Exploring Expect

Recording All Expect Output

It is possible to save in a file all the output that Expect produces. This includes not only the output from a spawned process, but anything that Expect itself generates such as diagnostics. Conceptually, this is akin to piping Expect’s standard output and standard error through the tee program. However, tee is fairly inflexible in that its piping cannot be modified once Expect has started.

Expect has a programmatic means to start and stop recording (or logging). Given a file name argument, the log_file command opens the file and begins recording to it. If a log file is already open, the old file is closed first. Using the -open or -leaveopen flag, the log_file command can also record to a file opened by the open command. The use of these flags are identical to their use in the spawn command (Chapter 13 (p. 285)) so I will not mention them further here.

Recording is done by appending to the file, so if anything has previously been stored in the file, it remains. To start over, use the -noappend flag (or simply remove the file first).

For example, the UNIX script command can be emulated with the following commands:

spawn $env(SHELL)
log_file -noappend typescript
interact

First a shell is started. Notice how the user’s desired shell is determined by getting it from the environment where it is stored in the variable SHELL. This is much more polite than simply starting up, say, /bin/csh. The log_file command then opens the file typescript. The -noappend flag ...