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.