dear unix diary,
i've been doing more devops/sysadmin stuff at work recently. i've been tasked with creating automation scripts, and i recently learned about expect

Quote:Expect is a program that "talks" to other interactive programs according to a script. Following the script, Expect knows what can be expected from a program and what the correct response should be. An interpreted language provides branching and high-level control structures to direct the dialogue. In addition, the user can take control and interact directly when desired, afterward returning control to the script.

so it's an evaluation engine that you can "pre-script" interactions with other software (mainly those which prompt for user input). it's a similar concept to the yes utility, but is far more flexible.

here's an example. say you connect to an open vpn server that interactively prompts you for your username/password after the initial handshake:


first: set the correct shebang for expect scripts. the spawn directive allows you to run a command. expect looks for a string (regex) returned from the previous command(s). when an expect is matched the next line will execute. in this case it's the send command, which mimics users input. here we're entering our username then pressing return (\r). there's another set of expects on the password, as well as the final expect for a successful login. the final line is the interact directive, which simple holds the program open waiting for manual user interaction. but in this case it simply keeps the vpn connection open. without this final command the script would have terminated and closed any programs it's run with the spawn method.

this is very cool, but it's a simple / dumb example. here i have my username and password hard coded into the script. this can be very dangerous / insecure, but i thought a simple script to explain the concept would work out better.