expect中为send语句上加变量的代码

代码如下:
#!/usr/bin/expect
set ssh_user "fivetrees"
set password "123456"
spawn ssh -i /root/.ssh/$ssh_user  root@xiaopp.3f.com
expect_before "no)?" {
send "yes\r" }
sleep 1
expect "Enter passphrase for key*"
send "$password\r"
for {set snum 0} {$snum <= 6} {incr snum} {
expect "*#"
send "ls -l /opt/backup/fivetreesdb_s$snum\r"
sleep 3
}
sleep 2
expect "*#"
send "echo\r"
exit

send expect 变量