Debugging with PS4 in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the PS4 variable in bash scripting?

PS4 is a special variable in bash that defines the prompt used when debugging with 'set -x'. It helps show detailed trace information for each command executed.

beginner

How do you enable debugging mode in a bash script to see command traces?

You enable debugging by running 'set -x' in the script or shell. This makes bash print each command and its arguments before executing them.

intermediate

How can you customize the debugging output prefix using PS4?

You can set PS4 to any string you want. For example, 'PS4="+ $(date +%T) "' adds a timestamp before each debug line, making it easier to follow.

intermediate

What does the following PS4 value do? PS4='+ ${BASH_SOURCE}:${LINENO}:${FUNCNAME[0]}: '

This PS4 setting shows the script file name, line number, and function name before each debug line. It helps locate where commands run in the script.

beginner

How do you turn off debugging mode after using 'set -x'?

You turn off debugging by running 'set +x'. This stops bash from printing commands before execution.

What does the PS4 variable control in bash debugging?

A The error message format

B The shell prompt when not debugging

C The output color of the script

D The prompt shown before each debug command trace

PS4 sets the prefix shown before each command line when debugging with 'set -x'.

Which command enables debugging mode in bash?

A debug on

B set +x

C set -x

D trace enable

'set -x' turns on debugging, showing commands as they run.

How can you add a timestamp to each debug line in bash?

A Set PS4 to include $(date +%T)

B Use set +x with a timestamp

C Change the shell prompt PS1

D Use echo before each command

Setting PS4 to '+ $(date +%T) ' adds a timestamp prefix to debug output.

What does 'set +x' do in a bash script?

A Starts debugging mode

B Stops debugging mode

C Prints all variables

D Runs the script silently

'set +x' disables debugging mode and stops printing commands.

Which PS4 setting helps show the script file and line number in debug output?

A PS4='+ ${BASH_SOURCE}:${LINENO} '

B PS4='DEBUG>'

C PS4='Line:'

D PS4='${FUNCNAME[0]}'

Using '${BASH_SOURCE}:${LINENO}' in PS4 shows the file and line number for each command.

Explain how to use PS4 and set -x together to debug a bash script with detailed trace information.

Think about how PS4 changes the prefix and set -x turns on tracing.

Describe how you would stop debugging output after enabling it with set -x in a bash script.

Consider the opposite of set -x.

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