Read-only variables (readonly) in Bash Scripting - Step-by-Step Execution

Concept Flow - Read-only variables (readonly)

Declare variable

↓

Make variable readonly

↓

Try to change variable

↓

Error: cannot modify readonly variable

↓

Use variable as is

↓

End

You declare a variable, mark it readonly, then any attempt to change it causes an error, preserving its value.

Execution Sample

Bash Scripting

myvar="hello"
readonly myvar
myvar="world"
echo "$myvar"

This script sets a variable, makes it readonly, tries to change it (which fails), then prints the variable.

Execution Table

Step	Command	Action	Variable State	Output	Notes
1	myvar="hello"	Assign 'hello' to myvar	myvar='hello'		Variable created with value 'hello'
2	readonly myvar	Make myvar readonly	myvar='hello' (readonly)		myvar is now readonly
3	myvar="world"	Attempt to change myvar	myvar='hello' (readonly)	bash: myvar: readonly variable	Error: cannot modify readonly variable
4	echo "$myvar"	Print myvar value	myvar='hello' (readonly)	hello	Value remains unchanged

💡 Script ends after printing 'hello'; modification attempt failed due to readonly.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
myvar	undefined	'hello'	'hello' (readonly)	'hello' (readonly)	'hello' (readonly)

Key Moments - 2 Insights

Why does the variable not change after trying to assign a new value?

Because the variable was made readonly at step 2, any assignment after that (step 3) causes an error and does not change the value, as shown in the execution_table.

Does the script stop when the error occurs at step 3?

No, the script continues after the error, so the echo command at step 4 runs and prints the original value.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'myvar' after step 3?

A 'world'

B 'hello'

C undefined

D empty string

💡 Hint

Check the 'Variable State' column at step 3 in the execution_table.

At which step does the script produce an error about modifying a readonly variable?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Output' and 'Notes' columns in the execution_table for the error message.

If we remove the 'readonly' command at step 2, what will be the output at step 4?

A world

B bash error

C hello

D empty

💡 Hint

Without readonly, the assignment at step 3 succeeds, changing the variable value.

Concept Snapshot

Declare a variable: myvar="value"
Make it readonly: readonly myvar
Trying to change it later causes an error
Readonly variables keep their value safe
Useful to prevent accidental changes

Full Transcript

This example shows how to create a readonly variable in bash. First, we assign 'hello' to myvar. Then we mark myvar as readonly using the readonly command. When we try to assign 'world' to myvar, bash gives an error and the value stays 'hello'. Finally, echo prints the unchanged value. This protects variables from accidental changes.