shift

Shift positional parameters.

Source: src/execution/builtins.f90:2373-2404

Synopsis

shift [n]

Description

The shift builtin removes the first N positional parameters, shifting the remaining parameters down. $2 becomes $1, $3 becomes $2, and so on.

Arguments

Argument	Description
`n`	Number of positions to shift (default: 1)

Usage

Basic Shift

set -- one two three four
echo $1    # one

shift
echo $1    # two
echo $2    # three

Shift Multiple

set -- a b c d e
shift 3
echo $1    # d
echo $@    # d e

Examples

Process All Arguments

while [[ $# -gt 0 ]]; do
    echo "Processing: $1"
    shift
done

Option Parsing

while [[ $# -gt 0 ]]; do
    case $1 in
        -v) verbose=1; shift ;;
        -o) output="$2"; shift 2 ;;
        --) shift; break ;;
        -*) echo "Unknown: $1"; exit 1 ;;
        *)  break ;;
    esac
done

Skip First N Arguments

#!/usr/bin/env fortsh
# First arg is command, rest are files

cmd="$1"
shift

for file in "$@"; do
    $cmd "$file"
done

# Usage: ./script process file1 file2 file3

Pair Processing

while [[ $# -ge 2 ]]; do
    key="$1"
    value="$2"
    echo "$key = $value"
    shift 2
done

Error Handling

# Shift more than available
set -- a b
shift 5
# Error: cannot shift that many

# Check before shifting
if [[ $# -ge 3 ]]; then
    shift 3
fi

Common Patterns

Consume Arguments

process_args() {
    while [[ $# -gt 0 ]]; do
        arg="$1"
        shift
        # Process $arg
    done
}

Save and Shift

first="$1"
shift
rest=("$@")

Until Separator

# Process until -- is found
while [[ $# -gt 0 && $1 != "--" ]]; do
    options+=("$1")
    shift
done
[[ $1 == "--" ]] && shift
# Remaining are non-option args

Implementation

Source: builtins.f90:2373-2404

subroutine builtin_shift(cmd, shell)
  ! Parse shift count (default 1)
  ! Validate against num_positional
  ! Shift parameters in positional_params array
  ! Update num_positional
end subroutine

Exit Status

Status	Condition
0	Success
1	Shift count greater than `$#`

Notes

shift 0 does nothing (valid)
Negative values are errors
In functions, shifts function's positional parameters
Does not affect $0 (script/shell name)

# Alternative: use array slicing
args=("$@")
args=("${args[@]:1}")   # Remove first
args=("${args[@]:3}")   # Remove first 3