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Advanced Programming in Expect:
A Bulletproof Interface


Introduction:

This article assumes the reader has a thorough understanding of the basics of the Expect scripting language and is looking for advanced solutions.  For more on Expect, see: 

http://www.cotse.com/dlf/man/expect/index.html
In the design of automated systems in Expect, one of the more difficult hurdles many programmers encounter is ensuring communication with ill-behaved connections and remote terminals. The send_expect procedure detailed in this article provides a means of ensuring communication with remote systems and handles editing and rebroadcast of the command line. Where a programmer would usually send a command line and then expect the echo from the remote system, this procedure replaces those lines of code and provides the most reliable interface I have come across.  Features of this interface include:
  • Guarantees transmission via remote system echo
  • Tolerates remote terminal control codes and garbage characters in the echo of the sent string
  • Persistence of attempts and hierarchy of methods before declaring a failure
  • Interactively edits and retransmits command lines that cannot be verified
  • Maintains its own moving-window diagnostics files, so they are small and directly associated with the errors
Communication with local processes (i.e. those running on the same workstation as the expect process) is typically not problematic and does not require the solutions detailed in this article.  External processes, however, can create a number of problems that may or may not affect communication, but will affect an automated system's ability to determine the success of the communication.  In cases where it is corrupted, it is not always immediately obvious: a corrupted command may trigger an error message, but data which has been corrupted may still be considered valid and the error would not show up immediately, and may cause a variety of problems.  This is why it is necessary to ensure that the entire string that is transmitted is properly received echoed by the remote system.

The basic idea of this interface is to send the command string except for its terminating character (usually, a carriage return) and look at the echo from the remote system.  If the two can be matched using the regular expressions in the expect clauses, then the terminating character is sent and transmission is considered successful. If success cannot be determined, the command line is cleared instead of being sent, and alternative transmission modes are used.

In many cases, nothing more than expecting the exact echo of the string is sufficient.  If you're reading this article, though, I suspect that you've encountered some of the problems I have when programming in Expect, and you're looking for the solution here.  If you're just reading out of interest, the problems arise when automating a session on a machine off in a lab, or on the other side of the world.  Strange characters pop up over the connection, and the terminal you're connected to does weird things with its echo, but everything is working.  It becomes very difficult to determine if what was sent was properly received when you have noise on the connection, terminal control codes inserted in the echo, and even server timeouts between the automation program and the remote session.  This interface survives all of that, and if it can't successfully transmit the string, it means that the connection to the remote system has been lost. 

The code provided in this article is executable, but needs to be incorporated into any system in which it is to be used.  Ordinarily, system-dependent commands need to be added based on the needs of the target system.  Also, this code uses simple calls to the puts command to output status messages - these should be changed to use whatever logging mechanism is used by the rest of the system.  A final caveat, and I can't emphasize this enough: always wear eye protection. 

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