Bash Script for toggling prefix in the last "n" lines of a file

toggle_prefix.sh

#!/bin/bash

# This script can be used to switch off some features from the configuration file.

# Usage:
# ./toggle_prefix.sh filename prefix num_lines
# Example:
# ./toggle_prefix.sh yourfile.txt ";" 4

# Default values
default_file="custom-configuration.ini"
default_prefix=";"
default_num_lines=4

# Assign positional parameters or use defaults
input_file="${1:-$default_file}"
toggle_prefix="${2:-$default_prefix}"
num_lines="${3:-$default_num_lines}"

# Temp file
temp_file=$(mktemp)

# Validate input file
if [[ ! -f "$input_file" ]]; then
  echo "Error: File '$input_file' not found."
  exit 1
fi

# Count total lines in file
total_lines=$(wc -l < "$input_file")
start_line=$((total_lines - num_lines + 1))

# Toggle prefix on the last N lines
awk -v start="$start_line" -v prefix="$toggle_prefix" '{
  if (NR >= start) {
    if (index($0, prefix) == 1) {
      sub("^" prefix, "", $0)
    } else {
      $0 = prefix $0
    }
  }
  print
}' "$input_file" > "$temp_file"

# Replace the original file
mv "$temp_file" "$input_file"
echo "✅ Updated '$input_file': Toggled prefix '$toggle_prefix' on last $num_lines lines."