Debugging with PS4 in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this script with PS4 set?

Consider this bash script with PS4 set to show line numbers. What will be the first line printed when running with bash -x script.sh?

Bash Scripting

#!/bin/bash
PS4='+$LINENO: '

function greet() {
  echo "Hello, $1"
}
greet World

A

+2: function greet() {
+3: greet World
Hello, World

B

+3: greet World
+4: echo Hello, World
Hello, World

C

+4: greet World
+5: echo Hello, World
Hello, World

D

+1: PS4='+$LINENO: '
+2: function greet() {
+3: echo Hello, World
Hello, World

💡 Hint

Remember PS4 shows the line number of the command being executed.

PS4 is set to '+$LINENO: ' so each debug line shows the line number. The first command executed after setting PS4 is the function call at line 7.

📝 Syntax

intermediate

Which PS4 assignment is valid to show timestamp before each command?

You want to set PS4 to show the current time in HH:MM:SS format before each command in bash debugging. Which assignment is valid?

A PS4='$(date +"%T") '

B PS4='$(date +%T) '

C PS4='$(date +%T)\n'

D PS4='$(date +%T)\t'

💡 Hint

Check how to properly quote the format string inside date command.

Option A correctly quotes the format string with double quotes inside single quotes, so date +"%T" runs properly. Option A lacks quotes around %T causing error.

🔧 Debug

advanced

Why does this PS4 setting cause no output in bash -x?

Given this script: #!/bin/bash PS4='+$LINENO: ' set -x for i in {1..3}; do echo $i PS4='+$LINENO: ' done Why does the debug output not show line numbers inside the loop?

Bash Scripting

#!/bin/bash
PS4='+$LINENO: '
set -x

for i in {1..3}; do
  echo $i
  PS4='+$LINENO: '
done

A Because PS4 is expanded once when set, so changing it inside the loop has no effect

B Because set -x disables PS4 inside loops

C Because PS4 is reset inside the loop after set -x, so it doesn't apply to commands inside the loop

D Because the shell ignores PS4 when used inside loops

💡 Hint

Think about when PS4 is expanded and how set -x uses it.

PS4 is expanded once when set -x is enabled. Changing PS4 inside the loop does not affect the debug prompt because the shell uses the original PS4 value.

🚀 Application

advanced

How to modify PS4 to show function names in bash debug output?

You want your bash debug output to include the function name before each command. Which PS4 setting achieves this?

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

B PS4='+${FUNCNAME}: '

C PS4='+${FUNCNAME[-1]}: '

D PS4='+${FUNCNAME[1]}: '

💡 Hint

FUNCNAME array holds the call stack, index 0 is current function.

FUNCNAME[1] is the caller function name, so to show the function where the command runs, use FUNCNAME[1]. FUNCNAME[0] is the current function (the debug function itself).

🧠 Conceptual

expert

What is the effect of this PS4 setting with command substitution?

Consider this PS4 setting: PS4='+$(date +"%T"): ' What happens when you run bash -x script.sh with this PS4?

A The timestamp is ignored and PS4 shows only '+'

B The timestamp is evaluated before each command, showing current time dynamically

C The timestamp is evaluated once when PS4 is set, so all debug lines show the same time

D The script fails with syntax error due to command substitution in PS4

💡 Hint

Think about when command substitution runs in variable assignments.

Command substitution in PS4 is evaluated once when PS4 is assigned, so the timestamp is fixed for the entire debug session.