if-elif-else in Bash Scripting - Time & Space Complexity

Time Complexity: if-elif-else

O(n)

Understanding Time Complexity

Let's see how the time it takes to run an if-elif-else decision changes as input changes.

We want to know how many steps the script takes when checking conditions.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

read -p "Enter a number: " num
if [ "$num" -eq 1 ]; then
  echo "Number is one"
elif [ "$num" -eq 2 ]; then
  echo "Number is two"
else
  echo "Number is something else"
fi
    

This script checks a number and prints a message based on its value.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Checking conditions one by one.
• How many times: At most, it checks each condition once in order.

How Execution Grows With Input

Each input number causes the script to check conditions until one matches or all are checked.

Input Size (n)	Approx. Operations
1	1 check (first condition matches)
2	2 checks (first fails, second matches)
3	2 checks (both conditions fail, else runs)

Pattern observation: The number of checks grows linearly with how far down the conditions we go.

Final Time Complexity

Time Complexity: O(n)

This means the time grows linearly with the number of conditions checked.

Common Mistake

[X] Wrong: "If statements always run in constant time no matter how many conditions there are."

[OK] Correct: The script checks conditions one by one, so more conditions mean more checks and more time.

Interview Connect

Understanding how condition checks add up helps you explain script efficiency clearly and confidently.

Self-Check

"What if we replaced the if-elif-else with a case statement? How would the time complexity change?"

Practice

1. What does the if-elif-else structure do in a Bash script?

easy

A. It repeats a set of commands multiple times.

B. It comments out lines of code.

C. It defines a function to reuse code.

D. It lets the script choose between multiple actions based on conditions.

Solution

1. Step 1: Understand the purpose of if-elif-else

   The if-elif-else structure checks conditions one by one and runs the matching block.

2. Step 2: Compare with other options

   Loops repeat commands, functions reuse code, comments disable code lines. Only if-elif-else chooses actions.

3. Final Answer:

   It lets the script choose between multiple actions based on conditions. -> Option D

4. Quick Check:

   if-elif-else = choose actions [OK]

Hint: If you want choices, use if-elif-else [OK]

Common Mistakes:

• Confusing if-elif-else with loops
• Thinking it defines functions
• Mixing it up with comments

2. Which of the following is the correct syntax to start an if-elif-else block in Bash?

easy

A. if [ condition ]; then

B. if (condition) {

C. if condition then

D. if condition:

Solution

1. Step 1: Recall Bash if syntax

   Bash requires square brackets around conditions and 'then' after the condition, with semicolons or newlines.

2. Step 2: Check each option

   if [ condition ]; then uses correct syntax: if [ condition ]; then. Others use wrong brackets or missing keywords.

3. Final Answer:

   if [ condition ]; then -> Option A

4. Quick Check:

   Correct if syntax = if [ condition ]; then [OK]

Hint: Use brackets and 'then' with semicolon or newline [OK]

Common Mistakes:

• Omitting brackets around condition
• Using parentheses instead of brackets
• Missing 'then' keyword

3. What will be the output of this script?

#!/bin/bash
num=7
if [ $num -lt 5 ]; then
  echo "Less than 5"
elif [ $num -eq 7 ]; then
  echo "Equal to 7"
else
  echo "Other"
fi

medium

A. Less than 5

B. Equal to 7

C. Other

D. Syntax Error

Solution

1. Step 1: Check the value of num and conditions

   num=7. First condition checks if num < 5 (false). Second checks if num == 7 (true).

2. Step 2: Determine which block runs

   Since second condition is true, it prints "Equal to 7" and skips else.

3. Final Answer:

   Equal to 7 -> Option B

4. Quick Check:

   num=7 matches elif condition [OK]

Hint: Check conditions top to bottom, first true runs [OK]

Common Mistakes:

• Assuming first condition runs
• Confusing -lt and -eq operators
• Missing fi causes syntax error

4. Identify the error in this Bash script snippet:

if [ $value -gt 10 ]
  echo "Greater than 10"
elif [ $value -eq 10 ]; then
  echo "Equal to 10"
else
  echo "Less than 10"
fi

medium

A. Missing fi at the end

B. Incorrect use of elif keyword

C. Missing 'then' after first if condition

D. Wrong comparison operator

Solution

1. Step 1: Check syntax after if condition

   In Bash, 'then' must follow the if condition, either on same line after semicolon or next line.

2. Step 2: Verify other parts

   elif has 'then', fi is present, operators are correct. Only missing 'then' after if.

3. Final Answer:

   Missing 'then' after first if condition -> Option C

4. Quick Check:

   if needs 'then' keyword [OK]

Hint: Always put 'then' after if condition [OK]

Common Mistakes:

• Forgetting 'then' after if
• Misplacing semicolons
• Confusing elif syntax

5. You want a script to print "Positive", "Zero", or "Negative" based on a variable num. Which script correctly uses if-elif-else to do this?

hard

A. #!/bin/bash
if [ $num -gt 0 ]; then
echo "Positive"
elif [ $num -eq 0 ]; then
echo "Zero"
else
echo "Negative"
fi

B. #!/bin/bash
if [ $num > 0 ]; then
echo "Positive"
elif [ $num == 0 ]; then
echo "Zero"
else
echo "Negative"
fi

C. #!/bin/bash
if $num > 0 then
echo "Positive"
elif $num == 0 then
echo "Zero"
else
echo "Negative"
fi

D. #!/bin/bash
if [ $num -ge 0 ]; then
echo "Positive"
elif [ $num -eq 0 ]; then
echo "Zero"
else
echo "Negative"
fi

Solution

1. Step 1: Check comparison operators

   Bash uses -gt, -eq, -lt for numeric comparisons. #!/bin/bash if [ $num -gt 0 ]; then echo "Positive" elif [ $num -eq 0 ]; then echo "Zero" else echo "Negative" fi uses correct operators.

2. Step 2: Verify logic and syntax

   #!/bin/bash if [ $num -gt 0 ]; then echo "Positive" elif [ $num -eq 0 ]; then echo "Zero" else echo "Negative" fi correctly checks >0, ==0, else negative. Others use wrong operators or syntax.

3. Final Answer:

   A -> Option A

4. Quick Check:

   Use -gt, -eq for numbers in if-elif-else [OK]

Hint: Use -gt, -eq for numbers; avoid > or == in [ ] [OK]

Common Mistakes:

• Using > or == inside [ ]
• Missing then keyword
• Wrong operator for numeric comparison