February 1, 2011

Martin Lundberg

Using expect to work with interactive programs

At work the other day I needed to create a script which would connect to a server through sftp and download a couple of files. I’m sure there are great tools for doing this and I would love to hear how you would solve it in the comments but for several reasons I needed to use expect and sftp which were available on the server from which the script would run.

Expect is a tools for automating working with interactive tools such as fsck, passwd or in my case, sftp. In the most simple case you use spawn to start a process, send to send input to the process and expect to wait for output from the process. In my case it looked something like this:

set timeout 600
spawn sftp -oPort=12345 username@server.tld
expect "password:"
send "yourpassword"
expect "sftp>"
send "get ./download/* ./files\n"
expect "sftp>"

An important thing to know is that by default expect only wait for 10 seconds before silently timing out. Because I downloaded files which could potentially take more then 10 seconds to download I needed to handle this. After some reading I found that all you needed to do was to add set timeout <time> to the script. Setting the time to -1 makes it wait forever.