rexpect

rexpect is an R package that allows you to automate interactions with programs that expose a text terminal interface. The API is inspired by the original Expect tool by Don Libes.

Installation

You can install the development version from GitHub with:

# install.packages("remotes")
remotes::install_github("jeroenjanssens/rexpect")

Example

Let’s say we have a Python script that repeatedly asks for a number. When the user enters sum, the script prints the sum of all the numbers. The script exits when the user presses CTRL-C. Here’s the contents of bin/sum.py:

#!/usr/bin/env python3

numbers = []

while True:
    user_input = input("Enter a number: ")
    
    if user_input == "sum":
        print(f"The sum is: {sum(numbers)}")
        continue

    try:
        numbers.append(float(user_input))
    except ValueError:
        print("Please enter a valid number.")

We can use rexpect to run this script and interact with it. Let’s enter the numbers 7, 13, and 22 and see what the sum is:

library(rexpect)
#> 
#> Attaching package: 'rexpect'
#> The following object is masked from 'package:utils':
#> 
#>     prompt

session <- spawn("bin/sum.py", prompt = "^(.*):$")
send_lines(session, c("7", "13", "22", "sum"), wait = TRUE)

Under the hood, rexpect starts a tmux session using tmuxr:

session
#> tmuxr pane 42:1.0: [80x23] [history 0/50000, 3912 bytes] %48 (active)

We can capture the output and extract the sum:

print(output <- read_all(session))
#> [1] "Enter a number: 7"   "Enter a number: 13"  "Enter a number: 22" 
#> [4] "Enter a number: sum" "The sum is: 42.0"    "Enter a number:"
as.numeric(gsub("[^0-9.]", "", output[length(output) - 1]))
#> [1] 42

We’ve accomplished our goal, so let’s exit:

send_lines(session, "exit")
read_all(session)
#> [1] "Enter a number: 7"            "Enter a number: 13"          
#> [3] "Enter a number: 22"           "Enter a number: sum"         
#> [5] "The sum is: 42.0"             "Enter a number: exit"        
#> [7] "Please enter a valid number." "Enter a number:"

Oh that’s right, we need to press CTRL-C to exit the script:

send_control_c(session)

This was a rather simple example to demonstrate some of the capabilities of rexpect. The session could have taken place inside a Docker container or over SSH. We could also have recorded the session using asciinema. Have a look at the function reference to learn more about what rexpect has to offer.

License

The rexpect package is licensed under the MIT License.