现有一段 shell

!/bin/bash

mobile=/var/www/html/mobile
desk=/var/www/html/sbg
if [ "$1" = "m" ] ; then
cd $mobile
pwd
elif [ "$1" = "d" ] ; then
cd $desk
pwd
else
echo "action is empty"
exit 0
fi
git pull origin
echo "success"

使用 php exec system shell_exec 调用 shell 但是 只能得到 success ， git pull origin 没能执行。请问这是什么原因