Read-only variables (readonly) in Bash Scripting

Introduction

Read-only variables help keep important values safe from accidental changes in your script.

When you want to store a value that should never change during the script.

To protect configuration settings from being modified by mistake.

When sharing variables between functions but want to keep them constant.

To avoid bugs caused by accidentally changing critical data.

When you want to make your script easier to understand by marking fixed values.

Syntax

Bash Scripting

readonly VARIABLE_NAME=VALUE
# or
readonly VARIABLE_NAME

Use 'readonly' before a variable name to make it unchangeable.

You can set the value when declaring or make an existing variable readonly.

Examples

Sets PI as a read-only variable with value 3.14.

Bash Scripting

readonly PI=3.14
# Now PI cannot be changed

Makes the existing variable NAME read-only.

Bash Scripting

NAME="Alice"
readonly NAME
# NAME is now read-only

Trying to change a read-only variable causes an error.

Bash Scripting

readonly MAX_USERS=100
MAX_USERS=200
# This will cause an error

Sample Program

This script sets GREETING as read-only, prints it, then tries to change it and prints again. The change will fail.

Bash Scripting

#!/bin/bash
readonly GREETING="Hello"
echo $GREETING
# Trying to change GREETING
GREETING="Hi"
echo $GREETING

Important Notes

Trying to change a read-only variable will cause an error and the variable keeps its original value.

Read-only variables help prevent accidental mistakes in your scripts.

You cannot unset a read-only variable with 'unset'.

Summary

Use 'readonly' to make variables unchangeable.

Read-only variables protect important values in your script.

Trying to modify a read-only variable causes an error.

Practice

1. What does the readonly command do in a bash script?

easy

A. It prints the value of a variable.

B. It deletes a variable from the script.

C. It makes a variable unchangeable after assignment.

D. It allows a variable to be changed multiple times.

Solution

1. Step 1: Understand the purpose of readonly

   The readonly command is used to mark a variable so it cannot be changed later in the script.

2. Step 2: Compare options with the command's function

   Only "It makes a variable unchangeable after assignment." correctly describes that the variable becomes unchangeable after assignment.

3. Final Answer:

   It makes a variable unchangeable after assignment. -> Option C

4. Quick Check:

   readonly = makes variable unchangeable [OK]

Hint: Readonly means variable cannot be changed after set [OK]

Common Mistakes:

• Thinking readonly deletes the variable
• Confusing readonly with printing variable
• Assuming readonly allows changes

2. Which of the following is the correct syntax to make a variable MYVAR read-only in bash?

easy

A. readonly MYVAR =value

B. readonly MYVAR

C. MYVAR readonly

D. set readonly MYVAR

Solution

1. Step 1: Recall the syntax of readonly

   The correct syntax is to first assign the variable, then use readonly MYVAR to make it read-only.

2. Step 2: Analyze each option

   readonly MYVAR =value tries to assign and readonly together, which is invalid. readonly MYVAR correctly uses readonly MYVAR. Options C and D are not valid bash syntax.

3. Final Answer:

   readonly MYVAR -> Option B

4. Quick Check:

   Use readonly VAR after assignment [OK]

Hint: Assign first, then use readonly VAR to lock it [OK]

Common Mistakes:

• Trying to assign value inside readonly command
• Placing readonly after variable name
• Using invalid commands like set readonly

3. What will be the output of this script?

MYVAR=hello
readonly MYVAR
MYVAR=world
echo $MYVAR

medium

A. world

B. Error: cannot modify readonly variable

C. Syntax error

D. hello

Solution

1. Step 1: Understand the script flow

   The variable MYVAR is assigned "hello" and then made readonly. The script then tries to change it to "world".

2. Step 2: Predict the behavior when modifying a readonly variable

   Bash will produce an error message when trying to assign a new value to a readonly variable, but the script will continue and the value of MYVAR remains "hello". The echo command outputs the original value.

3. Final Answer:

   hello -> Option D

4. Quick Check:

   Readonly variable value remains unchanged; echo prints original value [OK]

Hint: Changing readonly variable causes error message but value unchanged [OK]

Common Mistakes:

• Assuming variable changes silently
• Expecting syntax error instead of runtime error
• Thinking echo prints new value

4. Identify the error in this script:

readonly MYVAR =10
MYVAR=20
echo $MYVAR

medium

A. Cannot assign value during readonly declaration

B. Variable name is invalid

C. Missing export keyword

D. No error, script runs fine

Solution

1. Step 1: Check the syntax of readonly with assignment

   In bash, readonly cannot be combined with assignment like readonly MYVAR =10. The variable must be assigned first, then made readonly.

2. Step 2: Analyze the error caused by this syntax

   This syntax causes an error because bash expects only variable names after readonly, not assignments.

3. Final Answer:

   Cannot assign value during readonly declaration -> Option A

4. Quick Check:

   Assign first, then readonly VAR separately [OK]

Hint: Assign before readonly; no assignment inside readonly [OK]

Common Mistakes:

• Trying to assign value inside readonly command
• Ignoring syntax error messages
• Confusing export with readonly

5. You want to protect a variable CONFIG_PATH so it cannot be changed accidentally in your script, but you also want to print its value later. Which sequence of commands is correct?

hard

A. CONFIG_PATH=/etc/config
readonly CONFIG_PATH
echo $CONFIG_PATH

B. readonly CONFIG_PATH=/etc/config
echo $CONFIG_PATH

C. CONFIG_PATH=/etc/config
echo $CONFIG_PATH
readonly CONFIG_PATH

D. echo $CONFIG_PATH
CONFIG_PATH=/etc/config
readonly CONFIG_PATH

Solution

1. Step 1: Assign the variable before making it readonly

   You must first assign CONFIG_PATH a value, then use readonly to lock it.

2. Step 2: Print the variable after it is readonly

   Printing the variable after making it readonly is valid and will show the assigned value.

3. Final Answer:

   CONFIG_PATH=/etc/config readonly CONFIG_PATH echo $CONFIG_PATH -> Option A

4. Quick Check:

   Assign, then readonly, then print [OK]

Hint: Assign first, readonly second, then use variable [OK]

Common Mistakes:

• Trying to assign inside readonly
• Printing variable before assignment
• Making variable readonly before assignment