Debugging with PS4 in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable debugging output with PS4.

Bash Scripting

export PS4=[1]
set -x
ls
set +x

Drag options to blanks, or click blank then click option'

A"+ $BASH_SOURCE:$LINENO:"

B"- $BASH_SOURCE:$LINENO:"

C"$BASH_SOURCE-$LINENO>"

D"$LINENO:$BASH_SOURCE>"

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect variable names in PS4.

Not quoting the PS4 value properly.

Setting PS4 to "+ $BASH_SOURCE:$LINENO:" shows the script name and line number before each command when debugging is enabled.

2fill in blank

medium

Complete the code to print the current function name in PS4 during debugging.

Bash Scripting

export PS4='+ $BASH_SOURCE:$LINENO:$[1]: '
set -x
myfunc() {
  echo "Hello"
}
myfunc
set +x

Drag options to blanks, or click blank then click option'

AFUNCNAME[0]

BFUNCNAME()

CFUNCNAME

DFUNCNAME[1]

Attempts:

3 left

💡 Hint

Common Mistakes

Using FUNCNAME without index which prints the whole array.

Using FUNCNAME[0] which is the current function, not the caller.

FUNCNAME[1] gives the name of the current function being executed, useful in PS4 for debugging function calls.

3fill in blank

hard

Fix the error in the PS4 assignment to correctly show timestamp and line number.

Bash Scripting

export PS4='$(date +"%T") [1] $LINENO: '
set -x
ls
set +x

Drag options to blanks, or click blank then click option'

A"+"

B"-"

C"|"

D"=>"

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' which can be confused with options.

Using symbols that clutter the debug output.

Using '+' as a separator in PS4 is common and clear; it separates the timestamp and line number nicely.

4fill in blank

hard

Fill both blanks to create a PS4 that shows script name and function call depth.

Bash Scripting

export PS4='+ $[1]:$[2]> '
set -x
func() {
  echo "Test"
}
func
set +x

Drag options to blanks, or click blank then click option'

ABASH_SOURCE

BFUNCNAME

CLINENO

DBASH_LINENO

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing LINENO with function name.

Using BASH_LINENO which shows line numbers of functions, not names.

BASH_SOURCE shows the script name; FUNCNAME shows the function call stack, useful for debugging depth.

5fill in blank

hard

Fill all three blanks to create a PS4 that shows timestamp, script name, and line number.

Bash Scripting

export PS4='[[1]] $[2]:[3]: '
set -x
ls
set +x

Drag options to blanks, or click blank then click option'

Adate +"%H:%M:%S"

BBASH_SOURCE

CLINENO

DFUNCNAME

Attempts:

3 left

💡 Hint

Common Mistakes

Using FUNCNAME instead of LINENO for line number.

Not quoting the date command properly.

This PS4 shows the current time, script name, and line number for detailed debugging info.

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