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.

$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:

$#       # 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.

# 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.

# 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

Tired of Poor Bash Skills? Fix That With Deep Grasping!Learn More

Pipelines

Command Lists

Command Groups

Functions

Pattern Matching

Parameters

Variables

Signal Handling

Job Control

Arrays

Loops

Control Flow

Conditional Expressions

Arithmetic

Brace Expansion

Script Structure

Shell Builtins

Built-In Types

Redirection Operators

Pipeline & Process Operators

Parameter Expansion Operators

Numeric Comparison Operators

String Comparison Operators

File Test Operators

Logical Operators

Arithmetic Operators

Assignment Operators

Increment & Decrement Operators

Bitwise Operators

Bash Positional Parameter

Syntax and Indexing

Aggregate Special Parameters

Expansion Behavior: `"$*"` vs `"$@"`

Parameter Manipulation

The `shift` Builtin

The `set` Builtin

​Syntax and Indexing

​Aggregate Special Parameters

​Expansion Behavior: "$*" vs "$@"

​Parameter Manipulation

​The shift Builtin

​The set Builtin

Syntax and Indexing

Aggregate Special Parameters

Expansion Behavior: `"$*"` vs `"$@"`

Parameter Manipulation

The `shift` Builtin

The `set` Builtin