Read-only variables (readonly) in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this Bash script using readonly?

Consider this Bash script:

readonly VAR=10
VAR=20
echo $VAR

What will be the output when you run it?

Bash Scripting

readonly VAR=10
VAR=20
echo $VAR

A 10

B 20

C bash: VAR: readonly variable

D Empty output

💡 Hint

readonly variables cannot be changed after being set.

The script tries to change a readonly variable VAR. Bash will stop with an error: 'bash: VAR: readonly variable'.

💻 Command Output

intermediate

What happens when you unset a readonly variable in Bash?

Given this script:

readonly MYVAR=5
unset MYVAR
echo $MYVAR

What will be the output or error?

Bash Scripting

readonly MYVAR=5
unset MYVAR
echo $MYVAR

A 5

B bash: unset: MYVAR: cannot unset: readonly variable

C Empty output

D bash: MYVAR: unbound variable

💡 Hint

readonly variables cannot be unset.

Trying to unset a readonly variable causes an error: 'bash: unset: MYVAR: cannot unset: readonly variable'.

📝 Syntax

advanced

Which option correctly declares a readonly variable with value 'hello' in Bash?

Choose the correct syntax to declare a readonly variable GREETING with value 'hello'.

A readonly GREETING:='hello'

B readonly GREETING=hello

C readonly 'GREETING=hello'

D readonly GREETING='hello'

💡 Hint

Use the readonly command followed by variable assignment without quotes around the whole expression.

Option D correctly assigns 'hello' to GREETING and marks it readonly. Option D misses quotes (allowed but risky if value has spaces). Option D quotes the whole expression causing syntax error. Option D uses invalid syntax ':='.

🚀 Application

advanced

How to protect a variable from accidental changes in a Bash script?

You want to make sure a variable CONFIG_PATH cannot be changed later in your script. Which approach achieves this?

A Set CONFIG_PATH and then run: readonly CONFIG_PATH

B Set CONFIG_PATH and then run: export CONFIG_PATH

C Set CONFIG_PATH and then run: unset CONFIG_PATH

D Set CONFIG_PATH and then run: declare -i CONFIG_PATH

💡 Hint

readonly prevents reassignment, export shares variable with child processes.

readonly marks the variable as unchangeable. export shares it with child processes but does not protect from changes. unset removes the variable. declare -i makes it integer type but does not protect it.

🔧 Debug

expert

Why does this Bash script fail to protect the variable with readonly?

Look at this script:

CONFIG=default
readonly CONFIG
CONFIG=custom
echo $CONFIG

Why does it not prevent changing CONFIG?

Bash Scripting

CONFIG=default
readonly CONFIG
CONFIG=custom
echo $CONFIG

A readonly was set without assigning a value, so CONFIG remains changeable

B readonly only works on exported variables

C CONFIG=custom is ignored silently by Bash

D readonly must be declared before any assignment

💡 Hint

readonly without assignment locks current value, but here it was declared after assignment.

readonly CONFIG locks the current value of CONFIG. Since CONFIG was assigned before readonly, the value 'default' is locked. But the script tries to assign 'custom' after readonly, which causes an error. However, in some shells, readonly without assignment after assignment locks the variable. The error should occur, but if it doesn't, it means readonly was not effective because it was declared without value. The correct explanation is that readonly without assignment locks the variable but the assignment after readonly causes error. So the script should fail at CONFIG=custom. If it doesn't, the readonly was not effective.

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