Debugging with PS4 in Bash Scripting - Step-by-Step Execution

Concept Flow - Debugging with PS4

Set PS4 variable

↓

Enable debugging with 'set -x'

↓

Run script commands

↓

Each command prints with PS4 prefix

↓

Observe output with debug info

↓

Disable debugging with 'set +x'

↓

Script continues normally

Set PS4 to customize debug output prefix, enable debugging with 'set -x', run commands to see debug info, then disable debugging.

Execution Sample

Bash Scripting

PS4='+ $BASH_SOURCE:$LINENO:${FUNCNAME[0]}: '
set -x
myfunc() {
  echo "Hello"
}
myfunc
set +x

This script sets PS4 to show file, line, and function name before each command, enables debugging, runs a function, then disables debugging.

Execution Table

Step	Command Run	PS4 Prefix Output	Debug Output	Notes
1	PS4='+ $BASH_SOURCE:$LINENO:${FUNCNAME[0]}: '			Set PS4 variable for debug prefix
2	set -x			Enable debugging mode
3	myfunc() {	+ script.sh:3::	myfunc() {	Start function definition
4	echo "Hello"	+ script.sh:4::	echo "Hello"	During function definition: echo command
5	}	+ script.sh:5::	}	End function definition
6	myfunc	+ script.sh:7::	myfunc	Call function
7	echo "Hello"	+ script.sh:4:myfunc:	Hello	Function prints Hello
8	set +x	+ script.sh:8::	set +x	Disable debugging mode
9				Debugging off, normal script continues

💡 Debugging stops after 'set +x', script runs normally without debug output.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 8	Final
PS4	default	+ $BASH_SOURCE:$LINENO:${FUNCNAME[0]}:	+ $BASH_SOURCE:$LINENO:${FUNCNAME[0]}:	+ $BASH_SOURCE:$LINENO:${FUNCNAME[0]}:	+ $BASH_SOURCE:$LINENO:${FUNCNAME[0]}:
set -x (debug mode)	off	off	on	off	off

Key Moments - 3 Insights

Why does each debug line start with the PS4 prefix?

Because PS4 defines the prefix shown before each command when debugging is enabled with 'set -x', as seen in execution_table rows 3-7.

What happens if we don't disable debugging with 'set +x'?

Debug output continues for all commands, cluttering output. Execution_table row 9 shows debugging stops after 'set +x'.

Why does the function name appear in the PS4 prefix during the function call?

Because PS4 uses ${FUNCNAME[0]} which shows the current function name, visible in row 7 of execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the PS4 prefix output when the echo command inside the function runs?

A + script.sh:4:myfunc:

B + script.sh:7::

C + script.sh:3::

D + script.sh:8::

💡 Hint

Check execution_table row 7 under 'PS4 Prefix Output' column.

At which step does debugging get disabled?

A Step 2

B Step 6

C Step 8

D Step 9

💡 Hint

Look at execution_table row 8 where 'set +x' is run.

If PS4 was set to just '+ ', how would the debug output prefix change?

A It would show file and line number before each command

B It would show only '+ ' before each command

C It would show function name only

D No debug output would appear

💡 Hint

PS4 defines the prefix; changing it to '+ ' removes file, line, and function info.

Concept Snapshot

PS4 sets the prefix for debug output in bash.
Use 'set -x' to enable debugging and see commands with PS4 prefix.
Use 'set +x' to disable debugging.
PS4 can include variables like $BASH_SOURCE, $LINENO, ${FUNCNAME[0]}.
This helps trace script execution step-by-step.

Full Transcript

Debugging with PS4 in bash means setting the PS4 variable to customize the prefix shown before each command when debugging is enabled. You enable debugging with 'set -x', which prints each command with the PS4 prefix before running it. This helps you see exactly what the script is doing and where. You can include file name, line number, and function name in PS4 for detailed tracing. When done, disable debugging with 'set +x' to stop the extra output. This method helps find errors by showing the script's flow clearly.