/dev/null for discarding output in Linux CLI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is /dev/null in Linux?

/dev/null is a special file that discards all data written to it. It acts like a black hole for output.

beginner

How do you discard the standard output of a command in Linux?

By redirecting the output to /dev/null using command > /dev/null.

intermediate

What does this command do? ls > /dev/null 2>&1

It runs ls and discards both standard output and standard error by redirecting them to /dev/null.

beginner

Why would you want to use /dev/null in a script?

To ignore unwanted output or errors so they don’t clutter the screen or logs.

intermediate

What is the difference between command > /dev/null and command &> /dev/null?

command > /dev/null discards only standard output.
command &> /dev/null discards both standard output and standard error.

What happens when you redirect output to /dev/null?

A The output is discarded and not shown

B The output is saved to a file

C The output is displayed on the screen

D The output is sent to the printer

Redirecting to /dev/null discards the output, so it is not saved or shown anywhere.

How do you discard both standard output and standard error of a command?

A command > /dev/null

B command 2> /dev/null

C command < /dev/null

D command &> /dev/null

> redirects standard output, 2> redirects standard error, and >> appends. &> redirects both output and error.

Which of these commands discards only the error messages?

A command 2> /dev/null

B command > /dev/null 2>&1

C command &> /dev/null

D command > /dev/null

2> /dev/null redirects only standard error to /dev/null.

Why might you want to discard output in a script?

A To save output for later

B To print output to the screen

C To prevent cluttering the screen or logs

D To send output to another program

Discarding output helps keep the screen or logs clean from unnecessary messages.

What does /dev/null behave like?

A A printer device

B A black hole that swallows data

C A file that stores data permanently

D A network socket

/dev/null acts like a black hole that swallows any data sent to it.

Explain how to use /dev/null to discard both output and errors from a command.

Think about redirecting both outputs using > and 2>&1 or &>.

Describe a real-life situation where discarding output with /dev/null is helpful.

Consider running a command repeatedly in a script where output is not needed.

Practice

1. What does redirecting output to /dev/null do in Linux?

easy

A. It displays the output on the screen

B. It saves the output to a file named /dev/null

C. It discards the output so nothing is shown or saved

D. It sends the output to the printer

Solution

1. Step 1: Understand what /dev/null is

   /dev/null is a special file that discards anything written to it.

2. Step 2: Effect of redirecting output

   Redirecting output to /dev/null means the output is thrown away and not shown or saved.

3. Final Answer:

   It discards the output so nothing is shown or saved -> Option C

4. Quick Check:

   /dev/null discards output = D [OK]

Hint: Remember: /dev/null is like a black hole for output [OK]

Common Mistakes:

• Thinking /dev/null saves output to a file
• Confusing /dev/null with a display device
• Assuming output is printed or logged

2. Which of the following commands correctly discards only the standard error output?

easy

A. command > /dev/null

B. command 2> /dev/null

C. command > /dev/null 2>&1

D. command 1> /dev/null

Solution

1. Step 1: Identify standard error redirection syntax

   Standard error is file descriptor 2, so redirecting errors uses '2>'.

2. Step 2: Match command to discard only errors

   Using 'command 2> /dev/null' discards error output only, leaving normal output visible.

3. Final Answer:

   command 2> /dev/null -> Option B

4. Quick Check:

   2> /dev/null hides errors only = A [OK]

Hint: Use '2>' to redirect errors, '>' for normal output [OK]

Common Mistakes:

• Using '>' instead of '2>' to hide errors
• Confusing '2>&1' with error-only redirection
• Redirecting both outputs when only errors needed

3. What will be the output of this command?

ls /nonexistent 2> /dev/null

medium

A. Shows no output at all

B. Shows an error message about missing directory

C. Lists files in /nonexistent directory

D. Creates the /nonexistent directory

Solution

1. Step 1: Understand the command and redirection

   ls /nonexistent tries to list a directory that does not exist, producing an error on standard error.

2. Step 2: Effect of '2> /dev/null'

   The error message is redirected to /dev/null, so it is discarded and not shown.

3. Final Answer:

   Shows no output at all -> Option A

4. Quick Check:

   Error hidden by 2> /dev/null = C [OK]

Hint: Errors go to 2>, normal output to >; here errors hidden [OK]

Common Mistakes:

• Expecting error message to appear
• Thinking directory is created
• Confusing standard output with error output

4. You want to discard both standard output and error from a command, but your script still shows errors. What is wrong with this command?

command > /dev/null 2>1

medium

A. The command will only discard standard output

B. The command does not discard any output

C. The output is redirected to the wrong file

D. The error redirection syntax is incorrect; it should be 2>&1

Solution

1. Step 1: Analyze the error redirection syntax

   The command uses '2>1' which redirects standard error to a file named '1', not to standard output.

2. Step 2: Correct syntax for redirecting error to output

   The correct syntax to redirect standard error to the same place as standard output is '2>&1'.

3. Final Answer:

   The error redirection syntax is incorrect; it should be 2>&1 -> Option D

4. Quick Check:

   Use 2>&1 to redirect errors to output [OK]

Hint: Use '2>&1' to combine error with output redirection [OK]

Common Mistakes:

• Writing '2>1' instead of '2>&1'
• Forgetting '&' in error redirection
• Assuming '2>1' redirects errors to output

5. You want to run a script that produces both output and errors, but you want to log only errors to a file and discard normal output. Which command achieves this?

hard

A. script.sh > /dev/null 2> errors.log

B. script.sh > errors.log 2> /dev/null

C. script.sh > /dev/null 2>&1

D. script.sh 2> /dev/null > errors.log

Solution

1. Step 1: Understand output and error redirection

   '>' redirects standard output, '2>' redirects standard error.

2. Step 2: Match requirement to command

   To discard normal output, redirect it to /dev/null. To save errors, redirect standard error to errors.log.

3. Final Answer:

   script.sh > /dev/null 2> errors.log -> Option A

4. Quick Check:

   Output discarded, errors logged = B [OK]

Hint: Redirect output to /dev/null, errors to file with 2> [OK]

Common Mistakes:

• Swapping output and error redirections
• Redirecting both outputs to /dev/null
• Using wrong order causing errors to be discarded