! (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.
2. Logical Negation (Test Constructs)
Within thetest builtin ([ ]) or the extended test keyword ([[ ]]), ! acts as a unary operator that inverts the boolean evaluation of the subsequent 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.
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.
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.
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.
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.
8. Pattern Matching Negation (Extglob)
When theextglob 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).
9. History Expansion (Interactive Shells)
In interactive shells (or whenset -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.
- 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%).
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