cd ..是返回上一层目录， cd -是返回到上一次的工作目录。

msg 文件：是一个简单的text文本文件， 该文本文件被用来描述ros message
常常被用来生成不同的源代码; 对应与 msg 文件夹

				int8, int16, int32, int64 (plus uint*)
				float32, float64
				string
				time, duration（持续时间）
				other msg files
				variable-length array[] and fixed-length array[C]

srv 文件： 描述一个服务 service， 该文件包含两个内容：request 和 response；这两个部分被 ‘—’ 分割开来，上面三request 下面对应的是 response ； 对应于 srv 文件夹

# 一个简单的 msg 文件
	Header header
	string child_frame_id
	geometry_msgs/PoseWithCovariance pose
	geometry_msgs/TwistWithCovariance twist

# 一个简单的 srv 文件
#  A and B are the request, and Sum is the response.
	int64 A
	int64 B
	---
	int64 Sum

创建一个msg文件

fengxuewei@fengxuewei:~/C++_myself/catkin_ws$ roscd beginner_tutorials
fengxuewei@fengxuewei:~/C++_myself/catkin_ws/src/beginner_tutorials$ mkdir msg
fengxuewei@fengxuewei:~/C++_myself/catkin_ws/src/beginner_tutorials$ ls
CMakeLists.txt  include  launch  msg  package.xml  src

# 将数据写入到 msg 文件中
fengxuewei@fengxuewei:~/C++_myself/catkin_ws/src/beginner_tutorials$ echo "int64 num" > msg/Num.msg
fengxuewei@fengxuewei:~/C++_myself/catkin_ws/src/beginner_tutorials$ cd msg/
fengxuewei@fengxuewei:~/C++_myself/catkin_ws/src/beginner_tutorials/msg$ ls
Num.msg

我们需要把 msg 文件转换为为 c++ 代码， python代码等

1. 在package.xml确保增加 两行 
	<!-- 在 build 的时候，只需要加上 message_generation -->
	<build_depend>message_generation</build_depend>
	<!-- 在 run 的时候，只需要加上 message_runtime -->  
 	<exec_depend>message_runtime</exec_depend>
2. 增加 message_generation 的依赖项添加到 find_package 的调用中（存在在CMakeLists.txt）,方便接下来生成消息，为了这个可以简单的将 message_generation 添加到 下面的代码中（COMPONENTS 队列中）
# Do not just add this to your CMakeLists.txt, modify the existing text to add message_generation before the closing parenthesis
find_package(catkin REQUIRED COMPONENTS
   roscpp
   rospy
   std_msgs
   message_generation
)
3. 增加 message_runtime 依赖项到 catkin_package 中（在CMakeLists.txt中）
	catkin_package(
  ...
  CATKIN_DEPENDS message_runtime ...
  ...)
4. 在CMakeLists.txt中寻找 add_message_files, 取消其 “#” 注释， 添加 上面创建的消息文件到该点
	add_message_files(
  		FILES
  		Num.msg
	) 
	通过手动增加 .msg 文件， 确保 CMake 知道在增加其他的 .msg 文件的之后，什么时候重新初始化 该项目
	
5. 确保 generate_messages() 函数被调用
	generate_messages(
	  DEPENDENCIES
	  std_msgs
	)
(6. # rosbuild_genmsg() 在CMakeLists.txt文件中)

创建一个 srv 文件

$ roscd beginner_tutorials
$ mkdir srv

# 使用 copy 指令 从其他地方 复制一个service文件到srv文件夹下
roscp [package_name] [file_to_copy_path] [copy_path]
# 复制 rospy_tutorials 包下的 AddTwoInts.srv 文件到 该目录下的 srv/AddTwoInts.srv 文件中
roscp rospy_tutorials AddTwoInts.srv srv/AddTwoInts.srv

fengxuewei@fengxuewei:~/C++_myself/catkin_ws/src/beginner_tutorials/srv$ cat AddTwoInts.srv 
	int64 a
	int64 b
	---
	int64 sum

1. package.xml 文件中
  <build_depend>message_generation</build_depend>
  <exec_depend>message_runtime</exec_depend>
2. # Do not just add this line to your CMakeLists.txt, modify the existing line
	find_package(catkin REQUIRED COMPONENTS
	  roscpp
	  rospy
	  std_msgs
	  message_generation
	)
3. add_service_files(
	  FILES
	  AddTwoInts.srv
	)

compile

编译：在 catkin_ws/ 下运行 catkin_make install 即可
# In your catkin workspace
$ roscd beginner_tutorials
$ cd ../..
$ catkin_make install
$ cd - # 返回上一次工作目录
# Any .msg file in the msg directory will generate code for use in all supported languages. 
# The C++ i消息的头文件：	 ~/catkin_ws/devel/include/beginner_tutorials/. 
# The Python 脚本：		 ~/catkin_ws/devel/lib/python2.7/dist-packages/beginner_tutorials/msg. 
# The lisp file（和标准输入输出有关系）： ~/catkin_ws/devel/share/common-lisp/ros/beginner_tutorials/msg/.

rosmsg

Commands:
	rosmsg show		Show message description
	rosmsg info		Alias for rosmsg show
	rosmsg list		List all messages
	rosmsg md5		Display message md5sum
	rosmsg package	List messages in a package
	rosmsg packages	List packages that contain messages


# rosmsg show [message type]
# 	beginner_tutorials -- the package where the message is defined
# 	Num -- The name of the msg Num
fengxuewei@fengxuewei:~/C++_myself/catkin_ws/src/beginner_tutorials/include/beginner_tutorials$ rosmsg show beginner_tutorials/Num
	string first_name
	string last_name
	uint8 age
	uint32 score
	uint64 num

# 如何不能记住所处的具体包，可以使用下面的指令
fengxuewei@fengxuewei:~/C++_myself/catkin_ws/src/beginner_tutorials/include/beginner_tutorials$ rosmsg show Num
	[beginner_tutorials/Num]:
	string first_name
	string last_name
	uint8 age
	uint32 score
	uint64 num

rossrv

Commands:
	rossrv show		Show service description
	rossrv info		Alias for rossrv show
	rossrv list		List all services
	rossrv md5		Display service md5sum
	rossrv package	List services in a package
	rossrv packages	List packages that contain services


# rossrv show <service type>
fengxuewei@fengxuewei:~/C++_myself/catkin_ws/src/beginner_tutorials$ rossrv show beginner_tutorials/AddTwoInts
	int64 a
	int64 b
	---
	int64 sum

# rossrv show service_name
fengxuewei@fengxuewei:~/C++_myself/catkin_ws/src/beginner_tutorials$ rossrv show AddTwoInts
	[beginner_tutorials/AddTwoInts]:
	int64 a
	int64 b
	---
	int64 sum

	[rospy_tutorials/AddTwoInts]:
	int64 a
	int64 b
	---
	int64 sum

rospack = ros+pack(age) : 
	provides information related to ROS packages
roscd = ros+cd :
	changes directory to a ROS package or stack
rosls = ros+ls : 
	lists files in a ROS package
roscp = ros+cp : 
	copies files from/to a ROS package

消息和服务
rosmsg = ros+msg : 
	provides information related to ROS message definitions
rossrv = ros+srv : 
	provides information related to ROS service definitions
	
编译一个 ros 包
catkin_make : 
	makes (compiles) a ROS package
rosmake = ros+make : 
	makes (compiles) a ROS package (if you're not using a catkin workspace)