13-bash-basics

Bash Basics

Linux for DevOps · Module 4 — Bash Scripting

📺 Watch: video link coming soon

What you'll learn

• The shebang, making a script executable, and running it
• Variables and command substitution
• Reading input and printing output
• Conditions (if, test, [[ ]])
• Loops (for and while)

Concepts

A shell script is just a file full of the commands you'd otherwise type by hand. The first line — the shebang #!/usr/bin/env bash — tells the system which interpreter should run the file.

Variables hold values (name="Rashid") and you read them back with $name. Wrap expansions in quotes ("$name") so spaces don't break things. Command substitution $(...) captures the output of a command, e.g. today=$(date +%F).

Bash conditions run on exit codes, not the words true/false: a command that exits 0 counts as success (truthy), anything else is failure. That's what drives if.

What the names mean

Name	Stands for
bash	Bourne Again SHell — a pun: the original shell was written by Steve Bourne
shebang #!	"sharp" (#) + "bang" (!)
echo	prints back what you give it — like an echo
read	reads what the user types

Commands

# The full script lives in scripts/hello.sh — here's the shape of it:

name="${1:-}"                    # first argument, or empty
if [[ -z "$name" ]]; then        # if it's empty...
  read -rp "Your name: " name    # ...ask for it
fi
echo "Hello, $name — today is $(date +%F)"   # command substitution

for i in 1 2 3; do               # loop over a list
  echo "count $i"
done

count=0
while [[ $count -lt 3 ]]; do     # loop while a condition holds
  echo "while $count"
  count=$((count + 1))           # $(( )) = do math: add 1
done

Make it executable and run it:

chmod +x hello.sh     # chmod +x = make the file executable
./hello.sh            # run it — prompts for your name
./hello.sh Rashid     # run it with the name as an argument

In production

Your very first useful script is a tiny status checker: it asks whether a service is running and prints a clear answer. That pattern — check something, report it plainly — is the seed of every health-check and bootstrap script you'll write later.

Practice

• Write a script that takes a directory as an argument and prints how many files it contains. If no directory is given, default to the current directory.

Cheat sheet

Syntax	Meaning
#!/usr/bin/env bash	Shebang
var="value" / $var	Set / use a variable
$(cmd)	Command substitution
if [[ cond ]]; then … fi	Condition
for x in …; do … done	Loop

Scripts

• scripts/hello.sh — greeting script showing variables, input, conditions, and both loop styles.

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