shell pwd -P 显示当前路径的物理路径，与pwd命令的结果，在以下这种情况是不同的：
Ethan@Ethan-mac > ~/self_work/learn/shell > ln -s ~/env/data ./tmp Ethan@Ethan-mac > ~/self_work/learn/shell > ll total 0 lrwxr-xr-x 1 Ethan staff 21B 9 22 17:53 tmp -> /Users/Ethan/env/data Ethan@Ethan-mac > ~/self_work/learn/shell > cd tmp Ethan@Ethan-mac > ~/self_work/learn/shell/tmp > pwd /Users/Ethan/self_work/learn/shell/tmp Ethan@Ethan-mac > ~/self_work/learn/shell/tmp > pwd -P /Users/Ethan/env/data
