Skip to content

Exercise 08-04 — Shell Scripting: Functions & Best Practices

Navigation: ← Exercise 08-03 | Exercises Index | Next → Exercise 09 Note: 08-04 — Shell Scripting: Functions

Before You Start

Read Note 08-04 first.

All scripts go in ~/linux-practice/scripts/.

Exercise 8.13 — Your First Function

Step 1: Create a script with a simple function:

nano ~/linux-practice/scripts/functions-demo.sh

Content:

#!/bin/bash

# Define the function BEFORE calling it
print_section() {
    local TITLE=$1
    local WIDTH=40
    echo ""
    printf '=%.0s' $(seq 1 $WIDTH)
    echo ""
    echo "  $TITLE"
    printf '=%.0s' $(seq 1 $WIDTH)
    echo ""
}

# Now call the function
print_section "System Information"
echo "  Hostname: $(hostname)"
echo "  User:     $USER"

print_section "Disk Usage"
df -h / | tail -1

print_section "Memory"
free -h | grep Mem

chmod +x ~/linux-practice/scripts/functions-demo.sh
~/linux-practice/scripts/functions-demo.sh

Expected output: 

========================================
  System Information
========================================
  Hostname: myserver
  User:     jahid

========================================
  Disk Usage
========================================
/dev/sda1        50G   18G   30G  38% /

========================================
  Memory
========================================
Mem:          7.7G    2.1G    3.2G    120M      2.4G       5.2G

Understand: - Functions must be defined before they are called — bash reads top to bottom - local TITLE=$1 — the argument passed to the function lands in $1 inside the function - local keeps TITLE and WIDTH private — they won't clash with any variables outside the function

Exercise 8.14 — Functions That Return Values

Step 1: Create a script where functions compute and return results:

nano ~/linux-practice/scripts/math-functions.sh

Content:

#!/bin/bash

# Returns the larger of two numbers
max() {
    if [ $1 -ge $2 ]; then
        echo $1
    else
        echo $2
    fi
}

# Returns the absolute value of a number
abs() {
    if [ $1 -lt 0 ]; then
        echo $(( -$1 ))
    else
        echo $1
    fi
}

# Returns 0 (true) if number is even, 1 (false) if odd
is_even() {
    [ $(( $1 % 2 )) -eq 0 ]
}

# ── Main ──────────────────────────────────────────────────────────────────────

BIGGEST=$(max 42 17)
echo "The larger of 42 and 17 is: $BIGGEST"

DIFF=$(( 42 - 17 ))
echo "Absolute value of -$DIFF is: $(abs -$DIFF)"

for n in 1 2 3 4 5 6; do
    if is_even $n; then
        echo "$n is even"
    else
        echo "$n is odd"
    fi
done

chmod +x ~/linux-practice/scripts/math-functions.sh
~/linux-practice/scripts/math-functions.sh

Expected output: 

The larger of 42 and 17 is: 42
Absolute value of -25 is: 25
1 is odd
2 is even
3 is odd
4 is even
5 is odd
6 is even

Understand the two ways to return from a function: - echo $1 then RESULT=$(max 42 17) — returns a string/number by printing it - [ $(( $1 % 2 )) -eq 0 ] — returns an exit code (0=true, 1=false); used directly in if

Exercise 8.15 — local vs Global Variables

Step 1: See the difference in action:

nano ~/linux-practice/scripts/scope.sh

Content:

#!/bin/bash

COLOUR="blue"    # global variable

change_with_local() {
    local COLOUR="red"    # local — only changes inside this function
    echo "Inside function (local):  COLOUR = $COLOUR"
}

change_without_local() {
    COLOUR="green"        # no local — changes the global variable!
    echo "Inside function (global): COLOUR = $COLOUR"
}

echo "Before any function:       COLOUR = $COLOUR"
change_with_local
echo "After change_with_local:   COLOUR = $COLOUR"
change_without_local
echo "After change_without_local: COLOUR = $COLOUR"

chmod +x ~/linux-practice/scripts/scope.sh
~/linux-practice/scripts/scope.sh

Expected output: 

Before any function:       COLOUR = blue
Inside function (local):  COLOUR = red
After change_with_local:   COLOUR = blue
Inside function (global): COLOUR = green
After change_without_local: COLOUR = green

Key takeaway: Always use local inside functions. Without it, a function can silently change a variable that the rest of your script depends on — a hard-to-find bug.

Exercise 8.16 — set -euo pipefail in Action

Step 1: First, see what happens WITHOUT safe mode:

nano ~/linux-practice/scripts/unsafe.sh

Content:

#!/bin/bash
# NO set -euo pipefail

echo "Step 1: Starting..."
ls /this/does/not/exist     # this will fail
echo "Step 2: Still running after the error!"
echo "Step 3: Done."

chmod +x ~/linux-practice/scripts/unsafe.sh
~/linux-practice/scripts/unsafe.sh

Expected output: 

Step 1: Starting...
ls: cannot access '/this/does/not/exist': No such file or directory
Step 2: Still running after the error!
Step 3: Done.

The script kept going even though a command failed. In a real backup or deploy script, this could cause serious damage.

Step 2: Now add safe mode:

nano ~/linux-practice/scripts/safe.sh

Content:

#!/bin/bash
set -euo pipefail

echo "Step 1: Starting..."
ls /this/does/not/exist     # this will fail
echo "Step 2: This will NOT run."
echo "Step 3: This will NOT run either."

chmod +x ~/linux-practice/scripts/safe.sh
~/linux-practice/scripts/safe.sh

Expected output: 

Step 1: Starting...
ls: cannot access '/this/does/not/exist': No such file or directory

The script stopped immediately at the first failure. Step 2 and Step 3 never ran.

Step 3: Check the exit code:

~/linux-practice/scripts/safe.sh
echo "Exit code: $?"

A non-zero exit code tells the caller (or a CI system) that something went wrong.

Exercise 8.17 — A Complete Script with Functions

Step 1: Create a full system-info script:

nano ~/linux-practice/scripts/system-info.sh

Content:

#!/bin/bash
set -euo pipefail

# ── Helper functions ──────────────────────────────────────────────────────────

print_header() {
    local TITLE=$1
    echo ""
    echo "=== $TITLE ==="
}

check_command() {
    local CMD=$1
    if command -v "$CMD" &>/dev/null; then
        echo "  [FOUND]   $CMD  →  $(which $CMD)"
    else
        echo "  [MISSING] $CMD"
    fi
}

info_line() {
    printf "  %-20s %s\n" "$1:" "$2"
}

# ── Main ──────────────────────────────────────────────────────────────────────

print_header "System"
info_line "Hostname"     "$(hostname)"
info_line "OS"           "$(grep PRETTY_NAME /etc/os-release | cut -d= -f2 | tr -d '"')"
info_line "Kernel"       "$(uname -r)"
info_line "Uptime"       "$(uptime -p)"
info_line "Current user" "$(whoami)"

print_header "Hardware"
info_line "CPU cores"    "$(nproc)"
info_line "RAM total"    "$(free -h | awk 'NR==2{print $2}')"
info_line "Disk (root)"  "$(df -h / | awk 'NR==2{print $2 " total, " $4 " free"}')"

print_header "Installed Tools"
check_command bash
check_command python3
check_command git
check_command curl
check_command docker
check_command nginx

echo ""

chmod +x ~/linux-practice/scripts/system-info.sh
~/linux-practice/scripts/system-info.sh

Expected output (example): 

=== System ===
  Hostname:            myserver
  OS:                  Ubuntu 22.04.3 LTS
  Kernel:              6.5.0-41-generic
  Uptime:              up 3 days, 4 hours
  Current user:        jahid

=== Hardware ===
  CPU cores:           4
  RAM total:           7.7G
  Disk (root):         50G total, 32G free

=== Installed Tools ===
  [FOUND]   bash  →  /usr/bin/bash
  [FOUND]   python3  →  /usr/bin/python3
  [MISSING] docker

Understand the functions: - command -v "$CMD" — the correct way to check if a command exists - &>/dev/null — discards both stdout and stderr (we only care about the exit code) - printf "%-20s %s\n" — %-20s = left-aligned, 20 characters wide; keeps all values in a tidy column

Challenge — A Useful Script From Scratch

Write ~/linux-practice/scripts/user-report.sh that:

  1. Defines a function check_user() that accepts a username and prints:
  2. Username
  3. Home directory path
  4. Whether the home directory exists ([EXISTS] or [MISSING])
  5. Reads /etc/passwd, finds real users (UID ≥ 1000), and calls check_user() for each
  6. At the end, prints the total count

Expected output: 

=== Real User Accounts ===
  jahid        /home/jahid     [EXISTS]
  alice        /home/alice     [MISSING]
===========================
Total: 2 real user(s)

Hints: - awk -F: '$3 >= 1000 {print $1, $6}' /etc/passwd prints username homedir for real users - Use while read USERNAME HOMEDIR; do ... done to process each line - Call check_user "$USERNAME" "$HOMEDIR" inside the loop

Navigation: ← Exercise 08-03 | Exercises Index | Next → Exercise 09 Note: 08-04 — Shell Scripting: Functions