01-bash-basics

01 · Bash Basics

Maps to Shell Scripting → Section 1: Introduction to Bash (shebang, run-from-anywhere, variables, parameters $1/$@, arithmetic expansion)

This is where commands become automation. A shell script is just a file of the commands you already know, run top to bottom - the backbone of every deploy, backup and CI job. Here you'll write your first scripts, parameterise them, and do maths.

How to use this set

# from the 04-shell-scripting/ folder
make build                       # once
make start  S=01-bash-basics     # shell with a clean ~/sandbox to write scripts in
#   ...work in the container; when done, type 'exit' (or Ctrl+D) to leave, then:
make verify S=01-bash-basics     # runs your scripts and grades them
make reset  S=01-bash-basics     # fresh start

The grader runs your scripts, so they have to actually work.

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Tier 1 - Your first scripts (hint: #!/bin/bash, chmod +x, $variable)

1.1 (graded) Write ~/sandbox/hello.sh that prints Hello, DevOps. It must start with a shebang and be executable (so ./hello.sh runs it).

1.2 (graded) Write ~/sandbox/count.sh that stores the number 3 in a variable and uses it to print Count is 3.

Tier 2 - Parameters & arithmetic (think: $1, and $(( )) for maths)

2.1 (graded) Write ~/sandbox/greet.sh that greets the name passed as its first parameter - e.g. ./greet.sh Sam prints Hello, Sam.

2.2 (graded) Write ~/sandbox/add.sh that takes two numbers as parameters and prints their sum - e.g. ./add.sh 3 4 prints 7 (real addition, not 34).

Tier 3 - Put it together (goal only)

3.1 (graded) Write ~/sandbox/rect.sh that takes a length and a width as parameters and prints both the area (length × width) and the perimeter (2 × (length + width)). For ./rect.sh 5 8 that's area 40 and perimeter 26.

Self-check questions

1. What does the shebang line do, and why does the file need to be executable?
2. How do you read the first argument passed to a script? All arguments?
3. Why does $((3 + 4)) give 7 but "3 + 4" doesn't?