Backticks and $() for command substitution in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to store the current date in the variable.

Bash Scripting

current_date=[1]date[2]

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

A[

B$(

C{

D`

Attempts:

3 left

💡 Hint

Common Mistakes

Using curly braces {} instead of backticks.

Using square brackets [] which are for tests, not command substitution.

Backticks (`) are used to run a command and capture its output in a variable.

2fill in blank

medium

Complete the code to store the current user name using modern command substitution.

Bash Scripting

user_name=[1]whoami

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

A$(

B`

C)

D[

Attempts:

3 left

💡 Hint

Common Mistakes

Using backticks instead of $() which is the newer preferred style.

Using braces or brackets which are not for command substitution.

The modern way to do command substitution is using $(command).

3fill in blank

hard

Fix the error in the code to correctly assign the list of files to a variable.

Bash Scripting

files=[1]ls -1

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

A$(

B`

C)

D[

Attempts:

3 left

💡 Hint

Common Mistakes

Using braces or brackets which cause syntax errors.

Missing closing backtick or parenthesis.

Use $(command) to correctly capture the output of ls -1 into the variable.

4fill in blank

hard

Complete the code to print the current directory using command substitution.

Bash Scripting

echo [1]pwd

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

A$(

B`

C)

D}

Attempts:

3 left

💡 Hint

Common Mistakes

Using backticks without closing them.

Using braces or missing closing parenthesis.

Use $(pwd) to run pwd and print the current directory.

5fill in blank

hard

Complete the code to assign the number of lines in a file to a variable.

Bash Scripting

line_count=[1]wc -l filename.txt

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

A$(

B-l

C)

D`

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing $() and backticks incorrectly.

Forgetting to close the command substitution.

Use $(wc -l filename.txt) to count lines and assign output to variable.

Practice

1. What does the command substitution syntax $(date) do in a bash script?

easy

A. Creates a new variable named date

B. Runs the date command and replaces $(date) with its output

C. Comments out the date command

D. Prints the literal string $(date) without running the command

Solution

1. Step 1: Understand command substitution

   Command substitution runs the command inside $() and replaces it with the command's output.

2. Step 2: Apply to $(date)

   The date command outputs the current date and time, so $(date) is replaced by that output.

3. Final Answer:

   Runs the date command and replaces $(date) with its output -> Option B

4. Quick Check:

   Command substitution = runs command and inserts output [OK]

Hint: Remember: $() runs command and inserts output [OK]

Common Mistakes:

• Thinking it creates a variable
• Confusing it with comments
• Expecting literal string output

2. Which of the following is the preferred syntax for command substitution in bash?

easy

A. echo `ls -l`

B. echo $(ls -l

C. echo $[ls -l]

D. echo $(ls -l)

Solution

1. Step 1: Identify valid command substitution syntax

   Command substitution uses either backticks `command` or $(command). Both the backtick version and the incomplete $() version look valid at first.

2. Step 2: Check for best practice and syntax correctness

   echo `ls -l` uses backticks which are valid but less readable and harder to nest. echo $(ls -l) uses $() which is modern and preferred. echo $[ls -l] uses invalid syntax with $[]. echo $(ls -l misses closing parenthesis.

3. Final Answer:

   echo $(ls -l) -> Option D

4. Quick Check:

   Use $() for clear command substitution [OK]

Hint: Use $() for command substitution, it's clearer and nestable [OK]

Common Mistakes:

• Using $[] which is arithmetic expansion, not command substitution
• Missing closing parenthesis
• Confusing backticks with other symbols

3. What will be the output of this script snippet?

files=$(ls /tmp)
echo "$files"

medium

A. Shows an error because of missing quotes

B. Prints the literal string '$(ls /tmp)'

C. Lists all files and directories inside /tmp one per line

D. Prints nothing because variable is empty

Solution

1. Step 1: Understand command substitution with ls

   The command ls /tmp lists files in /tmp. Using files=$(ls /tmp) stores that output as a string with filenames separated by newlines.

2. Step 2: Echo the variable with quotes

   Using echo "$files" prints the content of files preserving newlines, so it lists all files and directories one per line.

3. Final Answer:

   Lists all files and directories inside /tmp one per line -> Option C

4. Quick Check:

   Command substitution stores output string = list of files [OK]

Hint: Command substitution stores output as string, echo prints it [OK]

Common Mistakes:

• Expecting literal command string output
• Confusing quotes causing errors
• Assuming variable is empty

4. Identify the error in this script snippet:

current_dir=`pwd
 echo "Current directory is: $current_dir"

medium

A. Missing closing backtick for command substitution

B. Incorrect use of double quotes

C. Variable name is invalid

D. No error, script runs fine

Solution

1. Step 1: Check command substitution syntax

   The command substitution using backticks must start and end with backticks. Here, the closing backtick is missing after pwd.

2. Step 2: Analyze impact of missing backtick

   Without the closing backtick, bash will throw a syntax error and not run the script properly.

3. Final Answer:

   Missing closing backtick for command substitution -> Option A

4. Quick Check:

   Backticks must be paired to run commands [OK]

Hint: Always close backticks to avoid syntax errors [OK]

Common Mistakes:

• Forgetting to close backticks
• Misplacing quotes inside command substitution
• Assuming variable names cause errors

5. You want to store the number of files in your home directory in a variable file_count. Which command correctly uses command substitution to do this?

hard

A. file_count=$(ls ~ | wc -l)

B. file_count=`ls ~ wc -l`

C. file_count=$(ls ~ wc -l)

D. file_count=ls ~ | wc -l

Solution

1. Step 1: Understand the command to count files

   To count files, we list them with ls ~ and pipe to wc -l to count lines (files).

2. Step 2: Use correct command substitution syntax

   file_count=$(ls ~ | wc -l) correctly uses $(ls ~ | wc -l) to run the full command and store output in file_count. The other options miss the pipe or fail to use command substitution.

3. Final Answer:

   file_count=$(ls ~ | wc -l) -> Option A

4. Quick Check:

   Use $() with full command and pipe inside [OK]

Hint: Use $() with full command including pipes for substitution [OK]

Common Mistakes:

• Missing pipe between commands
• Using backticks without pipe
• Assigning command without substitution