Debugging with PS4 in Bash Scripting - Mini Project: Build & Apply

Debugging Bash Scripts Using PS4

📖 Scenario: You are writing a bash script to calculate the total price of items bought. Sometimes the script does not work as expected. You want to learn how to debug it easily.

🎯 Goal: Learn how to use the PS4 variable and set -x to debug bash scripts by showing detailed command execution steps.

📋 What You'll Learn

Create a bash script with variables for item prices

Add a PS4 variable to customize debug output

Enable debugging with set -x

Calculate the total price using arithmetic

Print the total price

💡 Why This Matters

🌍 Real World

Debugging bash scripts is essential when automating tasks on servers or your computer. Using PS4 and set -x helps find errors quickly.

💼 Career

Many IT and DevOps jobs require writing and debugging shell scripts. Knowing how to use PS4 for debugging makes troubleshooting faster and easier.

1

Create variables for item prices

Create three variables called price1, price2, and price3 with values 10, 20, and 30 respectively.

Bash Scripting

# Create variables price1, price2, and price3 with values 10, 20, and 30
# Your code here

Hint

Use simple assignment like price1=10 without spaces.

2

Add PS4 variable for debug prefix

Add a variable called PS4 and set it to the string '+ Line: $LINENO: ' to show the line number in debug output.

Bash Scripting

price1=10
price2=20
price3=30
# Set PS4 to '+ Line: $LINENO: '
# Your code here

Hint

Use single quotes around the string and include $LINENO to show the current line number.

3

Enable debugging and calculate total

Add set -x to enable debugging. Then create a variable total that sums price1, price2, and price3 using arithmetic expansion.

Bash Scripting

price1=10
price2=20
price3=30
PS4='+ Line: $LINENO: '
# Enable debugging
# Calculate total as sum of price1, price2, and price3
# Your code here

Hint

Use set -x on its own line. Use $(( )) for arithmetic.

4

Print the total price

Print the value of total using echo.

Bash Scripting

price1=10
price2=20
price3=30
PS4='+ Line: $LINENO: '
set -x
total=$((price1 + price2 + price3))
# Print total
# Your code here

Hint

Use echo $total to display the total.

Practice

1. What does the PS4 variable control in a bash script when debugging?

easy

A. The number of commands to debug before stopping

B. The output file where debug logs are saved

C. The timeout duration for debugging

D. The prefix shown before each command when debugging is enabled

Solution

1. Step 1: Understand the role of PS4 in bash debugging

   PS4 sets the prefix string that appears before each command when debugging with set -x is active.

2. Step 2: Differentiate PS4 from other debugging settings

   PS4 does not control output files, timeouts, or command counts; it only changes the debug output prefix.

3. Final Answer:

   The prefix shown before each command when debugging is enabled -> Option D

4. Quick Check:

   PS4 = debug output prefix [OK]

Hint: PS4 sets what you see before each debug command line [OK]

Common Mistakes:

• Confusing PS4 with output file settings
• Thinking PS4 controls debug duration
• Assuming PS4 limits number of debug commands

2. Which of the following is the correct way to set a custom PS4 prefix to show the current line number in bash debugging?

easy

A. PS4='Line \$LINENO: '

B. PS4='Line $LINENO: '

C. PS4='Line \n: '

D. PS4='Line $LINENO\n'

Solution

1. Step 1: Understand variable expansion in PS4

   To show the current line number dynamically, you must escape the dollar sign in PS4 so it expands at runtime, not when assigned.

2. Step 2: Identify correct escaping

   Using PS4='Line \$LINENO: ' escapes the dollar sign, so $LINENO expands during debugging, showing the actual line number.

3. Final Answer:

   PS4='Line \$LINENO: ' -> Option A

4. Quick Check:

   Escape $ for runtime expansion in PS4 [OK]

Hint: Escape $ in PS4 to expand variables during debugging [OK]

Common Mistakes:

• Not escaping $ causes literal $LINENO output
• Using \n instead of $LINENO for line number
• Placing newline characters incorrectly

3. Given this script snippet with debugging enabled:

PS4='+ $BASH_SOURCE:$LINENO:'
set -x
x=5
y=10
z=$((x + y))
set +x

What is the first debug output line shown after set -x?

medium

A. + script.sh:2: x=5

B. + : x=5

C. + :2: x=5

D. + script.sh:x=5

Solution

1. Step 1: Analyze PS4 value and variable expansion

   PS4 is set to '+ $BASH_SOURCE:$LINENO:' but $BASH_SOURCE and $LINENO are not escaped, so they expand immediately when assigned, likely empty or current shell values.

2. Step 2: Determine actual debug prefix

   Because variables are not escaped, $BASH_SOURCE is empty and $LINENO is 2 (line of set -x), so prefix becomes '+ :2:'.

3. Final Answer:

   + :2: x=5 -> Option C

4. Quick Check:

   Unescaped vars expand early, prefix = '+ :2:' [OK]

Hint: Unescaped vars in PS4 expand immediately, not during debug [OK]

Common Mistakes:

• Assuming $BASH_SOURCE expands to script filename
• Expecting line number to be dynamic without escaping
• Ignoring immediate expansion on assignment

4. You wrote this script to debug with line numbers:

PS4='Line $LINENO: '
set -x
foo=bar
set +x

But the debug output shows Line $LINENO: foo=bar literally. What is the fix?

medium

A. Change PS4 to 'Line \$LINENO: ' to delay variable expansion

B. Remove quotes around PS4 assignment

C. Use single quotes instead of double quotes for PS4

D. Set PS4 after set -x

Solution

1. Step 1: Identify why $LINENO is literal

   Using single quotes with unescaped $LINENO causes it to be treated as literal text, not expanded during debugging.

2. Step 2: Fix by escaping $ for runtime expansion

   Escaping $ as \$ in PS4 delays expansion until debugging runs, showing actual line numbers.

3. Final Answer:

   Change PS4 to 'Line \$LINENO: ' to delay variable expansion -> Option A

4. Quick Check:

   Escape $ in PS4 to expand at debug time [OK]

Hint: Escape $ in PS4 to see line numbers during debugging [OK]

Common Mistakes:

• Not escaping $ in PS4
• Changing quotes incorrectly
• Setting PS4 after enabling debugging

5. You want to debug a bash script showing the filename, line number, and function name before each command. Which PS4 setting achieves this correctly?

hard

A. PS4='+ ${BASH_SOURCE}:${LINENO}:${FUNCNAME[0]}: '

B. PS4='+ \${BASH_SOURCE}:\${LINENO}:\${FUNCNAME[0]}: '

C. PS4='+ $BASH_SOURCE:$LINENO:$FUNCNAME: '

D. PS4='+ \$BASH_SOURCE:\$LINENO:\$FUNCNAME: '

Solution

1. Step 1: Understand variable expansion timing in PS4

   To show dynamic info like filename, line number, and function name during debugging, variables must be escaped with backslashes to delay expansion until debug time.

2. Step 2: Choose correct syntax for array variable FUNCNAME

   Use \${FUNCNAME[0]} to get current function name properly escaped.

3. Step 3: Verify full PS4 string

   PS4='+ \${BASH_SOURCE}:\${LINENO}:\${FUNCNAME[0]}: ' correctly escapes all variables and uses braces for array indexing, ensuring dynamic debug info appears.

4. Final Answer:

   PS4='+ \${BASH_SOURCE}:\${LINENO}:\${FUNCNAME[0]}: ' -> Option B

5. Quick Check:

   Escape all vars with braces for dynamic debug info [OK]

Hint: Escape variables with braces in PS4 for dynamic debug info [OK]

Common Mistakes:

• Not escaping variables causing static output
• Missing braces for array FUNCNAME
• Using $FUNCNAME without index for function name