shell-scripting.md

Shell scripting

Some introduction to the topic

Learning checklist

• what is a shell script
• #!/bin/bash and #!/usr/bin/env bash
• set -e and set -x
• how to make scripts executable chmod +x my_script.sh
• how to execute scripts
• $(( )) for arithmetic operations
• how to define and use variables
• how to refer to arguments
• exit codes
• be careful with destructive commands (e.g. rm) and overwriting files (outside a version-controlled directory). it will execute and you can delete important things!

Resources

Recommended reading

• Unix Philosophy
• 101 Bash Commands and Tips for Beginners to Experts
• Bash handbook
• Bash guide
• https://explainshell.com

References

• Assignment

Instructions: Write the shell scripts detailed below. At each step, execute the shell script to verify that the script does what it's supposed to

• Write a shell script (hello_world.sh) to:
  • Write 'hello world' to a file named hello.txt
  • echo the contents of hello.txt to standard output (a.k.a. stdout, i.e. the terminal screen)
  • Copy hello.txt to hello2.txt
  • Overwrite the contents of hello2.txt to contain only 'goodbye'
  • Add a new line 'world' to hello2.txt
  • echo the contents of hello2.txt
  • exit with code 0
• Write a shell script (greet_user.sh) to:
  • take in a string as the first argument (e.g. ./greet_user.sh obama)
  • echo "hello, nice to meet you obama!!!" to standard output
  • add a if statement do 2 things if no name/string is provided when ./greet_user.sh is executed:
    • echo to the screen "ERROR: No input is provided. Exiting now..."
    • exit with status code 1
  • Extend the if statement to include a second check to ensure that the length of the string greater than 1
    • hint: use [[ ]], || and $(echo -n $1 | wc -m)
• Write a shell script (add_numbers.sh) to:
  • take in 2 arguments (e.g. add_numbers.sh 1 2)
  • add the 2 numbers and print the results to screen as such: The sum is: <output>
• Write a shell script (run_everything.sh) to run all the shell scripts above:
  • Version 1. Run the following:
    • hello_world.sh
    • greet_user.sh bob
    • add_numbers.sh 1 2
  • Version 2 (add the set -e option and run the following:
    • hello_world.sh
    • greet_user.sh
    • add_numbers.sh 1 2
    • (this should exit with error at greet_user.sh, and you should not see the results of add_numbers.sh)
• Commit and push your changes to github!

More fun assignments

• http://overthewire.org/wargames/bandit/