Shell Scripting Basics

Introduction

A shell script is a text file of commands. Anything you can type in the terminal you can save, repeat and share. Scripts turn boring jobs, such as backups, renaming files or deployments, into one command.

Your first script

Create a file named hello.sh:

#!/usr/bin/env bash
echo "Hello, $1!"

• The first line, the shebang, tells the system which program runs the file.
• $1 is the first argument given to the script.

Make it executable and run it:

chmod +x hello.sh
./hello.sh Sam

Hello, Sam!

Variables

name="Sam"
echo "Hello, $name"
today=$(date +%F)          # store a command's output
echo "Backup for $today"

No spaces around = when assigning. $(...) runs a command and uses its output.

Always quote variables

Without quotes, a value with spaces breaks into several words:

file="my notes.txt"
rm $file       # WRONG: tries to remove "my" and "notes.txt"
rm "$file"     # right: removes "my notes.txt"

Make double quotes around "$variable" a habit.

Arguments

• $1, $2 and so on are the arguments.
• $# is how many were given.
• $@ is all of them.

Conditions

if [ "$#" -lt 1 ]; then
  echo "Usage: $0 name"
  exit 1
fi

if [ -f "$1" ]; then
  echo "$1 is a file"
elif [ -d "$1" ]; then
  echo "$1 is a folder"
else
  echo "$1 does not exist"
fi

Common tests: -f file exists, -d directory exists, -z string is empty, and for numbers -eq, -lt, -gt. Spaces inside the brackets are required. In Bash, [[ ... ]] is a more forgiving alternative.

Loops

for f in *.txt; do
  echo "Found $f"
done

count=1
while [ "$count" -le 3 ]; do
  echo "Round $count"
  count=$((count + 1))
done

$((...)) does arithmetic.

Reading input

read -r -p "Your name? " answer
echo "Nice to meet you, $answer"

Try it yourself

Write a script backup.sh that takes a file name as an argument, checks that it exists, and copies it to a folder called backup with today's date added to the name. Print a helpful message if no argument is given.

Summary

• A script starts with a shebang, and chmod +x makes it executable.
• Use variables, arguments ($1, $#), if tests and for or while loops.
• Quote your variables, always.