Read-only variables (readonly) in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a read-only variable named MY_VAR with value 10.

Bash Scripting

readonly [1]=10

Drag options to blanks, or click blank then click option'

Avar1

Bmy_var

CMY_VAR

Dreadonly

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'readonly' as the variable name.

Using lowercase variable names when uppercase is expected.

The readonly command followed by the variable name MY_VAR makes it read-only.

2fill in blank

medium

Complete the code to print the value of the read-only variable MY_VAR.

Bash Scripting

echo [1]

Drag options to blanks, or click blank then click option'

AMY_VAR

B$MY_VAR

CMY_VAR$

D${MY_VAR}

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting the dollar sign before the variable name.

Using variable name without braces which can cause errors in some contexts.

Use ${MY_VAR} to correctly reference the variable value in bash.

3fill in blank

hard

Fix the error in the code that tries to change a read-only variable MY_VAR to 20.

Bash Scripting

readonly MY_VAR=10
[1]
MY_VAR=20

Drag options to blanks, or click blank then click option'

Aunset MY_VAR

Breadonly MY_VAR

Cexport MY_VAR

DMY_VAR

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to assign a new value directly to a readonly variable.

Using 'export' which does not remove readonly status.

You cannot assign a new value to a read-only variable. You must unset it first to remove the read-only attribute.

4fill in blank

hard

Fill both blanks to declare a read-only variable VAR with value 100 and then print it.

Bash Scripting

[1] VAR=100
[2] $VAR

Drag options to blanks, or click blank then click option'

Areadonly

Becho

Cprint

Dexport

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'print' instead of 'echo' to display the value.

Using 'export' which does not make the variable read-only.

Use 'readonly' to declare the variable and 'echo' to print its value.

5fill in blank

hard

Fill all three blanks to declare a read-only variable NAME with value 'Alice', print it, and then try to change it (which should fail).

Bash Scripting

[1] NAME="Alice"
[2] $NAME
NAME=[3]

Drag options to blanks, or click blank then click option'

Areadonly

Becho

C"Bob"

Dprint

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'print' instead of 'echo' to display the value.

Trying to change the readonly variable without unsetting it first.

Declare NAME as readonly, print it with echo, then attempt to assign a new value "Bob" which will fail.

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