由于使用了/bin/sh 导致找不到pushd命令
切换回/bin/bash即可，但是我在shell中声明#!/bin/bash还是继续报错
看到一句解释如下：
This has to do with /bin/sh being linked to /bin/dash instead of /bin/bash in ubuntu’s latest releases. To fix it once and for all do the following:
使用下面的方法：
# rm -f /bin/sh
# ln -s /bin/bash /bin/sh