function cmd_help() {
    // Display 'help' message
    return "Commands list:<br>" +
        "   -about    Information about this website<br>"+
        "   -clear    Clear terminal screen<br>" +
        "   -history  Displays the command history of this session<br>" +
        "   -exec     Execute arbitrary math and logic equations";
}

function cmd_about() {
    // Display 'about' message
    return "This website is based on the general idea and design of a terminal.<br>" +
        "It serves the purpose of a homepage. It exists just for the fun of creating it.";
}

function cmd_clear() {
    // Clear terminal by reloading page
    window.location.reload();
    return "";
}

function cmd_history() {
    // Display the command history line by line
    let output = "";

    const hl = history.list;
    for (let i = 0; i < hl.length; i++) {
        const lineBreak = (i !== hl.length - 1) ? "<br>" : "";
        output += `${i+1} ${hl[i]}${lineBreak}`;
    }

    return output;
}

function cmd_exec(input) {
    // No input was given
    if (input === undefined)
        return "You must enter parameters!";
    // Input contains letters or invalid characters
    const str = input.join(' ');
    if (/[',:;a-zA-Z]/.test(str) || str === "")
        return "Invalid input!";
    // Input is inside of character range with exceptions
    const chars = str.split('').filter(e => {
        const code = e.charCodeAt(0);
        const s = code ===  32;
        const a = code >    36;
        const b = code <    63;
        const c = code ===  94;
        const d = code === 124;
        return !(s || a && b || c || d);
    });
    // If exceptions remain, invalid input was given
    if (chars.length > 0)
        return "Invalid input!";
    // Execute input and return output
    return (new Function(`return ${str}`))().toString();
}

function cmd_echo(input) {
    if (input === undefined)
        return " ";
    return input.join(' ');
}