奇怪的for 循环

[mysql@master ~]$ cat a1.sh 
for file 
do
echo $file
done
[mysql@master ~]$ sh ./a1.sh 1 2 3 4 5 6
1
2
3
4
5
6

相当于
[mysql@master ~]$ cat a1.sh 
for file  in  $@ 
do
echo $file
done
[mysql@master ~]$ sh ./a1.sh 
[mysql@master ~]$ sh ./a1.sh  1 2 3 4 5 6
1
2
3
4
5
6