# Automation & security

Bash scripting basics

Variables, conditionals, loops and making a script executable.

The shebang and making it executable

The first line tells the shell which interpreter to run the script with. chmod +x makes the file executable so you can run it directly instead of typing bash script.sh.

bash

#!/usr/bin/env bash
echo "hello from a script"

Variables

No spaces around =. Reference a variable with $NAME or ${NAME}; the braces are needed when concatenating right up against other text.

bash

NAME="world"
echo "hello, ${NAME}!"

Conditionals

if/then/fi with [[ ]] for the test — the double-bracket form is more forgiving than the classic single-bracket [ ] and is what modern scripts should use.

bash

if [[ -f /etc/nginx/nginx.conf ]]; then
  echo "nginx is installed"
else
  echo "not found"
fi

Loops

for iterates over a list of values; while repeats as long as a condition holds. Both are common in deploy and backup scripts for looping over files or hosts.

bash

for f in /etc/nginx/sites-available/*; do
  echo "found: $f"
done

Fail fast

Add set -euo pipefail near the top of any real script: it exits on the first error, on use of an unset variable, and on a failure anywhere in a pipeline — instead of silently continuing after something breaks.

bash

#!/usr/bin/env bash
set -euo pipefail