Expect scripting language is used to feed input automatically to an interactive program. It is easy to learn compared to other scripting languages. Using expect script sysadmins and developers can automate redundant tasks easily. It works by expecting specific strings, and sending or responding strings accordingly.
Following three expect commands are used when automating any interactive processes.
- send – to send the strings to the process
- expect – wait for the specific string from the process
- spawn – to start the command
Make sure to install expect packages on your system, as it does not get installed by default. Once installed, you’ll see the expect interpreter as “/usr/bin/expect”. Generally, expect script files has .exp as extensions.
1. Expect “Hello World” Example
The following expect script is expecting the specific string “hello”. When it finds it (after user enters it), “world” string will be send as response.
#!/usr/bin/expect expect "hello" send "world"
2. Timeout On Expect String
By default, the expect timeout is 10 seconds. If you don’t enter anything for the expect command, it times out in 20 seconds. You can also change the timeout as shown below.
#!/usr/bin/expect set timeout 10 expect "hello" send "world"
3. Automate User Processes With Expect
With the help of expect, you can automate the user processes and get the expected output. For example, you might use this to simplify the project testing process by writing scripts for the test cases.
The below example does the addition program automation.
#!/usr/bin/expect
set timeout 20
spawn "./addition.pl"
expect "Enter the number1 :" { send "12\r" }
expect "Enter the number2 :" { send "23\r" }
interact
Execute this as shown below.
$ ./user_proc.exp spawn ./addition.pl Enter the number1 : 12 Enter the number2 : 23 Result : 35
In case, if you have written the code without interact command, then the script would exit immediately after sending the string “23\r”. Interact command does the control, hands over the job to the addition process, and produces the expected result.
4. Match and No Match Contents in $expect_out Variables
On the successful matching of string expect returns, but before that it stores the matched string in $expect_out(0,string). The string that are received prior plus the matched string are stored in $expect_out(buffer). The below example shows you the value of these two variable on match.
#!/usr/bin/expect set timeout 20 spawn "./hello.pl" expect "hello" send "no match : <$expect_out(buffer)> \n" send "match : <$expect_out(0,string)>\n" interact
The hello.pl program just prints only two lines as shown below.
#!/usr/bin/perl print "Perl program\n"; print "hello world\n";
Execute it as shown below.
$ ./match.exp spawn ./hello.pl Perl program hello world no match : <Perl program hello> match : <hello>
5. Automate SU login into Other User Accounts
Expect allows you to pass the password for the Linux login account from the program, instead of entering the password on the terminal. In the below program, su login is automated to login into desired accounts.
#!/usr/bin/expect set timeout 20 set user [lindex $argv 0] set password [lindex $argv 1] spawn su $user expect "Password:" send "$password\r"; interact
Execute the above expect program as shown below.
bala@localhost $ ./su.exp guest guest spawn su guest Password: guest@localhost $
After running the above script, it logged into the guest user account from bala user account.
6. SSH Login into Another Machine
The example expect program shown below automates the ssh login from one machine to another machine.
#!/usr/bin/expect set timeout 20 set ip [lindex $argv 0] set user [lindex $argv 1] set password [lindex $argv 2] spawn ssh "$user\@$ip" expect "Password:" send "$password\r"; interact
Execute the above expect program as shown below.
guest@host1 $ ./ssh.exp 192.168.1.2 root password spawn ssh root@192.168.1.2 Password: Last login: Sat Oct 9 04:11:35 2010 from host1.geetkstuff.com root@host2 #
Originally from http://www.thegeekstuff.com/2010/10/expect-examples/
