Default values (${var:-default}) in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print the value of VAR or 'default' if VAR is empty or unset.

Bash Scripting

echo ${VAR[1]default}

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

A:=

B:-

C-

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' instead of ':-' which does not check for empty values.

Using ':=' which assigns the default value to VAR.

Using '+' which behaves differently.

The syntax ${VAR:-default} prints VAR if set and not empty; otherwise, it prints 'default'.

2fill in blank

medium

Complete the code to assign 'default' to VAR only if VAR is unset or empty.

Bash Scripting

VAR=[1]

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

A${VAR:-}

B${VAR:=}

C${VAR:-default}

D${VAR:=default}

Attempts:

3 left

💡 Hint

Common Mistakes

Using ':-' which only provides a default for output but does not assign.

Using empty default values which do nothing.

The '${VAR:=default}' syntax assigns 'default' to VAR if it is unset or empty.

3fill in blank

hard

Fix the error in the code to print 'default' if VAR is unset or empty.

Bash Scripting

echo ${VAR[1]default}

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

A:=

B-

C:-

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' which does not handle empty values.

Using ':=' which assigns instead of just printing.

The correct syntax is '${VAR:-default}' to print 'default' if VAR is unset or empty.

4fill in blank

hard

Fill both blanks to print VAR if set and not empty, else print 'default'.

Bash Scripting

echo $[1][2]default}

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

AVAR

B:-

C-

DVAR:-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' instead of ':-' which ignores empty values.

Putting the operator before the variable name.

Use '${VAR:-default}' to print VAR if set and not empty, else 'default'.

5fill in blank

hard

Fill both blanks to assign 'default' to VAR only if VAR is unset or empty, then print VAR.

Bash Scripting

[1]={{BLANK_2}default
echo $}

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

AVAR

B${VAR:=

D}

Attempts:

3 left

💡 Hint

Common Mistakes

Not closing the brace in the assignment syntax.

Using ':-' instead of ':=' which does not assign.

The code assigns 'default' to VAR if unset or empty using '${VAR:=default}', then prints VAR.

Practice

1. What does the expression ${var:-default} do in a bash script?

easy

A. It uses the value of var if set; otherwise, it uses default without changing var.

B. It sets var to default if var is unset or empty.

C. It always replaces var with default.

D. It throws an error if var is unset or empty.

Solution

1. Step 1: Understand the syntax of ${var:-default}

   This syntax means: use var if it is set and not empty; otherwise, use default.

2. Step 2: Check if var is changed

   This expression does not assign or change var, it only provides a fallback value for use.

3. Final Answer:

   It uses the value of var if set; otherwise, it uses default without changing var. -> Option A

4. Quick Check:

   Default value used only if var is empty or unset [OK]

Hint: Remember: :- means use default if empty or unset [OK]

Common Mistakes:

• Thinking it changes the variable's value
• Confusing :- with := which assigns default
• Assuming it throws an error if unset

2. Which of the following is the correct syntax to print a variable name or use "Guest" if name is empty or unset?

easy

A. echo ${name=Guest}

B. echo ${name?Guest}

C. echo ${name+Guest}

D. echo ${name:-Guest}

Solution

1. Step 1: Identify the correct default value syntax

   The syntax ${var:-default} prints var if set; otherwise, prints default.

2. Step 2: Check each option

   echo ${name=Guest} assigns default if unset, not just prints. echo ${name+Guest} prints Guest only if name is set. echo ${name?Guest} throws error if name is unset. Only echo ${name:-Guest} prints name or "Guest" safely.

3. Final Answer:

   echo ${name:-Guest} -> Option D

4. Quick Check:

   Use :- for default fallback [OK]

Hint: Use :- to print default without assignment [OK]

Common Mistakes:

• Using = which assigns instead of fallback
• Using + which prints default only if var is set
• Using ? which causes error if var unset

3. What is the output of this script?

unset user
user=""
echo "User: ${user:-anonymous}"

medium

A. User: anonymous

B. User:

C. User: user

D. User: ${user:-anonymous}

Solution

1. Step 1: Analyze variable state

   The variable user is first unset, then set to an empty string.

2. Step 2: Apply default value logic

   Since user is empty (empty string counts as unset for :-), the expression uses the default "anonymous".

3. Final Answer:

   User: anonymous -> Option A

4. Quick Check:

   Empty variable triggers default value [OK]

Hint: Empty or unset triggers default with :- [OK]

Common Mistakes:

• Thinking empty string is treated as set
• Expecting empty output instead of default
• Confusing unset and empty behavior

4. Identify the error in this script snippet:

echo "Name: ${name-default}"

What is the problem?

medium

A. The dash should be a plus sign: ${name+default}.

B. The syntax is correct and will work as expected.

C. It should be ${name:-default} to use default if name is empty or unset.

D. The variable name is missing a $ sign.

Solution

1. Step 1: Understand the difference between - and :-

   Using ${name-default} uses default only if name is unset, but not if empty.

2. Step 2: Correct syntax for empty or unset

   To cover both unset and empty, ${name:-default} is needed.

3. Final Answer:

   It should be ${name:-default} to use default if name is empty or unset. -> Option C

4. Quick Check:

   Use :- to cover empty and unset [OK]

Hint: Use :- not - to cover empty and unset [OK]

Common Mistakes:

• Using - which ignores empty variables
• Confusing + and - operators
• Assuming syntax is always correct

5. You want to print the value of path if set and not empty, otherwise print "/usr/local/bin". But if path is set to an empty string, you want to treat it as unset. Which command correctly does this?

hard

A. echo ${path-/usr/local/bin}

B. echo ${path:-/usr/local/bin}

C. echo ${path:=/usr/local/bin}

D. echo ${path:+/usr/local/bin}

Solution

1. Step 1: Understand the requirement

   We want to print path if it is set and not empty; otherwise, print the default "/usr/local/bin".

2. Step 2: Choose correct parameter expansion

   ${path:-default} uses default if path is unset or empty. ${path-default} only uses default if unset, not empty. ${path:=default} assigns default to path. ${path:+default} prints default only if path is set.

3. Final Answer:

   echo ${path:-/usr/local/bin} -> Option B

4. Quick Check:

   :- covers unset and empty [OK]

Hint: Use :- to treat empty as unset and provide default [OK]

Common Mistakes:

• Using - which ignores empty strings
• Using := which changes variable value
• Using + which prints default only if set