Shift Command

The shift command is a Bash shell builtin that reassigns the positional parameters by shifting them to the left. When executed, the value of the positional parameter $n+1 is assigned to $1, $n+2 is assigned to $2, and so forth. This operation permanently discards the first n positional parameters from the current shell execution context and decrements the total parameter count ($#) by n.

shift [n]

Mechanics and State Changes

• Argument n: An optional non-negative integer dictating the shift offset.
• Default Behavior: If n is omitted, the command defaults to shift 1. If n is 0, no parameters are modified.
• Immutability of $0: The special parameter $0 (the script or shell name) is strictly excluded from the shift operation and remains unchanged.
• Parameter Count ($#): The variable $# is dynamically updated to equal $# - n.
• Unsetting Parameters: The highest n positional parameters that previously held values are unset.

Exit Status

• 0 (Success): The shift operation completes successfully, meaning n was less than or equal to $#.
• >0 (Failure): If n is strictly greater than the current number of positional parameters ($#), the command fails. In this state, no positional parameters are modified, and $# remains unchanged.

State Visualization

# Initial execution state: ./script.sh alpha bravo charlie delta echo

# $0 = "./script.sh"

# $1 = "alpha"

# $2 = "bravo"

# $3 = "charlie"

# $4 = "delta"

# $5 = "echo"

# $# = 5

shift 2


# Post-shift execution state:

# $0 = "./script.sh" (Unchanged)

# $1 = "charlie"     (Previously $3)

# $2 = "delta"       (Previously $4)

# $3 = "echo"        (Previously $5)

# $4 = unset

# $5 = unset

# $# = 3             (Decremented by 2)

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