Read-only variables (readonly) in Bash Scripting - Mini Project: Build & Apply

Using Read-only Variables in Bash Scripts

📖 Scenario: You are writing a bash script to manage a simple configuration where some values must not change after being set. This is common in scripts that rely on fixed settings like file paths or version numbers.

🎯 Goal: Learn how to create read-only variables in bash so that their values cannot be changed later in the script.

📋 What You'll Learn

Create a variable with a fixed value

Make the variable read-only using the readonly command

Try to change the read-only variable (to see it fail)

Print the value of the read-only variable

💡 Why This Matters

🌍 Real World

Many bash scripts use read-only variables to protect important settings from accidental changes during execution.

💼 Career

Understanding read-only variables helps you write safer and more reliable automation scripts in system administration and DevOps roles.

1

Create a variable with a fixed value

Create a variable called config_path and set it to the string /etc/myapp/config.

Bash Scripting

# Create the variable 'config_path' with the value '/etc/myapp/config'
# Your code here

Hint

Use = to assign the value without spaces, and wrap the path in double quotes.

2

Make the variable read-only

Use the readonly command to make the variable config_path read-only.

Bash Scripting

config_path="/etc/myapp/config"
# Make 'config_path' read-only
# Your code here

Hint

Type readonly followed by the variable name.

3

Try to change the read-only variable

Try to assign the string /tmp/config to the variable config_path after it is made read-only.

Bash Scripting

config_path="/etc/myapp/config"
readonly config_path
# Try to change 'config_path' to '/tmp/config'
# Your code here

Hint

Just write the assignment line again after readonly.

4

Print the value of the read-only variable

Print the value of config_path using echo.

Bash Scripting

config_path="/etc/myapp/config"
readonly config_path
config_path="/tmp/config"
# Print the value of 'config_path'
# Your code here

Hint

Use echo "$config_path" to print the variable's value.

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