Git必须知道当前版本是哪个版本，在Git中，用HEAD表示当前版本，上一个版本就是HEAD^，上上一个版本就是HEAD^^，当然往上100个版本写100个^比较容易数不过来，所以写成HEAD~100。
例子
zhangyang@ubuntu:/mnt/hgfs/wwwroot/sichuan_guangyuan_cangxixian_zhongyiyuan$ git reset --hard
Checking out files: 100% (5776/5776), done.
HEAD is now at ed6f951 remove lajiwenjian
zhangyang@ubuntu:/mnt/hgfs/wwwroot/sichuan_guangyuan_cangxixian_zhongyiyuan$ git reset --hard HEAD^
Checking out files: 100% (7586/7586), done.
HEAD is now at acdd161 add first
zhangyang@ubuntu:/mnt/hgfs/wwwroot/sichuan_guangyuan_cangxixian_zhongyiyuan$ git pull origin master
Username for 'http://10.81.23.51': zhangy
Password for 'http://zhangy@10.81.23.51':
From http://10.81.23.51/php/sichuan_guangyuan_cangxixian_zhongyiyuan
* branch master -> FETCH_HEAD
Updating acdd161..ed6863d
Fast-forward
README.md | 1 -
zpdydhl/README.md | 4 ++--
2 files changed, 2 insertions(+), 3 deletions(-)
