背景

我最近开始使用mavros，按照官网的教程进行安装采用二进制的方式安装，一切进行的很顺利，接下来我就按照PX4官网的去执行一个让无人机自动起飞的例子，完全按照官网的代码和配置到最后一步执行roslaunch offboard_py start_offb.launch 的时候不出意外的，我的终端提示我

    Resource not found: px4 ROS path [0] = ... ...
    然后就有一些ROS path的命令的提示...

教程下面也给出了解决方法：

source ~/PX4-Autopilot/Tools/simulation/gazebo/setup_gazebo.bash ~/PX4-Autopilot ~/PX4-Autopilot/build/px4_sitl_default
export ROS_PACKAGE_PATH=$ROS_PACKAGE_PATH:~/PX4-Autopilot
export ROS_PACKAGE_PATH=$ROS_PACKAGE_PATH:~/PX4-Autopilot/Tools/simulation/gazebo/sitl_gazebo
export GAZEBO_PLUGIN_PATH=$GAZEBO_PLUGIN_PATH:/usr/lib/x86_64-linux-gnu/gazebo-9/plugins

教程说把上面的命令加入到自己的.bashrc 下面去。

注意事项1：其中要注意的是~/PX4-Autopilot 这个路径有可能你和别人的不一样，你需要指定到你自己的目录下。
注意事项2：最后一行export GAZEBO_PLUGIN_PATH=$GAZEBO_PLUGIN_PATH:/usr/lib/x86_64-linux-gnu/gazebo-9/plugins里的gazebo-9也有可能和你的电脑不一样，有可能是gazwbo-11你找到对应的目录，然后看看自己的目录下面到底是啥。

做到这里，按理说我们再次执行roslaunch offboard_py start_offb.launch 就可以把无人机起飞的程序跑起来了。但事与愿违，我的还是不成功。

问题追踪及解决方案

问题是经过上述的配置仍然报：

    Resource not found: px4 ROS path [0] = ... ...

的错误，所以我就排查问题，首先，我在roslaunch offboard_py start_offb.launch 命令执行前手动在命令行中分别输入下面这几行，问题就解决了。

source ~/PX4-Autopilot/Tools/simulation/gazebo/setup_gazebo.bash ~/PX4-Autopilot ~/PX4-Autopilot/build/px4_sitl_default
export ROS_PACKAGE_PATH=$ROS_PACKAGE_PATH:~/PX4-Autopilot
export ROS_PACKAGE_PATH=$ROS_PACKAGE_PATH:~/PX4-Autopilot/Tools/simulation/gazebo/sitl_gazebo
export GAZEBO_PLUGIN_PATH=$GAZEBO_PLUGIN_PATH:/usr/lib/x86_64-linux-gnu/gazebo-9/plugins

所以我分析，在~/.bashrc中设置的变量在哪里被冲掉了。

为了验证我的猜想，我又重新打开一个命令行，输入echo $ROS_PACKAGE_PATH 发现是有我设置的那些的。说明环境变量设置的没有问题，那问题就在接下来的步骤中。我想起来，我在执行roslaunch offboard_py start_offb.launch之前先进行了source ~/catkin_ws/devel/setup.bash，所以就是这个命令导致ROS_PACKAGE_PATH环境变量被修改了。问题找到了，那为什么我电脑上工作空间的环境变量配置会影响ROS_PACKAGE_PATH这个变量呢？

问题就在source */setup.bash上，除了catkin_ws工程中有这个操作，ROS启动也需要source /opt/ros/noetic/setup.bash 其中noetic是我的ROS版本，你的可能不一样。

通过我的实验，我发现：

无论你之前的ROS_PACKAGE_PATH内容是什么，在执行source /opt/ros/noetic/setup.bash 后都只有/opt/ros/noetic/share这一个结果，也就是会冲掉其他设置的内容。
无论你之前的ROS_PACKAGE_PATH内容是什么，在执行source ~/catkin_ws/devel/setup.bash 后都只有~/catkin_ws/src:/opt/ros/noetic/share，相对于上面增加了 ~/catkin_ws/src,也就是会冲掉其他设置的内容。注意~/catkin_ws是我自己的ROS工作空间目录，你的可能和这个不一样。

根据上述分析，我发现，设置环境变量的操作是有顺序的，我原来的顺序是最后执行source ~/catkin_ws/devel/setup.bash 导致我在~/.bashrc中设置的被冲掉。所以我最后总结了一个执行顺序：

第一步：~/.bashrc中添加source /opt/ros/noetic/setup.bash ROS的设置
第二步：~/.bashrc中添加source ~/catkin_ws/devel/setup.bash 工作空间的设置
第三步：~/.bashrc中添加下面的内容： PX4环境变量设置

source ~/PX4-Autopilot/Tools/simulation/gazebo/setup_gazebo.bash ~/PX4-Autopilot ~/PX4-Autopilot/build/px4_sitl_default
export ROS_PACKAGE_PATH=$ROS_PACKAGE_PATH:~/PX4-Autopilot
export ROS_PACKAGE_PATH=$ROS_PACKAGE_PATH:~/PX4-Autopilot/Tools/simulation/gazebo/sitl_gazebo
export GAZEBO_PLUGIN_PATH=$GAZEBO_PLUGIN_PATH:/usr/lib/x86_64-linux-gnu/gazebo-9/plugins

上面所有的设置请不要直接复制粘贴，根据实际情况修改你的设置，这样才不会出错。

深层分析

最后我想再了解一下，为什么source */setup.bash会把原本ROS_PACKAGE_PATH的设置冲掉，因此我打开了我工作空间中的setup.bash内容如下：

#!/usr/bin/env bash
# generated from catkin/cmake/templates/setup.bash.in

CATKIN_SHELL=bash

# source setup.sh from same directory as this file
_CATKIN_SETUP_DIR=$(builtin cd "`dirname "${BASH_SOURCE[0]}"`" > /dev/null && pwd)
. "$_CATKIN_SETUP_DIR/setup.sh"

我发现这个文件没执行啥，就是再执行一遍setup.sh文件，所以我们打开setup.sh文件，内容如下：

#!/usr/bin/env sh
# generated from catkin/cmake/template/setup.sh.in

# Sets various environment variables and sources additional environment hooks.
# It tries it's best to undo changes from a previously sourced setup file before.
# Supported command line options:
# --extend: skips the undoing of changes from a previously sourced setup file
# --local: only considers this workspace but not the chained ones
# In plain sh shell which doesn't support arguments for sourced scripts you can
# set the environment variable `CATKIN_SETUP_UTIL_ARGS=--extend/--local` instead.

# since this file is sourced either use the provided _CATKIN_SETUP_DIR
# or fall back to the destination set at configure time
: ${_CATKIN_SETUP_DIR:=/home/facri/Documents/ROS/catkin_ws/devel}
_SETUP_UTIL="$_CATKIN_SETUP_DIR/_setup_util.py"
unset _CATKIN_SETUP_DIR

if [ ! -f "$_SETUP_UTIL" ]; then
  echo "Missing Python script: $_SETUP_UTIL"
  return 22
fi

# detect if running on Darwin platform
_UNAME=`uname -s`
_IS_DARWIN=0
if [ "$_UNAME" = "Darwin" ]; then
  _IS_DARWIN=1
fi
unset _UNAME

# make sure to export all environment variables
export CMAKE_PREFIX_PATH
if [ $_IS_DARWIN -eq 0 ]; then
  export LD_LIBRARY_PATH
else
  export DYLD_LIBRARY_PATH
fi
unset _IS_DARWIN
export PATH
export PKG_CONFIG_PATH
export PYTHONPATH

# remember type of shell if not already set
if [ -z "$CATKIN_SHELL" ]; then
  CATKIN_SHELL=sh
fi

# invoke Python script to generate necessary exports of environment variables
# use TMPDIR if it exists, otherwise fall back to /tmp
if [ -d "${TMPDIR:-}" ]; then
  _TMPDIR="${TMPDIR}"
else
  _TMPDIR=/tmp
fi
_SETUP_TMP=`mktemp "${_TMPDIR}/setup.sh.XXXXXXXXXX"`
unset _TMPDIR
if [ $? -ne 0 -o ! -f "$_SETUP_TMP" ]; then
  echo "Could not create temporary file: $_SETUP_TMP"
  return 1
fi
CATKIN_SHELL=$CATKIN_SHELL "$_SETUP_UTIL" $@ ${CATKIN_SETUP_UTIL_ARGS:-} >> "$_SETUP_TMP"
_RC=$?
if [ $_RC -ne 0 ]; then
  if [ $_RC -eq 2 ]; then
    echo "Could not write the output of '$_SETUP_UTIL' to temporary file '$_SETUP_TMP': may be the disk if full?"
  else
    echo "Failed to run '\"$_SETUP_UTIL\" $@': return code $_RC"
  fi
  unset _RC
  unset _SETUP_UTIL
  rm -f "$_SETUP_TMP"
  unset _SETUP_TMP
  return 1
fi
unset _RC
unset _SETUP_UTIL
. "$_SETUP_TMP"
rm -f "$_SETUP_TMP"
unset _SETUP_TMP

# source all environment hooks
_i=0
while [ $_i -lt $_CATKIN_ENVIRONMENT_HOOKS_COUNT ]; do
  eval _envfile=\$_CATKIN_ENVIRONMENT_HOOKS_$_i
  unset _CATKIN_ENVIRONMENT_HOOKS_$_i
  eval _envfile_workspace=\$_CATKIN_ENVIRONMENT_HOOKS_${_i}_WORKSPACE
  unset _CATKIN_ENVIRONMENT_HOOKS_${_i}_WORKSPACE
  # set workspace for environment hook
  CATKIN_ENV_HOOK_WORKSPACE=$_envfile_workspace
  . "$_envfile"
  unset CATKIN_ENV_HOOK_WORKSPACE
  _i=$((_i + 1))
done
unset _i

unset _CATKIN_ENVIRONMENT_HOOKS_COUNT

内容挺多是吧，但我觉得核心只有一行

CATKIN_SHELL=$CATKIN_SHELL "$_SETUP_UTIL" $@ ${CATKIN_SETUP_UTIL_ARGS:-} >> "$_SETUP_TMP"

意思是去执行当前目录下的python 脚本。
我上网上搜了一下这位博主也分析到这里了：链接，但是也没找到最终原因，那接下来让我们分析分析这个文件里面具体执行的信息。我发现这个功能好深呀，后面又调用了pyton脚本，python脚本的内容也很多。有哪位大神可以帮我分析分析，通过注释看，这些配置文件都是通过其他配置文件自动生成的，一环套一环。

如果有人研究明白了上面的流程可以留言交流，我暂时先不去深究了，奥卡姆剃刀，先解决问题就行。总之，今天的问题解决了，总结了设置环境变量的顺序，大家一定要记住。