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Bash beginner series · Part 2

Bash Scripting for Beginners: Arguments, If Statements and Exit Codes

Part 2 teaches scripts that accept input, make decisions and report success or failure. You will use positional arguments, if statements, test operators and exit codes.

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Arguments If statements Test operators Exit codes File checker script Service checker script Line by line Try it yourself Common mistakes Next part

Arguments

Using script arguments

Arguments are values you pass to a script when you run it.

./check-file.sh /etc/passwd

Variable	Meaning
`$0`	The script name
`$1`	The first argument
`$2`	The second argument
`$#`	The number of arguments
`$@`	All arguments, safely handled when quoted

Logic

Bash if statements

An if statement lets a script choose what to do.

if [ -f "$1" ]; then
  echo "File exists"
else
  echo "File not found"
fi

Tests

Useful test operators

Test	Meaning
`-f`	Regular file exists
`-d`	Directory exists
`-e`	Path exists
`-x`	File is executable
`-z`	String is empty
`-n`	String is not empty
`-eq`	Numbers are equal
`-gt`	Number is greater than

Exit codes

Success and failure with exit codes

Linux commands return an exit code. 0 usually means success. Anything else usually means failure.

echo "$?"

In your own scripts, use exit 0 for success and exit 1 for a general failure.

Example script

Script 1: check whether a file or directory exists

#!/usr/bin/env bash

PATH_TO_CHECK="$1"

if [ -z "$PATH_TO_CHECK" ]; then
  echo "Usage: $0 path"
  exit 1
fi

if [ -f "$PATH_TO_CHECK" ]; then
  echo "Regular file exists: $PATH_TO_CHECK"
  exit 0
elif [ -d "$PATH_TO_CHECK" ]; then
  echo "Directory exists: $PATH_TO_CHECK"
  exit 0
else
  echo "Path not found: $PATH_TO_CHECK"
  exit 1
fi

$ ./check-path.sh /etc/passwd

Regular file exists: /etc/passwd

$ ./check-path.sh /nope

Path not found: /nope

Example script

Script 2: check whether a Linux service is running

#!/usr/bin/env bash

SERVICE="$1"

if [ -z "$SERVICE" ]; then
  echo "Usage: $0 service-name"
  exit 1
fi

if systemctl is-active --quiet "$SERVICE"; then
  echo "[OK] $SERVICE is running"
  exit 0
else
  echo "[WARN] $SERVICE is not running"
  exit 1
fi

$ ./check-service.sh sshd

[OK] sshd is running

$ ./check-service.sh nginx

[WARN] nginx is not running

This links neatly with systemd service troubleshooting and the systemctl cheat sheet.

Line by line

What the service checker is doing

Line	Meaning
`SERVICE="$1"`	Stores the first argument in a clearer variable name.
`[ -z "$SERVICE" ]`	Checks whether the variable is empty.
`systemctl is-active --quiet "$SERVICE"`	Checks whether the service is active without printing normal output.
`exit 0`	Reports success to the shell.
`exit 1`	Reports failure to the shell.

Try it yourself

Beginner exercises

Exercise 1: executable file checker

Write a script that accepts a path and prints whether it exists and is executable.

./check-executable.sh /usr/bin/bash

Expected style of output:

$ ./check-executable.sh /usr/bin/bash

[OK] /usr/bin/bash exists and is executable

Exercise 2: disk warning script

Write a script that accepts a number and prints whether it is greater than or equal to 90.

$ ./check-number.sh 94

[WARN] value is 94

Downloads

Download the example scripts

Quality check

Check your script with bash -n and ShellCheck

Before running a script on a real server, check the syntax:

bash -n check-service.sh

If available, ShellCheck can find common Bash mistakes such as missing quotes, unused variables and unsafe patterns:

shellcheck check-service.sh

$ shellcheck check-service.sh

No issues detected!

Mistakes

Common mistakes

Using [ -f $FILE ] instead of [ -f "$FILE" ].
Forgetting spaces inside test brackets. Use [ -f "$FILE" ], not [-f "$FILE"].
Using string operators for numbers, or number operators for strings.
Not exiting with a failure code when required input is missing.

Next: loops, functions and real admin scripts

Part 3 shows how to repeat tasks, create reusable functions and build scripts for multiple services, domains and files.

Practice script

Practice: check a website from an argument

This script combines arguments, if statements and exit codes into a small real task.

#!/usr/bin/env bash

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

url="$1"

if curl -fsS "$url" > /dev/null; then
  echo "OK: $url responded"
  exit 0
else
  echo "ERROR: $url did not respond"
  exit 2
fi

Example output:

$ ./check-url.sh
Usage: ./check-url.sh URL

$ ./check-url.sh https://example.com
OK: https://example.com responded

Learning path

Where this fits in the Bash learning path

Part 2 is where scripts start making decisions. After this, continue with for loops, while loops, functions and the full Bash Scripting Hub.

FAQ

Frequently Asked Questions

What should a Bash script do when arguments are missing?

It should print a short usage message and exit with a non-zero status.

What exit code means success?

Exit code 0 means success. Non-zero exit codes indicate failure or a specific error condition.

Why are if statements important in Bash scripts?

They allow scripts to check conditions and make decisions instead of blindly running every command.

Should beginner scripts use set -e?

It can be useful later, but beginners should first understand exit codes and command failures before relying on set -e.

$ practise_next --topic bash

Practise this next

Turn the guide into practice with a related quiz, builder, cheat sheet or learning path.

[1]Open the Bash Scripting HubFollow the main Bash learning path.[2]Try the Bash Script CheckerCheck simple Bash scripts and practise safer habits.[3]Read: Bash Functions ExplainedLearn how to organise scripts with functions.[4]Open the Bash Cheat SheetReview common Bash syntax and scripting patterns.