1. md5sum
2. 解决Could not get lock /var/cache/apt/archives/lock
在ubuntu apt-get upgrade的时候，遇到：
E: Could not get lock /var/cache/
apt/archives/lock - open (11 Resource temporarily unavailable)
E: Unable to lock the download directory
解决办法如下：
sudo rm -rf /var/cache/apt/archives/lock
sudo apt-get update
然后apt-get就恢复正常了。