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# Bash Indirect Expansion
The `!` (bang or exclamation) operator in Bash is a highly overloaded token whose behavior is strictly determined by its syntactic context. It functions primarily as a logical negator for exit statuses and expressions, a modifier for parameter expansion, a character class negator in pattern matching, and a trigger for interactive history expansion.
## 1. Pipeline Negation (Reserved Word)
When used as a reserved word at the beginning of a pipeline, `!` logically inverts the exit status (`$?`) of that pipeline. If the pipeline evaluates to `0` (success), the `!` forces the shell to return `1` (failure). If the pipeline evaluates to any non-zero value, `!` forces it to return `0`.
```bash theme={"dark"}
! command [arg ...] [ | command2 ... ]
```
## 2. Logical Negation (Test Constructs)
Within the `test` builtin (`[ ]`) or the extended test keyword (`[[ ]]`), `!` acts as a unary operator that inverts the boolean evaluation of the subsequent expression.
```bash theme={"dark"}
[ ! expression ]
[[ ! expression ]]
test ! expression
```
## 3. Arithmetic Logical Negation
Within arithmetic evaluation contexts, `!` functions as a logical NOT unary operator. It evaluates to `1` if the subsequent arithmetic expression evaluates to `0`, and evaluates to `0` if the expression evaluates to any non-zero value.
```bash theme={"dark"}
(( ! expression ))
$(( ! expression ))
```
## 4. Indirect Parameter Expansion
When used as a prefix inside parameter expansion braces, `!` introduces a level of indirection (dereferencing). The shell evaluates the value of the variable, treats that value as the name of a *second* variable, and expands to the value of the second variable.
```bash theme={"dark"}
${!parameter}
```
## 5. Array Index/Key Expansion
When applied to an array variable combined with the `@` or `*` subscript within parameter expansion, `!` modifies the expansion to yield the assigned indices (for indexed arrays) or keys (for associative arrays) rather than the array's values.
```bash theme={"dark"}
${!array_name[@]}
${!array_name[*]}
```
## 6. Variable Name Prefix Expansion
When combined with a string prefix and the `@` or `*` operators, `!` instructs the shell to expand to a list of all currently defined variable names that begin with the specified prefix.
```bash theme={"dark"}
${!prefix@}
${!prefix*}
```
## 7. Character Class Negation (Pathname Expansion)
In standard pathname expansion (globbing) and pattern matching, `!` placed immediately after an opening bracket `[` negates the character class. It matches any single character *not* enclosed in the brackets.
```bash theme={"dark"}
[!characters]*
[!a-z]*
```
## 8. Pattern Matching Negation (Extglob)
When the `extglob` shell option is enabled (`shopt -s extglob`), `!` is used in conjunction with parentheses to form a negated pattern matching operator. It matches anything that does *not* match the specified pattern(s).
```bash theme={"dark"}
!(pattern[|pattern...])
```
## 9. History Expansion (Interactive Shells)
In interactive shells (or when `set -H` is enabled), `!` is the history expansion character. It instructs the shell's history library to substitute text from previously executed commands into the current command line before lexical analysis occurs.
* **Event designators:** Refer to specific command lines.
```bash theme={"dark"}
!! # The immediately preceding command
!n # Command line number 'n'
!-n # The current command line minus 'n'
!string # Most recent command starting with 'string'
!?string[?] # Most recent command containing 'string'
```
* **Word designators:** Extract specific words from a designated history event. They are separated from the event designator by a colon `:` (which can be omitted if the word designator begins with `^`, `$`, `*`, `-`, or `%`).
```bash theme={"dark"}
!^ # The first argument (word 1) of the previous command
!$ # The last argument of the previous command
!* # All arguments of the previous command (excluding the command word)
!:n # The 'n'th word of the previous command
!event:n # The 'n'th word of the specified 'event'
```
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