attach:
将宿主机的stdin,stdout,stderr到指定容器，即并不会产生新的进程。

exec:
是在一个正在运行的容器中执行新的程序。

参考案例:

[root@centos201 ~]# docker run   --name c2 -itd alpine:latest
[root@centos201 ~]
[root@centos201 ~]# docker attach c2
/ # 
/ #  ctrl + p -->  ctrl +q   # 表示不中断当前的容器，让容器继续后台运行。



[root@centos201 ~]# docker run --name c3 -d alpine tail -f /etc/hosts
da91421f2248623b8d61a5f47b25c1b973cec2f0dc9e37649491d039023af85f
[root@centos201 ~]# 
[root@centos201 ~]# docker ps 
CONTAINER ID   IMAGE     COMMAND                CREATED         STATUS         PORTS     NAMES
da91421f2248   alpine    "tail -f /etc/hosts"   4 seconds ago   Up 3 seconds             c3
[root@centos201 ~]# 
[root@centos201 ~]# docker attach c3 
ls
pwd
exit




[root@centos201 ~]# docker ps 
CONTAINER ID   IMAGE     COMMAND                CREATED              STATUS              PORTS     NAMES
da91421f2248   alpine    "tail -f /etc/hosts"   About a minute ago   Up About a minute             c3
[root@centos201 ~]# 
[root@centos201 ~]# 
[root@centos201 ~]# docker exec -it c3 sh
/ # 
/ # ps -ef
PID   USER     TIME  COMMAND
    1 root      0:00 tail -f /etc/hosts
    7 root      0:00 sh
   13 root      0:00 ps -ef
/ # 
/ #