在expect脚本中,grep的输出不保存在expect\u out(缓冲区)中

我有一个脚本,它将文件按特定值进行grep,但grep的响应不会保存在expect\u out(缓冲区)中。以下是我的代码:

#!/usr/bin/expect 

set prompt "(%|>|#|\\$)"
#set prompt "(%|>|\\$)"

set pidBash [ spawn bash ]
puts "\nStarting script....."

send -- "\n"
expect -re $prompt {}
set output $expect_out(buffer)
send -- "grep '^option' /home/pb791b/udhcpd.conf | grep lease\n"
sleep 4
expect -re $prompt {}

set output $expect_out(buffer)
set outputLines [split $output "\n"]
set len [llength $outputLines]
puts "\nlen: $len\n"
for {set i 0} {$i<[expr $len]} {incr i} {
    set line [lindex $outputLines $i]
    puts "line $i: $line"
}

exec kill $pidBash
puts "\nprocess Bash terminated"

puts "End script."

pb791b@pb791b-VirtualBox:~/devtest/ngs/base/Tests/shellScripts$ ./temp2.sh 
spawn bash

Starting script.....

pb791b@pb791b-VirtualBox:~/devtest/ngs/base/Tests/shellScripts$ 
grep '^option' /home/pb791b/udhcpd.conf | grep lease
pb791b@pb791b-VirtualBox:~/devtest/ngs/base/Tests/shellScripts$ grep '^option' /home/pb791b/udhcpd.conf | grep lease
option  lease   864000      # 10 days of seconds
pb791b@pb791b-VirtualBox:~/devtest/ngs/base/Tests/shellScripts$ 
len: 3

line 0:  
line 1: grep '^option' /home/pb791b/udhcpd.conf | grep lease
line 2: pb791b@pb791b-VirtualBox:~/devtest/ngs/base/Tests/shellScripts$

process Bash terminated
End temp.sh script.
pb791b@pb791b-VirtualBox:~/devtest/ngs/base/Tests/shellScripts$ 

当我打印expect_out(buffer)时,它先打印grep命令,然后打印提示符,但缺少grep“option lease 864000#10天秒”的输出