在ROS下编写代码，利用编写的代码来控制小乌龟的运动，包括但不限于直线运动、圆周运动、转向，速度变化，并录制数据包。

好的，这个任务需要先安装ROS，并且需要安装Turtlesim模拟器和rosbag包。

安装Turtlesim模拟器：

sudo apt-get install ros-melodic-turtlesim

安装rosbag包：

sudo apt-get install ros-melodic-rosbag

接下来，我们可以在ROS中创建一个新的工作空间：

mkdir -p ~/turtlebot_ws/src
cd ~/turtlebot_ws/src
catkin_init_workspace

然后，我们需要创建一个ROS包来管理我们的代码：

cd ~/turtlebot_ws/src
catkin_create_pkg turtlebot_control rospy roscpp turtlesim

在src文件夹中创建一个名为turtlebot_control.py的Python文件，并将下面的代码复制到文件中：

#!/usr/bin/env python

import rospy
from geometry_msgs.msg import Twist
from turtlesim.msg import Pose
from math import pow, atan2, sqrt

class TurtleBot:
    def __init__(self):
        # 初始化节点
        rospy.init_node('turtlebot_control', anonymous=True)

        # 发布器和订阅器
        self.velocity_publisher = rospy.Publisher('/turtle1/cmd_vel', Twist, queue_size=10)
        self.pose_subscriber = rospy.Subscriber('/turtle1/pose', Pose, self.update_pose)

        self.pose = Pose()
        self.rate = rospy.Rate(10)

    # 回调函数，更新小乌龟的位置和朝向
    def update_pose(self, data):
        self.pose = data
        self.pose.x = round(self.pose.x, 4)
        self.pose.y = round(self.pose.y, 4)

    # 计算小乌龟和目标点之间的距离
    def euclidean_distance(self, goal_pose):
        return sqrt(pow((goal_pose.x - self.pose.x), 2) + pow((goal_pose.y - self.pose.y), 2))

    # 计算小乌龟和目标点之间的角度
    def linear_vel(self, goal_pose, constant=1.5):
        return constant * self.euclidean_distance(goal_pose)

    # 计算小乌龟需要旋转的角度
    def steering_angle(self, goal_pose):
        return atan2(goal_pose.y - self.pose.y, goal_pose.x - self.pose.x)

    # 计算小乌龟需要旋转的方向
    def angular_vel(self, goal_pose, constant=6):
        return constant * (self.steering_angle(goal_pose) - self.pose.theta)

    # 小乌龟在直线上移动的函数
    def move2goal(self):
        goal_pose = Pose()
        goal_pose.x = input("Set your x goal: ")
        goal_pose.y = input("Set your y goal: ")

        distance_tolerance = 0.1
        vel_msg = Twist()

        while self.euclidean_distance(goal_pose) >= distance_tolerance:
            # 线性速度
            vel_msg.linear.x = self.linear_vel(goal_pose)
            vel_msg.linear.y = 0
            vel_msg.linear.z = 0

            # 角速度
            vel_msg.angular.x = 0
            vel_msg.angular.y = 0
            vel_msg.angular.z = self.angular_vel(goal_pose)

            # 发布速度消息
            self.velocity_publisher.publish(vel_msg)

            # 延迟，使循环以10Hz的速度运行
            self.rate.sleep()

        # 停止小乌龟运动
        vel_msg.linear.x = 0
        vel_msg.angular.z = 0
        self.velocity_publisher.publish(vel_msg)

    # 小乌龟在圆周上运动的函数
    def circle(self):
        radius = input("Set your radius: ")
        vel_msg = Twist()

        while not rospy.is_shutdown():
            # 线速度
            vel_msg.linear.x = 2 * 3.14 * radius / 10
            vel_msg.linear.y = 0
            vel_msg.linear.z = 0

            # 角速度
            vel_msg.angular.x = 0
            vel_msg.angular.y = 0
            vel_msg.angular.z = 2 * 3.14 / 20

            # 发布速度消息
            self.velocity_publisher.publish(vel_msg)

            # 延迟，使循环以10Hz的速度运行
            self.rate.sleep()

        # 停止小乌龟运动
        vel_msg.linear.x = 0
        vel_msg.angular.z = 0
        self.velocity_publisher.publish(vel_msg)

    # 小乌龟转向的函数
    def turn(self):
        angle = input("Set your angle: ")

        vel_msg = Twist()

        # 角速度为正，小乌龟向左转
        vel_msg.angular.z = 0.5

        # 弧度与角度的转换
        radians = angle * 3.14 / 180

        # 计算小乌龟需要旋转的时间
        t0 = rospy.Time.now().to_sec()
        current_angle = 0
        while current_angle < radians:
            self.velocity_publisher.publish(vel_msg)
            t1 = rospy.Time.now().to_sec()
            current_angle = 0.5 * (t1 - t0)

        # 停止小乌龟运动
        vel_msg.angular.z = 0
        self.velocity_publisher.publish(vel_msg)

    # 录制数据包的函数
    def record(self):
        filename = input("Please enter the filename: ")
        duration = input("Please enter the duration of recording: ")

        # 创建rosbag记录器
        command = 'rosbag record -O ' + filename + ' /turtle1/cmd_vel /turtle1/pose -q -a --duration=' + str(duration)
        os.system(command)

if __name__ == '__main__':
    try:
        turtlebot = TurtleBot()

        # 移动小乌龟
        print("Move to goal")
        turtlebot.move2goal()

        # 在圆周上运动
        print("Circle")
        turtlebot.circle()

        # 转向
        print("Turn")
        turtlebot.turn()

        # 录制数据包
        print("Record")
        turtlebot.record()

        rospy.spin()
    except rospy.ROSInterruptException:
        pass

这个代码中包含了一些函数来控制小乌龟的运动，包括直线运动、圆周运动、转向和录制数据包。你可以根据需要进行修改，并在终端中使用下面的命令来运行代码：

cd ~/turtlebot_ws
catkin_make
source devel/setup.bash
rosrun turtlebot_control turtlebot_control.py

如果需要查看数据包，可以在终端中使用下面的命令：

cd ~/turtlebot_ws/src/turtlebot_control
rosbag play <filename>