前提：给自己机器配置ssh免密登录
#!/bin/bash
cat << 'EOD' > list
1
2
3
4
EOD

while read line
do
if ssh myname@127.0.0.1 "test -f /tmp/a.sh"; then #配置了免密登录，此处不用输入密码
echo "$line file exist."
else
echo "$line file not exist."
fi
done < list

问题：按理，list文件里有4行，这个while循环应该会执行4次才对，为何这个只执行了一次呢？

答案如下：

检查ssh选项，发现

-n      Redirects stdin from /dev/null (actually, prevents reading from stdin).  This must be used when ssh is run in the background.  A
             common trick is to use this to run X11 programs on a remote
machine.  For example, ssh -n shadows.cs.hut.fi emacs & will start
             an emacs on shadows.cs.hut.fi, and the X11 connection will
be automatically forwarded over an encrypted channel.  The ssh pro-
             gram will be put in the background.  (This does not work if
ssh needs to ask for a password or passphrase; see also the -f
             option.)

可以说这个选项是专门用来解决这个问题的。用/dev/null来当ssh的输入，阻止ssh读取本地的标准输入内容。

#要想使本脚本执行次数正常，ssh命令需要添加-n选项即可。

https://blog.csdn.net/notsea/article/details/42028359