‘expect’ script in linux

Ever come across a situation where you want to access an interactive application like ssh from a bash script but do not want to enter password manually and let the script do it for you?

Yesterday I had the same situation and I found solution for this, with a tool called “expect”.

What is except?

“Expect is a tool for automating interactive applications like ssh, telnet, ftp etc.”

Let’s take an example,

Login to ssh on a system with,

• IP: 192.168.56.3
• username: batman
• password: batman

Now to do this you will have to do following steps manually.

• Open terminal
• type command, ssh batman@192.168.56.3
• Press Enter
• A password prompt will appear, like this
• Here’s the problem when you want to automate this activity, you will have to manually enter the password after “password:” prompt, then only you can access the system with ssh.

 

To automate this activity from script, you will need expect.

First install expect in your linux system with below command.

apt-get install expect

Now lets create a script to automate above activity.

• Open your terminal
• Create a script file, Let’s say ConnectToBatman
• Make it executable

chmod +x ./ConnectToBatman

• Then run the script,

./ConnectToBatman

• Here’s what script will look like,

#!/usr/bin/expect -f

#############################################################
# Notice here '/usr/bin/expect -f' is used in very first line
# to make define expect path
#############################################################

#spawn:- Starts a script or a program, here it starts ssh call
spawn ssh batman@192.168.56.3

#expect :- expect should be kept after spawn, 
# it will wait for ssh to respond, in case of ssh it will 
#respond
# by asking for password with word "password:" in the end
expect "password:"

#send:- To send a reply to your program.
# send is used after expect, to send response to program
# in this case we need to provide password for username: batman
# the password is 'batman'
# Notice a \n after password it works as
# Enter after typing password.
send "batman\n"

#interact:- Allowing you in interact with your program.
# interact should be added following above commands otherwise
# you won't be able to enter into the program after login
interact

• That’s it …..
• With above 4 commands you can access ssh with script,

References:

https://likegeeks.com/expect-command/