if-then-fi structure in Bash Scripting - Step-by-Step Execution

Concept Flow - if-then-fi structure

Start

↓

Evaluate condition

↓

Execute commands inside then

↓

End if

↓

Skip commands inside then

↓

End if

The script checks a condition. If true, it runs commands inside then. If false, it skips them and ends the if block.

Execution Sample

Bash Scripting

if [ "$num" -gt 0 ]; then
  echo "Positive number"
fi

Checks if num is greater than 0; if yes, prints 'Positive number'.

Execution Table

Step	Condition	Result	Branch Taken	Output
1	[ "$num" -gt 0 ]	True	Execute then block	Positive number
2	End of if	-	Exit if	-

💡 Condition false or commands inside then executed; if block ends with fi.

Variable Tracker

Variable	Start	After Step 1	Final
num	5	5	5

Key Moments - 2 Insights

Why does the script skip the echo command sometimes?

Because the condition in the if statement is false, so the then block is not executed (see execution_table step 1).

What does 'fi' mean in the script?

'fi' marks the end of the if block, telling the shell where the conditional commands finish.

Visual Quiz - 3 Questions

Look at the execution table, what happens when the condition is true?

A The commands inside then block are skipped

B The commands inside then block run

C The script ends immediately

D The condition is checked again

💡 Hint

See execution_table row 1, 'Branch Taken' and 'Output' columns

At which step does the script finish the if block?

A Step 2

B Step 1

C Before Step 1

D After Step 2

💡 Hint

Check execution_table row 2, 'Step' and 'Branch Taken' columns

If num was -3, what would happen at step 1?

A Condition true, then block runs

B Script crashes

C Condition false, then block skipped

D Condition true, but then block skipped

💡 Hint

Refer to variable_tracker for num value and execution_table step 1 condition result

Concept Snapshot

if-then-fi structure in bash:
if [ condition ]; then
  commands
fi
- Checks condition
- Runs commands if true
- Ends with fi
- Skips commands if false

Full Transcript

This visual execution shows how the bash if-then-fi structure works. The script starts by evaluating a condition inside the if statement. If the condition is true, it executes the commands inside the then block. If false, it skips those commands. The if block ends with fi. Variables like num keep their values unchanged during this process. Key points include understanding that fi marks the end of the if block and that commands inside then only run when the condition is true. The execution table traces these steps clearly, showing when commands run or are skipped.

Practice

1. What is the purpose of the if-then-fi structure in a bash script?

easy

A. To run commands only if a certain condition is true

B. To repeat commands multiple times

C. To define a function

D. To declare a variable

Solution

1. Step 1: Understand the role of if-then-fi

   The if-then-fi structure checks a condition and runs commands only if that condition is true.

2. Step 2: Compare with other options

   Repeating commands is done with loops, functions define reusable code, and variables store data, so these do not match if-then-fi.

3. Final Answer:

   To run commands only if a certain condition is true -> Option A

4. Quick Check:

   if-then-fi controls conditional execution = D [OK]

Hint: If you want to do something only when true, use if-then-fi [OK]

Common Mistakes:

• Confusing if-then-fi with loops
• Thinking if-then-fi defines functions
• Using if-then-fi to declare variables

2. Which of the following is the correct syntax for a simple if statement in bash?

easy

A. if $x > 5 then echo "Yes" fi

B. if ($x > 5) { echo "Yes" }

C. if [ $x -gt 5 ]; then echo "Yes"; fi

D. if $x -gt 5 echo "Yes" fi

Solution

1. Step 1: Check the correct if syntax in bash

   Bash requires conditions inside square brackets with spaces, then 'then', commands, and ending with 'fi'. if [ $x -gt 5 ]; then echo "Yes"; fi matches this.

2. Step 2: Identify errors in other options

   if ($x > 5) { echo "Yes" } uses wrong syntax with parentheses and braces (not bash). if $x > 5 then echo "Yes" fi misses semicolons and brackets. if $x -gt 5 echo "Yes" fi misses 'then' keyword.

3. Final Answer:

   if [ $x -gt 5 ]; then echo "Yes"; fi -> Option C

4. Quick Check:

   Correct bash if syntax = C [OK]

Hint: Remember: if [ condition ]; then ... fi with spaces [OK]

Common Mistakes:

• Missing spaces inside [ ]
• Omitting 'then' keyword
• Using wrong brackets or symbols

3. What will be the output of this script?

#!/bin/bash
x=10
if [ $x -lt 5 ]; then
echo "Less"
fi
echo "Done"

medium

A. No output

B. Less
Done

C. Less

D. Done

Solution

1. Step 1: Evaluate the condition

   The variable x is 10. The condition checks if 10 is less than 5, which is false.

2. Step 2: Determine which commands run

   Since the condition is false, the echo "Less" inside if does not run. The echo "Done" after if runs always.

3. Final Answer:

   Done -> Option D

4. Quick Check:

   Condition false, only "Done" prints = A [OK]

Hint: If condition false, skip if block, run commands after fi [OK]

Common Mistakes:

• Assuming if block runs even if condition false
• Confusing -lt with -gt
• Ignoring commands after fi

4. Identify the error in this bash script snippet:

if [ $num -eq 5 ]
then
  echo "Number is five"
fi

medium

A. Missing semicolon or newline after condition

B. No error, script is correct

C. Missing fi keyword

D. Wrong comparison operator

Solution

1. Step 1: Check if syntax correctness

   The condition is inside [ ] with spaces, '-eq' is correct for numeric equality, 'then' is on next line, and 'fi' closes the block.

2. Step 2: Confirm no syntax errors

   Newline after condition is valid instead of semicolon. All keywords are present and correctly placed.

3. Final Answer:

   No error, script is correct -> Option B

4. Quick Check:

   Proper if-then-fi syntax = B [OK]

Hint: Newline after condition is allowed instead of semicolon [OK]

Common Mistakes:

• Thinking newline after condition is error
• Confusing -eq with other operators
• Forgetting fi keyword

5. You want to write a script that prints "Even" if a number stored in n is even, and "Odd" otherwise. Which of these is the correct if-then-fi structure to do this?

hard

A. if [ $((n % 2)) -eq 0 ]; then echo "Even"; else echo "Odd"; fi

B. if [ $n % 2 == 0 ]; then echo "Even"; else echo "Odd"; fi

C. if (( n % 2 == 0 )) then echo "Even"; else echo "Odd"; fi

D. if [ $n / 2 -eq 0 ]; then echo "Even"; else echo "Odd"; fi

Solution

1. Step 1: Understand how to check evenness in bash

   We use arithmetic expansion $(( )) to calculate remainder of n divided by 2. If remainder is 0, number is even.

2. Step 2: Analyze each option's correctness

   if [ $((n % 2)) -eq 0 ]; then echo "Even"; else echo "Odd"; fi correctly uses [ $((n % 2)) -eq 0 ] with then, else, fi. if [ $n % 2 == 0 ]; then echo "Even"; else echo "Odd"; fi uses invalid syntax inside [ ]. if (( n % 2 == 0 )) then echo "Even"; else echo "Odd"; fi misses semicolon before 'then' causing syntax error. if [ $n / 2 -eq 0 ]; then echo "Even"; else echo "Odd"; fi incorrectly uses division instead of modulo.

3. Final Answer:

   if [ $((n % 2)) -eq 0 ]; then echo "Even"; else echo "Odd"; fi -> Option A

4. Quick Check:

   Modulo with $(( )) inside [ ] and proper if-then-fi = A [OK]

Hint: Use $(( )) for math inside [ ] in if-then-fi [OK]

Common Mistakes:

• Using % inside [ ] without $(( ))
• Using == instead of -eq for numbers
• Confusing division with modulo