Read-only variables (readonly) in Bash Scripting - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the readonly command do in bash scripting?

It marks a variable as read-only, so its value cannot be changed or unset after being set.

beginner

How do you declare a read-only variable named MY_VAR with value hello?

Use readonly MY_VAR="hello" or MY_VAR="hello"; readonly MY_VAR.

beginner

What happens if you try to change a read-only variable in bash?

Bash will show an error message and will not allow the variable's value to be changed.

beginner

Can you unset a read-only variable in bash?

No, once a variable is marked read-only, it cannot be unset or deleted.

beginner

Why use read-only variables in bash scripts?

To protect important values from accidental changes, making scripts more reliable and easier to debug.

Which command makes a variable read-only in bash?

A export

B readonly

C declare -i

D unset

The readonly command marks variables as read-only.

What happens if you try to assign a new value to a read-only variable?

A The value changes successfully

B The variable is unset

C Bash throws an error and keeps the original value

D The script exits immediately

Bash prevents changing read-only variables and shows an error.

How do you declare a read-only variable after assigning it a value?

A readonly VAR=value

B VAR=value; export readonly VAR

C declare readonly VAR=value

D VAR=value; readonly VAR

You can assign first, then mark it read-only with readonly VAR.

Can you unset a variable marked as read-only?

A No, read-only variables cannot be unset

B Yes, with <code>unset</code>

C Only if you use <code>readonly -u</code>

D Only in interactive shells

Read-only variables cannot be unset or deleted.

Why is it useful to use read-only variables in scripts?

A To prevent accidental changes to important values

B To save memory

C To speed up script execution

D To allow variables to be shared between scripts

Read-only variables protect important values from accidental changes.

Explain how to create and use a read-only variable in bash scripting.

Think about how to protect a variable from changes.

Describe what happens if you try to modify or unset a read-only variable in bash.

Consider bash's behavior when protecting variables.

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