作业三：
以操作文件的方式，新建一个用户alex
echo "alex:x:1200:1200::/home/alex/:/bin/bash" >> /etc/passwd
echo "alex:x:1200:" >> /etc/group
echo "alex::17239:0:99999:7:::" >> /etc/shadow
echo "alex:::" >> /etc/gshadow
mkdir /home/alex
touch /var/spool/mail/alex
cp -r /etc/skel/.[!.]* /home/alex
chown -R alex.alex /home/alex
chown alex.mail /var/spool/mail/alex