TIL: Bash Script Automation

Posted on Jun 7, 2022
description: Something I learnt today. Maybe more than one thing👾
tl;dr: I made a bash script that automates the running of a script, including automatic user input.

Shebang

First thing I learnt, is that you have to specify the interpreter you want to execute the file.

Shebang examples

  • Execute using Bourne Shell: #!/bin/sh
  • Execute using Bash Shell: #!/bin/bash

Installing & using Expect

Expect is a scripting language that allows for automation of interactive programs. src

To install it, sudo apt get expect. Then make a new file and open it, you can name that file testscript.sh

In that script, specify that we’re using the expect interpreter like so

#!/usr/bin/expect

Spawn, Expect, Send

The script I was running manually had a sequence like this:

  1. Start the script
  2. Get prompted for input #1, I type in my input, & press return
  3. Get prompted for input #2, I type in my input, & press return
  4. Long running script executes in the background
  • Spawn is what will take care of step 1
  • My best guess at the role of expect, is it let’s the interpreter know what string is going to stop execution and prompt for user input.
  • Send is what takes care of the automatic user input

this is how my 4 steps can be automated then

#!/usr/bin/expect

set timeout -1
spawn sudo scriptThatIHaveToStart
expect "Input #1"
send "Hey, this is the first input\r"
expect "Input #2"
send "this is the 2nd input\r"

expect eof
  • set timeout -1 Will make the process run forever, meaning you have to manually stop the process. This is important is your process is a long running script that you just want in the background.
  • That last line is expect end of file. I’m not yet sure what that’s for, hehe

Running the script

chmod to make it executable

Run chmod +x testscript.sh. replace testscript with whatever you named yours

Execution time baby

Then while you’re in the directory with the script, ./testscript.sh will execute it.

Fin.

Some resources I used

An error & it’s fix

Expect Script - bash script file not found

If you get this error, it’s likely because you haven’t installed expect. I experienced this, lol.

It’s either uninstalled, or you’d have supplied the wrong file path to the expect binary.