Why conditionals branch script logic in Bash Scripting - Visual Breakdown

Concept Flow - Why conditionals branch script logic

Start script

↓

Evaluate condition

↓

Run True

↓

Continue script

↓

End

The script starts by checking a condition. If true, it runs one set of commands; if false, it runs another. Then it continues.

Execution Sample

Bash Scripting

if [ "$age" -ge 18 ]; then
  echo "Adult"
else
  echo "Minor"
fi

This script checks if age is 18 or more, prints 'Adult' if yes, otherwise 'Minor'.

Execution Table

Step	Condition	Result	Branch Taken	Output
1	[ "$age" -ge 18 ]	True	Then branch	Adult
2	End of if	-	-	-

💡 Condition became true, so 'Then' branch ran and script continued after if.

Variable Tracker

Variable	Start	After Step 1	Final
age	20	20	20

Key Moments - 2 Insights

Why does the script run only one branch, not both?

Because the condition decides which branch runs. See execution_table step 1: only the 'Then' branch runs when condition is true.

What happens if the condition is false?

The script runs the 'else' branch instead. This is shown by the branching in concept_flow where false leads to a different path.

Visual Quiz - 3 Questions

Look at the execution_table, what output is printed when the condition is true?

A No output

B Adult

C Minor

D Error message

💡 Hint

Check the Output column in execution_table row 1 where condition is true.

At which step does the script decide which branch to run?

A Step 2

B Before Step 1

C Step 1

D After Step 2

💡 Hint

Look at the Condition and Branch Taken columns in execution_table row 1.

If age was 16, which branch would run?

A Else branch

B Then branch

C Both branches

D No branch

💡 Hint

Refer to concept_flow where false condition leads to else branch.

Concept Snapshot

if [ condition ]; then
  # commands if true
else
  # commands if false
fi

- Condition decides which commands run
- Only one branch runs per check
- Helps script choose actions based on data

Full Transcript

This script uses a conditional to decide what to do next. It checks if the variable age is 18 or more. If yes, it prints 'Adult'. If not, it prints 'Minor'. The condition acts like a fork in the road, sending the script down one path or the other. This way, the script can make choices and behave differently depending on the data it sees.

Practice

1. What is the main purpose of using if statements in a bash script?

easy

A. To make decisions and run different commands based on conditions

B. To repeat commands multiple times

C. To define functions

D. To declare variables

Solution

1. Step 1: Understand the role of if in scripts

   if statements check conditions and decide which commands to run next.

2. Step 2: Compare with other options

   Repeating commands is done by loops, functions define reusable code, and variables store data, so these are not the main purpose of if.

3. Final Answer:

   To make decisions and run different commands based on conditions -> Option A

4. Quick Check:

   Conditionals branch logic = A [OK]

Hint: If it chooses what to do next, it's an if statement [OK]

Common Mistakes:

• Confusing if with loops
• Thinking if defines functions
• Mixing variable declaration with conditionals

2. Which of the following is the correct syntax to start an if statement in bash?

easy

A. if condition {}

B. if (condition) {

C. if condition then

D. if [ condition ]; then

Solution

1. Step 1: Recall bash if syntax

   Bash requires square brackets around the condition and a semicolon before then.

2. Step 2: Check other options for syntax errors

   Options A, B, and C use incorrect syntax styles not valid in bash.

3. Final Answer:

   if [ condition ]; then -> Option D

4. Quick Check:

   Bash if syntax = D [OK]

Hint: Remember: if [ condition ]; then is bash style [OK]

Common Mistakes:

• Omitting semicolon before then
• Using parentheses instead of brackets
• Missing brackets around condition

3. What will be the output of this script?

#!/bin/bash
num=5
if [ $num -gt 10 ]; then
  echo "Greater than 10"
elif [ $num -eq 5 ]; then
  echo "Equals 5"
else
  echo "Less than 5"
fi

medium

A. Equals 5

B. Greater than 10

C. Less than 5

D. No output

Solution

1. Step 1: Check the value of num

   The variable num is set to 5.

2. Step 2: Evaluate the conditions in order

   First condition checks if 5 > 10 (false), second checks if 5 == 5 (true), so it prints "Equals 5".

3. Final Answer:

   Equals 5 -> Option A

4. Quick Check:

   num=5 matches elif condition = A [OK]

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

Common Mistakes:

• Assuming first condition is true
• Ignoring elif branch
• Confusing numeric comparison operators

4. Identify the error in this bash conditional snippet:

if [ $value = 10 ]; then
  echo "Value is 10"
fi

medium

A. Missing semicolon before then

B. Using single equals for numeric comparison

C. Brackets should be double [[ ]]

D. No error, script runs fine

Solution

1. Step 1: Understand comparison operators in bash

   For numeric comparison inside [ ], use -eq instead of =.

2. Step 2: Check the snippet

   The snippet uses = which is for string comparison, so numeric comparison is incorrect here.

3. Final Answer:

   Using single equals for numeric comparison -> Option B

4. Quick Check:

   Numeric compare needs -eq not = [OK]

Hint: Use -eq for numbers, = for strings in [ ] [OK]

Common Mistakes:

• Confusing string and numeric operators
• Thinking = works for numbers
• Ignoring bash operator rules

5. You want a script to print "Positive", "Zero", or "Negative" based on a variable num. Which conditional structure correctly branches all cases?

hard

A. if [ $num -ge 0 ]; then echo "Positive"; else echo "Negative"; fi

B. if [ $num = 0 ]; then echo "Zero"; else echo "Non-zero"; fi

C. if [ $num -gt 0 ]; then echo "Positive"; elif [ $num -eq 0 ]; then echo "Zero"; else echo "Negative"; fi

D. if [ $num -lt 0 ]; then echo "Negative"; elif [ $num -gt 0 ]; then echo "Positive"; fi

Solution

1. Step 1: Identify all required cases

   We need to handle positive (>0), zero (=0), and negative (<0) values.

2. Step 2: Check each option for coverage

   if [ $num -gt 0 ]; then echo "Positive"; elif [ $num -eq 0 ]; then echo "Zero"; else echo "Negative"; fi covers all three cases with if, elif, else branches. Others miss zero or one case.

3. Final Answer:

   if [ $num -gt 0 ]; then echo "Positive"; elif [ $num -eq 0 ]; then echo "Zero"; else echo "Negative"; fi -> Option C

4. Quick Check:

   All cases covered with if-elif-else = B [OK]

Hint: Use if-elif-else to cover all conditions [OK]

Common Mistakes:

• Missing zero case
• Using wrong comparison operators
• Not using else for last case