Exercise 08-02 — Shell Scripting: Conditionals¶

Navigation: ← Exercise 08-01 | Exercises Index | Next → Exercise 08-03 Note: 08-02 — Shell Scripting: Conditionals

Before You Start¶

Read Note 08-02 first.

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

Exercise 8.4 — Check If a Path Exists¶

Step 1: Create the script:

nano ~/linux-practice/scripts/check-path.sh

Content:

#!/bin/bash

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

PATH_TO_CHECK=$1

if [ -f "$PATH_TO_CHECK" ]; then
    echo "'$PATH_TO_CHECK' is a regular file."
    echo "Size: $(du -sh "$PATH_TO_CHECK" | cut -f1)"

elif [ -d "$PATH_TO_CHECK" ]; then
    echo "'$PATH_TO_CHECK' is a directory."
    FILE_COUNT=$(ls "$PATH_TO_CHECK" | wc -l)
    echo "It contains $FILE_COUNT items."

elif [ -e "$PATH_TO_CHECK" ]; then
    echo "'$PATH_TO_CHECK' exists but is neither a file nor a directory."

else
    echo "'$PATH_TO_CHECK' does not exist."
    exit 1
fi

chmod +x ~/linux-practice/scripts/check-path.sh

Step 2: Test it with several paths:

~/linux-practice/scripts/check-path.sh /etc/passwd
~/linux-practice/scripts/check-path.sh /home
~/linux-practice/scripts/check-path.sh /tmp
~/linux-practice/scripts/check-path.sh /nonexistent/path

Expected outputs (one per run):

'/etc/passwd' is a regular file.
Size: 4.0K

'/home' is a directory.
It contains 1 items.

'/tmp' is a directory.
It contains 14 items.

'/nonexistent/path' does not exist.

Understand the conditions: - -f = is a regular file - -d = is a directory - -e = exists (any type — file, directory, symlink, device, etc.) - The "quotes" around $PATH_TO_CHECK — if the path contains spaces, an unquoted variable would split into separate words and break the test

Reflect: Why does the script check -f before -e? What would happen if -e were checked first?

Exercise 8.5 — Number Comparisons¶

Step 1: Create a grade calculator:

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

Content:

#!/bin/bash

if [ $# -ne 1 ]; then
    echo "Usage: $0 <score (0-100)>"
    exit 1
fi

SCORE=$1

# Validate: must be a number between 0 and 100
if [ $SCORE -lt 0 ] || [ $SCORE -gt 100 ]; then
    echo "Error: score must be between 0 and 100"
    exit 1
fi

if [ $SCORE -ge 90 ]; then
    echo "Grade: A"
elif [ $SCORE -ge 80 ]; then
    echo "Grade: B"
elif [ $SCORE -ge 70 ]; then
    echo "Grade: C"
elif [ $SCORE -ge 60 ]; then
    echo "Grade: D"
else
    echo "Grade: F"
fi

echo "Score: $SCORE/100"

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

Step 2: Test several scores:

~/linux-practice/scripts/grade.sh 95
~/linux-practice/scripts/grade.sh 82
~/linux-practice/scripts/grade.sh 55
~/linux-practice/scripts/grade.sh 101

Expected output for 82:

Grade: B
Score: 82/100

Expected output for 101:

Error: score must be between 0 and 100

Understand the validation: - [ $SCORE -lt 0 ] || [ $SCORE -gt 100 ] — the || means OR: if either side is true, the whole condition is true - The elif chain stops at the first true condition — once $SCORE -ge 80 matches, it won't check the remaining conditions

Exercise 8.6 — String Comparisons¶

Step 1: Create a script that reacts to a user's answer:

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

Content:

#!/bin/bash

read -p "Do you want to continue? (yes/no): " ANSWER

if [ -z "$ANSWER" ]; then
    echo "You didn't type anything. Defaulting to 'no'."
    exit 0
fi

# Convert to lowercase so "YES", "Yes", "yes" all work
ANSWER=$(echo "$ANSWER" | tr '[:upper:]' '[:lower:]')

if [ "$ANSWER" = "yes" ] || [ "$ANSWER" = "y" ]; then
    echo "Great! Continuing..."
elif [ "$ANSWER" = "no" ] || [ "$ANSWER" = "n" ]; then
    echo "OK, stopping."
    exit 0
else
    echo "I don't understand '$ANSWER'. Please type yes or no."
    exit 1
fi

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

Step 2: Run it and try different inputs:

~/linux-practice/scripts/ask.sh

Try: yes, YES, y, no, n, maybe, (press Enter without typing)

Understand the string handling: - -z "$ANSWER" — true if the string is empty (user pressed Enter without typing) - echo "$ANSWER" | tr '[:upper:]' '[:lower:]' — converts the string to all lowercase so comparisons don't need to handle every capitalisation - Always quote string variables in comparisons: "$ANSWER" not $ANSWER

Challenge — System Requirement Checker¶

Write ~/linux-practice/scripts/check-system.sh that checks all of the following and prints PASS or FAIL for each:

Is there at least 100MB of free disk space on /?
df / | awk 'NR==2{print $4}' gives free space in 1K blocks
Is there at least 256MB of free RAM?
free | awk 'NR==2{print $7}' gives available RAM in KB
Does the file /etc/ssh/sshd_config exist?
Is bash version 4 or higher? (bash --version | awk 'NR==1{print $4}' gives the version string)

Expected output format:

[PASS] Disk space: 8.2G free
[FAIL] RAM: only 120MB available (need 256MB)
[PASS] sshd_config exists
[PASS] Bash version: 5.1.16

Navigation: ← Exercise 08-01 | Exercises Index | Next → Exercise 08-03 Note: 08-02 — Shell Scripting: Conditionals