Pipe operator (|) in Linux CLI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to list all files and then count the lines using the pipe operator.

Linux CLI

ls -l [1] wc -l

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

A|

B&

C>

D;

Attempts:

3 left

💡 Hint

Common Mistakes

Using '&' runs commands in background, not piping output.

Using '>' redirects output to a file, not to another command.

Using ';' runs commands sequentially but does not pipe output.

The pipe operator '|' sends the output of the first command as input to the second command.

2fill in blank

medium

Complete the code to find all '.txt' files and count them using the pipe operator.

Linux CLI

find . -name '*.txt' [1] wc -l

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

A;

B|

C&&

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '&&' instead of pipe, which does not pass output.

Using '>' redirects output to a file, not to another command.

Using ';' runs commands one after another without piping.

The pipe '|' passes the list of found files to 'wc -l' to count them.

3fill in blank

hard

Fix the error in the code to correctly pipe the output of 'ps aux' to 'grep bash'.

Linux CLI

ps aux [1] grep bash

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

A|

B>

C||

D&

Attempts:

3 left

💡 Hint

Common Mistakes

Using '&' runs command in background, not piping output.

Using '||' runs second command only if first fails, no piping.

Using '>' redirects output to a file, not to another command.

The pipe '|' correctly sends the output of 'ps aux' to 'grep bash' for filtering.

4fill in blank

hard

Fill both blanks to list all running processes and filter those containing 'ssh'.

Linux CLI

ps [1] [2] grep ssh

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

Aaux

B|

C-ef

D&&

Attempts:

3 left

💡 Hint

Common Mistakes

Using '&&' instead of pipe, which does not pass output.

Using 'aux' with '&&' instead of pipe.

Not using any option with 'ps' to list all processes.

Use 'ps -ef' to list all processes and pipe '|' to 'grep ssh' to filter them.

5fill in blank

hard

Fill all three blanks to list files, filter those with '.log', and count them.

Linux CLI

ls [1] [2] grep [3] | wc -l

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

A-l

B|

C.log

D-a

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-a' instead of '-l' for listing files.

Using ';' or '&&' instead of pipe '|'.

Not specifying '.log' to filter log files.

Use 'ls -l' to list files, pipe '|' to 'grep .log' to filter log files, then count lines with 'wc -l'.

Practice

1. What does the pipe operator | do in Linux command line?

easy

A. Sends the output of one command as input to another command

B. Runs two commands simultaneously

C. Redirects output to a file

D. Comments out a line in a script

Solution

1. Step 1: Understand the pipe operator function

   The pipe operator | takes the output from the command on its left side.

2. Step 2: Connect output to input of next command

   This output is then passed as input to the command on the right side of the pipe.

3. Final Answer:

   Sends the output of one command as input to another command -> Option A

4. Quick Check:

   Pipe operator = send output to next command [OK]

Hint: Pipe connects output to input of next command [OK]

Common Mistakes:

• Thinking pipe runs commands in parallel
• Confusing pipe with output redirection >
• Assuming pipe comments code lines

2. Which of the following is the correct syntax to use the pipe operator to list files and then count them?

easy

A. ls & wc -l

B. ls > wc -l

C. ls || wc -l

D. ls | wc -l

Solution

1. Step 1: Identify correct pipe syntax

   The pipe operator is |, which connects commands by passing output to input.

2. Step 2: Check command usage

   ls | wc -l lists files and counts lines, which is correct syntax.

3. Final Answer:

   ls | wc -l -> Option D

4. Quick Check:

   Pipe syntax uses | between commands [OK]

Hint: Use single | to pipe commands, not > or & [OK]

Common Mistakes:

• Using > which redirects output to file
• Using & which runs commands in background
• Using || which is logical OR, not pipe

3. What is the output of the command: echo -e "apple\nbanana\ncherry" | grep "an"?

medium

A. apple cherry

B. apple banana cherry

C. banana

D. No output

Solution

1. Step 1: Understand the echo command output

   echo -e "apple\nbanana\ncherry" outputs three lines: apple, banana, cherry.

2. Step 2: Apply grep filter with "an"

   grep "an" filters lines containing "an", which matches only "banana".

3. Final Answer:

   banana -> Option C

4. Quick Check:

   grep filters lines with 'an' = banana [OK]

Hint: grep filters lines containing the pattern [OK]

Common Mistakes:

• Expecting all words printed on one line
• Ignoring newline characters in echo
• Thinking grep matches partial words only

4. Identify the error in this command: cat file.txt | | grep "error"

medium

A. No error, command runs fine

B. Extra pipe operator causes syntax error

C. grep command is misspelled

D. Missing filename after cat

Solution

1. Step 1: Analyze the pipe usage

   The command has two pipe operators | | consecutively, which is invalid syntax.

2. Step 2: Understand correct pipe usage

   Only one pipe operator should separate commands; extra pipes cause errors.

3. Final Answer:

   Extra pipe operator causes syntax error -> Option B

4. Quick Check:

   Double pipes cause syntax error [OK]

Hint: Use single | between commands, no doubles [OK]

Common Mistakes:

• Ignoring extra pipe and expecting output
• Thinking missing filename is the error
• Assuming grep is misspelled

5. You want to find how many lines in log.txt contain the word "fail" but ignore case. Which command correctly uses pipes to do this?

hard

A. cat log.txt | grep -i fail | wc -l

B. grep fail log.txt | wc -l

C. cat log.txt > grep -i fail | wc -l

D. grep -i fail | cat log.txt | wc -l

Solution

1. Step 1: Use cat to output file content

   cat log.txt outputs the file content to pipe.

2. Step 2: Use grep with -i to ignore case

   grep -i fail filters lines containing "fail" ignoring case.

3. Step 3: Count lines with wc -l

   wc -l counts the filtered lines.

4. Final Answer:

   cat log.txt | grep -i fail | wc -l -> Option A

5. Quick Check:

   Pipe commands to filter and count lines [OK]

Hint: Pipe cat to grep -i then to wc -l for case-insensitive count [OK]

Common Mistakes:

• Using > instead of | breaks pipe chain
• Placing commands in wrong order
• Omitting -i for case-insensitive search