file_or_pipe

in.sh

#!/bin/bash

# Function to process input (file or stdin)
process_input() {
    while IFS= read -r line; do
        # Perform any processing here
        echo "$line"
    done
}

# Main logic
if [ $# -gt 0 ]; then
    # If arguments are provided, process each file
    for file in "$@"; do
        if [ -f "$file" ]; then
            process_input < "$file"
        else
            echo "Error: File '$file' does not exist." >&2
        fi
    done
else
    # If no arguments, process stdin
    process_input
fi

removed duplication of read block