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# Bash Positional Parameter
Positional parameters are a series of special, dynamically assigned variables in Bash that store the arguments passed to a script, shell, or function upon invocation. They are numerically indexed, read-only via standard assignment, and automatically populated by the shell's parser during execution.
## Syntax and Indexing
Positional parameters are referenced using digits. Single-digit parameters can be referenced directly with the `$` prefix. Multi-digit parameters strictly require curly braces `{}` to prevent the shell from misinterpreting the variable name.
```bash theme={"dark"}
$0 # The name of the calling script or shell (special parameter, not strictly positional)
$1 # The first positional parameter
$2 # The second positional parameter
...
$9 # The ninth positional parameter
${10} # The tenth positional parameter (braces are mandatory)
${11} # The eleventh positional parameter
```
## Aggregate Special Parameters
Bash provides specific special parameters to evaluate or expand the entire array of positional parameters at once:
```bash theme={"dark"}
$# # Expands to the decimal count of positional parameters currently set.
$* # Expands to all positional parameters.
$@ # Expands to all positional parameters.
```
### Expansion Behavior: `"$*"` vs `"$@"`
When unquoted, `$*` and `$@` behave identically, expanding to the list of arguments subject to standard word splitting and pathname expansion. When enclosed in double quotes, their parsing behavior diverges:
* `"$*"` expands to a **single string** containing all parameters, separated by the first character of the `IFS` (Internal Field Separator) variable. It evaluates as: `"$1c$2c$3"` (where `c` is the first character of `IFS`). If `IFS` is unset, the parameters are separated by spaces. If `IFS` is null (e.g., `IFS=`), the parameters are joined without any intervening separators.
* `"$@"` expands to **separate strings** for each positional parameter. It evaluates as: `"$1" "$2" "$3"`. This is the standard mechanism for preserving internal whitespace and exact argument boundaries.
## Parameter Manipulation
Because positional parameters cannot be reassigned using standard variable assignment syntax, Bash provides built-in commands to manipulate them. In Bash, variable names must begin with an alphabetic character or an underscore. Because a digit like `1` is not a valid identifier for assignment, the parser treats an expression like `1="value"` as a command execution. This results in a `command not found` error (e.g., `bash: 1=value: command not found`), rather than a syntax error.
### The `shift` Builtin
The `shift [n]` command shifts the positional parameters to the left by `n` positions (where `n` defaults to 1). The parameter at `$1` is discarded, `$2` becomes `$1`, and `$#` is decremented accordingly. If `n` is greater than the current number of positional parameters (`$#`), the command fails (returns a non-zero exit status) and the positional parameters remain completely unchanged.
```bash theme={"dark"}
# Initial state: $1="alpha", $2="beta", $3="gamma", $#=3
shift 1
# Resulting state: $1="beta", $2="gamma", $#=2
```
*Note: The `$0` parameter is immutable and is never affected by `shift`.*
### The `set` Builtin
The `set` builtin can be used to completely overwrite the current context's positional parameters. Using the `--` delimiter ensures that subsequent strings are treated strictly as positional parameters, even if they begin with a hyphen.
```bash theme={"dark"}
# Clears existing parameters and assigns new ones
set -- "new_arg1" "new_arg2" "new_arg3"
# Resulting state: $1="new_arg1", $2="new_arg2", $3="new_arg3", $#=3
```
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