Default values for input in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to assign a default value 'guest' to the variable 'user' if no input is given.

Bash Scripting

user=${1:-[1]
echo "User is $user"

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

A$1

Bdefault

Cguest

Dinput

Attempts:

3 left

💡 Hint

Common Mistakes

Using $1 directly without default value syntax.

Using = instead of :- inside the braces.

The syntax ${1:-guest} assigns 'guest' if $1 is empty or unset.

2fill in blank

medium

Complete the code to assign '8080' as the default port if no argument is provided.

Bash Scripting

port=${1:-[1]
echo "Server will run on port $port"

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

A8080

B3000

C443

D80

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to use the :- syntax for default values.

Using a port number that is not the intended default.

The default port 8080 is assigned if no input is given using ${1:-8080}.

3fill in blank

hard

Fix the error in the code to correctly assign 'default.txt' as the filename if no argument is given.

Bash Scripting

filename=${1[1]"default.txt"}
echo "Using file: $filename"

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

A:-

B:=

C=

D?

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' instead of ':-' causes wrong behavior when variable is empty.

Omitting the colon ':' changes the meaning.

The correct syntax for default value is ${var:-default}, so ':-' is needed.

4fill in blank

hard

Fill both blanks to assign 'admin' as default user and 'root' as default group if no inputs are given.

Bash Scripting

user=${1[1]"admin"}
group=${2[2]"root"}
echo "User: $user, Group: $group"

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

A:-

B:=

C-

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' or ':=' which do not cover empty values.

Mixing different default value syntaxes for user and group.

Use ':-' to assign default if variable is unset or empty for both user and group.

5fill in blank

hard

Fill all three blanks to create a script that assigns default values: 'localhost' for host, '3306' for port, and 'root' for user if no inputs are given.

Bash Scripting

host=${1[1]"localhost"}
port=${2[2]"3306"}
user=${3[3]"root"}
echo "Connecting to $host on port $port as $user"

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

A:-

B:=

C-

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' or ':=' which do not cover empty values.

Using '-' without colon which only covers unset variables.

The ':-' syntax is used for all three variables to assign defaults if unset or empty.

Practice

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

easy

A. It throws an error if name is not set.

B. It assigns 'Guest' to name permanently.

C. It deletes the variable name if it is empty.

D. It uses the value of name if set; otherwise, it uses 'Guest' as default.

Solution

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

   This syntax means: use the value of variable if it is set and not empty; otherwise, use default.

2. Step 2: Apply to ${name:-Guest}

   If name is set and not empty, it returns name. If name is unset or empty, it returns 'Guest'.

3. Final Answer:

   It uses the value of name if set; otherwise, it uses 'Guest' as default. -> Option D

4. Quick Check:

   Default value syntax = It uses the value of name if set; otherwise, it uses 'Guest' as default. [OK]

Hint: Use ${var:-default} to provide fallback values [OK]

Common Mistakes:

• Thinking it assigns the default permanently
• Confusing with ${var:=default} which assigns default
• Assuming it throws error if variable unset

2. Which of the following is the correct syntax to read a variable user with a default value 'admin' if input is empty?

easy

A. read user; user=${user:-admin}

B. read user || user=admin

C. read user && user=admin

D. read user ? user=admin

Solution

1. Step 1: Understand how to read input and set default

   The read command takes input into user. To set a default if empty, use user=${user:-admin}.

2. Step 2: Check each option

   read user; user=${user:-admin} correctly reads input then sets default if empty. Options A, C, D use invalid syntax for default assignment.

3. Final Answer:

   read user; user=${user:-admin} -> Option A

4. Quick Check:

   Correct default syntax after read = read user; user=${user:-admin} [OK]

Hint: Use read then assign default with ${var:-default} [OK]

Common Mistakes:

• Using || or && incorrectly for default assignment
• Trying to assign default inside read command
• Using invalid operators like ? for default

3. What is the output of this script if the user presses Enter without typing anything?

read -p "Enter your city: " city
city=${city:-Unknown}
echo "City: $city"

medium

A. City: city

B. City:

C. City: Unknown

D. Error: city not set

Solution

1. Step 1: Analyze read input behavior

   User presses Enter without input, so city is empty string.

2. Step 2: Apply default value substitution

   city=${city:-Unknown} sets city to 'Unknown' because city is empty.

3. Final Answer:

   City: Unknown -> Option C

4. Quick Check:

   Empty input triggers default = City: Unknown [OK]

Hint: Empty input triggers default with ${var:-default} [OK]

Common Mistakes:

• Expecting empty output instead of default
• Confusing ${var:-default} with ${var-default}
• Assuming error occurs on empty input

4. Identify the error in this script snippet:

read name
name=${name=John}
echo "Name: $name"

medium

A. The echo command syntax is wrong

B. The default value syntax is incorrect; should use :- instead of =

C. The variable name is reserved and cannot be used

D. The read command is missing a prompt

Solution

1. Step 1: Check default value syntax

   The script uses ${name=John} which is invalid for default substitution. The correct syntax is ${name:-John}.

2. Step 2: Confirm other parts

   read without prompt is valid, name is a valid variable, and echo syntax is correct.

3. Final Answer:

   The default value syntax is incorrect; should use :- instead of = -> Option B

4. Quick Check:

   Default syntax requires :- not = [OK]

Hint: Use ${var:-default} not ${var=default} for default values [OK]

Common Mistakes:

• Using = instead of :- for default values
• Thinking read needs a prompt
• Confusing variable names with reserved words

5. You want to write a script that asks for a username and a port number. If the user presses Enter without input, the username should default to 'guest' and the port to '8080'. Which script snippet correctly implements this?

hard

A. read -p "Username: " user
user=${user:-guest}
read -p "Port: " port
port=${port:-8080}
echo "User=$user, Port=$port"

B. read -p "Username: " user
user=${user=guest}
read -p "Port: " port
port=${port=8080}
echo "User=$user, Port=$port"

C. read user
if [ -z "$user" ]; then user=guest; fi
read port
if [ -z "$port" ]; then port=8080; fi
echo "User=$user, Port=$port"

D. read user
user=${user:-guest}
read port
port=${port-8080}
echo "User=$user, Port=$port"

Solution

1. Step 1: Check default value syntax for both variables

   read -p "Username: " user user=${user:-guest} read -p "Port: " port port=${port:-8080} echo "User=$user, Port=$port" uses ${var:-default} which correctly sets default if variable is empty or unset.

2. Step 2: Verify prompt usage and output

   read -p "Username: " user user=${user:-guest} read -p "Port: " port port=${port:-8080} echo "User=$user, Port=$port" uses read -p to prompt user, then sets defaults, then echoes both variables correctly.

3. Final Answer:

   read -p "Username: " user user=${user:-guest} read -p "Port: " port port=${port:-8080} echo "User=$user, Port=$port" -> Option A

4. Quick Check:

   Use ${var:-default} with read -p for defaults = read -p "Username: " user user=${user:-guest} read -p "Port: " port port=${port:-8080} echo "User=$user, Port=$port" [OK]

Hint: Use read -p with ${var:-default} for clean input with defaults [OK]

Common Mistakes:

• Using = instead of :- for defaults
• Not prompting user with read -p
• Using ${var-default} which skips empty but not unset