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Expect: automation and semi-automation of processes under Linux

Friday August 7th 2020 by SocraticDev

Expect is a TCL based utility which excels in automating interactive tasks.

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.

-- man page for Expect

This short script below (** strongly discouraged **) automatically provides your root password when requested by the system. This example illustrates the interactive aspect of automation with Expect. The script starts, performs certain actions, and returns control to the user.

expect -c '
spawn bash
sleep 0.3
send -- "sudo ls\n"
expect "password for"
send -- "<Your Password>\n"
interact
exit
'
Some use cases
  • Expect was used, in the time of telephone modem connections, for the server to call users and, thus, avoid charges for the call;
  • Reliability and sustainability: it is used as an automated testing tool by critical components of GNU/Linux such as gcc and LLVM. It is therefore worth the cost of investing time and effort to learn how to use it well and integrate it into your workflow;
  • Executing tasks usually performed by humans as they involve periods of inactivity, cursor movement, etc;
  • Automatically triggering completion with TAB and then hand over to the user;
  • Automatically invoking keyboard shortcuts in bash;
  • Interacting naturally with programs like a human would;
  • Filling in the gaps of a program where it would lack ways to interact programmatically with it, but on the other hand allowing a human user to do it manually;
  • Partial automation: performing certain operations and then handing over to a human user.
Why isn't Expect better known?
  • It is a very ecclectic application. First of all, its name, Expect, is really not great in terms of SEO. Rather difficult to look for help in Google by using this keyword;
  • Then, it's an interpreter who doesn't really have his own language. Expect uses TCL while adding extensions to it. Thus, it is sometimes difficult to determine if we are dealing with vanilla TCL or a peculiarity of Expect;
  • Despite its weak presence on the web, by reading the documentation related to TCL, we discover a universe of unsuspected possibilities to benifit from your UNIX or GNU/Linux systems.
Sources

Don Libes' Expect: A Surprisingly Underappreciated Unix Automation Tool

https://wiki.tcl-lang.org/page/Expect