Logical operators (-a, -o, !) in Bash Scripting - Mini Project: Build & Apply

Using Logical Operators in Bash Scripts

📖 Scenario: You are writing a simple bash script to check conditions on numbers, similar to checking if a number is in a certain range or if a file exists. This helps automate decisions in scripts.

🎯 Goal: Build a bash script that uses logical operators -a (and), -o (or), and ! (not) to test multiple conditions together.

📋 What You'll Learn

Create a variable num with the value 15

Create a variable file with the value example.txt

Use if statements with logical operators -a, -o, and !

Print messages based on combined conditions

💡 Why This Matters

🌍 Real World

Logical operators help automate decisions in scripts, like checking if files exist or if values meet certain criteria before running commands.

💼 Career

Understanding bash logical operators is essential for system administrators, DevOps engineers, and anyone automating tasks on Linux or Unix systems.

1

Set up variables

Create a variable called num and set it to 15. Create another variable called file and set it to example.txt.

Bash Scripting

# Create variables num and file
# Your code here

Hint

Use = to assign values without spaces, and quotes for the filename.

2

Add a threshold variable

Create a variable called threshold and set it to 10.

Bash Scripting

num=15
file="example.txt"
# Create variable threshold
# Your code here

Hint

Assign 10 to threshold without spaces.

3

Use logical operators in if statements

Write an if statement that checks if num is greater than threshold -a the file named by file does not exist (use ! and -e). Inside the if, print "Number is big and file missing".

Bash Scripting

num=15
file="example.txt"
threshold=10
# Write if statement with -a and ! -e
# Your code here

Hint

Use [ condition ] with spaces, and combine conditions with -a. Use ! -e "$file" to check file absence.

4

Add an else if with -o operator and print output

Add an elif that checks if num is less than or equal to threshold -o the file named by file exists (use -e). Inside this elif, print "Number is small or file exists". Add an else that prints "No conditions met". Finally, run the script so it prints the output.

Bash Scripting

num=15
file="example.txt"
threshold=10
if [ "$num" -gt "$threshold" -a ! -e "$file" ]; then
    echo "Number is big and file missing"
fi
# Add elif with -o and else with echo
# Your code here

Hint

Use elif [ condition ] with -o for OR. Use else for fallback. Run script to see output.

Practice

1. Which logical operator in bash scripting means AND when used inside [ ]?

easy

A. &&

B. -o

C. !

D. -a

Solution

1. Step 1: Understand bash logical operators inside [ ]

   Inside single brackets, -a means AND, -o means OR, and ! means NOT.

2. Step 2: Identify the AND operator

   The operator -a is used to combine two conditions with AND logic.

3. Final Answer:

   -a -> Option D

4. Quick Check:

   AND operator = -a [OK]

Hint: Remember: -a is AND, -o is OR, ! is NOT inside [ ] [OK]

Common Mistakes:

• Confusing -a with -o
• Using && inside [ ] which is invalid
• Thinking ! means AND

2. Which of the following is the correct syntax to check if file file.txt exists AND is readable in bash?

easy

A. [ -e file.txt -o -r file.txt ]

B. [ -e file.txt && -r file.txt ]

C. [ -e file.txt -a -r file.txt ]

D. [ ! -e file.txt -a -r file.txt ]

Solution

1. Step 1: Understand the condition to check

   We want to check if file.txt exists AND is readable, so both conditions must be true.

2. Step 2: Choose correct logical operator and syntax

   Inside [ ], -a means AND, so [ -e file.txt -a -r file.txt ] is correct. Using -o means OR, which is wrong here. Using && inside [ ] is invalid syntax.

3. Final Answer:

   [ -e file.txt -a -r file.txt ] -> Option C

4. Quick Check:

   AND check uses -a inside [ ] [OK]

Hint: Use -a for AND inside [ ], not && or -o for AND [OK]

Common Mistakes:

• Using -o instead of -a for AND
• Using && inside [ ] which causes syntax error
• Negating conditions incorrectly with !

3. What will be the output of this script snippet?

if [ -f myfile.txt -o ! -r myfile.txt ]; then echo "Check passed"; else echo "Check failed"; fi

Assuming myfile.txt exists but is NOT readable.

medium

A. Check passed

B. Check failed

C. Syntax error

D. No output

Solution

1. Step 1: Analyze conditions with given file state

   -f myfile.txt is true because file exists. ! -r myfile.txt is true because file is not readable.

2. Step 2: Evaluate OR (-o) logic

   Since OR means either condition true passes, and both are true here, the whole condition is true.

3. Final Answer:

   Check passed -> Option A

4. Quick Check:

   True OR True = True [OK]

Hint: OR (-o) passes if any condition is true [OK]

Common Mistakes:

• Thinking ! -r means readable
• Confusing -o with -a
• Expecting syntax error due to !

4. Identify the error in this bash condition:

if [ !-f myfile.txt -a -r myfile.txt ]; then echo "Valid"; fi

medium

A. Missing spaces around ! operator

B. Incorrect use of -a inside [ ]

C. No error, condition is correct

D. Missing closing bracket ]

Solution

1. Step 1: Check spacing around ! operator

   In bash, ! must have a space after it inside [ ]. The code [ ! -f myfile.txt -a -r myfile.txt ] is correct only if spaces are present. If missing, it causes error.

2. Step 2: Confirm other syntax parts

   The use of -a inside [ ] is valid. The closing bracket is present. So no error there.

3. Final Answer:

   Missing spaces around ! operator -> Option A

4. Quick Check:

   ! needs spaces inside [ ] [OK]

Hint: Always put spaces around ! inside [ ] [OK]

Common Mistakes:

• Using && or || inside [ ]
• Forgetting spaces around operators
• Assuming -a is invalid inside [ ]

5. You want to write a bash condition that checks if a variable var is NOT empty AND file1.txt exists OR file2.txt is readable. Which condition is correct?

hard

A. [ -n "$var" -a ( -e file1.txt -o -r file2.txt ) ]

B. [ -n "$var" -a -e file1.txt -o -r file2.txt ]

C. [ -z "$var" -a -e file1.txt -o -r file2.txt ]

D. [ -n "$var" -o -e file1.txt -a -r file2.txt ]

Solution

1. Step 1: Understand the logic to implement

   We want: var NOT empty AND file1 exists OR file2 readable, i.e., (NOT empty AND file1 exists) OR file2 readable.

2. Step 2: Check each option's logic and syntax

   A: Parentheses for grouping invalid inside [ ], syntax error.
   B: [ -n "$var" -o -e file1.txt -a -r file2.txt ] wrong logic: mixes OR then AND.
   C: [ -z "$var" -a -e file1.txt -o -r file2.txt ] uses -z (empty), wrong for NOT empty.
   D: [ -n "$var" -a -e file1.txt -o -r file2.txt ] correct: left-to-right ((non-empty -a exists) -o readable).

3. Final Answer:

   [ -n "$var" -a -e file1.txt -o -r file2.txt ] -> Option B

4. Quick Check:

   -a/-o left-to-right no grouping needed [OK]

Hint: -a/-o associate left-to-right inside [ ] [OK]

Common Mistakes:

• Trying to group with ( ) invalid in [ ]
• Mixing -o and -a without understanding associativity
• Using -z for non-empty check