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.

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