# (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.
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.
@ 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.
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.
@ 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
0and evaluates the string length of the first element (${parameter[0]}).
- Array Element Count: If the parameter is an array subscripted by
@or*, it evaluates to the total number of populated elements in the array.
- Positional Parameter Count: If the parameter is
@or*, it evaluates to the number of positional parameters currently set.
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 $# evaluates to a decimal integer representing the total number of positional parameters passed to the current script or the current function scope.
6. Arithmetic Evaluation: Base (Radix) Indicator
Within arithmetic expansion contexts (such as$(( )), (( )), or the let builtin), # acts as a delimiter separating an arithmetic base from its corresponding integer value.
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.
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