used as an argument prefix with the `set` builtin or variable declaration builtins (`declare`, `typeset`, `local`), the `+` operator disables shell options or removes variable attributes. This is the inverse of standard POSIX conventions where `-` enables options or sets attributes.
*Note: The `export` builtin does not accept `+` flags to remove attributes. To remove the export attribute from a variable, you must use `export -n` or `declare +x`.*
**Syntax:**
```bash theme={"dark"}
set +[short-option-flag]
set +o [long-option-name]
declare +[attribute-flag] variable_name
typeset +[attribute-flag] variable_name
```
## Directory Stack Indexing
Builtins that manipulate the directory stack (`pushd`, `popd`, `dirs`) use the `+` prefix followed by an integer to reference a specific directory index. The `+N` notation specifies the Nth directory from the left of the directory stack, starting with zero.
**Syntax:**
```bash theme={"dark"}
pushd +N
popd +N
dirs +N
```
## Extended Globbing Quantifier
When the `extglob` shell option is enabled (`shopt -s extglob`), the `+` operator functions as a pattern-matching quantifier. It evaluates strings against a delimited list of patterns.
**Syntax:**
```bash theme={"dark"}
+(pattern1|pattern2|...)
```
This construct matches one or more occurrences of any of the patterns specified within the parentheses.
## Regular Expression Quantifier
Within the conditional test construct `[[ ]]` utilizing the `=~` binary operator, the right-hand side is evaluated as a POSIX Extended Regular Expression (ERE). Here, `+` retains its standard regex definition.
**Syntax:**
```bash theme={"dark"}
[[ $string =~ pattern+ ]]
```
The `+` quantifies the preceding regex token, requiring it to match one or more times consecutively.
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