Understanding echo/spawn/send/expect construct

Question

Here is a line from a bash script I am trying to understand:

echo "spawn myscript.sh arg1 arg2; expect \"Please enter your value: \"; send \"myval\r\"; expect eof" | expect

I think I understand the first expect and the send.

Q1: Does expect work only with spawn?

Q2: What is the purpose of the last expect after the pipe?

Q3: In general, can you guess what the original author of this line was attempting to accomplish?

Answer 1

Q1: The purpose of expect is to automate interactive programs. To launch the program and interact with it, you use the spawn command.

Q2: the last expect after the pipe is the expect binary. If not given a file argument, it reads its stdin to get the script to execute.

Q3: The author wanted a mechanism to automatically pass a value when the script prompts for one. That's it.

I strongly suspect (depending on what myscript.sh does) that you don't need expect at all:

echo "myval" | myscript.sh arg1 arg2

Answer 2

Q1: I'm not an expect user, but it seems clear that spawn runs a script in the background and makes it controllable by expect, rather than just running without any interactivity.

Q2: The string is passed as a script to expect. I assume, like most scripting languages, this is equivalent to saving the string to a file and running expect file.

Q3: It looks like a test to see that myscript.sh, when run, prints a message and responds to a user provided value by exiting without printing anything.

In other words, I expect you should be able to run this script with the same result:

#!/usr/bin/expect
spawn myscript.sh arg1 arg2 # Run `myscript.sh arg1 arg2` and return control to `expect`
expect "Please enter your value: " # Succeeds if the script prints this value
send "myval\r" # Simulate entering this value
expect eof # Succeeds if the script ends without printing anything