stderr redirection (2>, 2>>) in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this command?

Consider the command:

ls /nonexistent 2> error.log; cat error.log

What will be the output of cat error.log?

Linux CLI

ls /nonexistent 2> error.log; cat error.log

A ls: cannot access '/nonexistent': No such file or directory

B bash: ls: command not found

C

ls: cannot access '/nonexistent': No such file or directory
ls: cannot access '/nonexistent': No such file or directory

D No output (empty file)

💡 Hint

The 2> operator redirects error messages to a file.

The command tries to list a non-existing directory. The error message is redirected to 'error.log'. Using 'cat error.log' shows the error message inside the file.

💻 Command Output

intermediate

What happens when appending stderr with 2>>?

Given the commands:

echo 'First error' 1>&2 2> error.log
 echo 'Second error' 1>&2 2>> error.log
 cat error.log

What will be the output of cat error.log?

Linux CLI

echo 'First error' 1>&2 2> error.log
 echo 'Second error' 1>&2 2>> error.log
 cat error.log

A Second error

B No output

C First error

D

First error
Second error

💡 Hint

2> overwrites, 2>> appends to the file.

The first echo sends 'First error' to stderr, redirected to error.log (overwrites). The second echo appends 'Second error' to error.log. So both lines appear in the file.

🔧 Debug

advanced

Why does this command not redirect stderr as expected?

You run:

grep 'pattern' file.txt | wc -l 2> errors.log

But errors.log remains empty even if file.txt does not exist. Why?

Linux CLI

grep 'pattern' file.txt | wc -l 2> errors.log

A Because wc -l overwrites errors.log

B Because 2> redirects stderr of wc, not grep

C Because file.txt exists and has no errors

D Because only stdout of grep is piped, stderr is not redirected properly

💡 Hint

Think about which command's stderr is redirected.

In shell pipelines, a redirection like 2> after a pipe applies to the preceding command (wc -l here), not the first command (grep). Grep's stderr goes to the terminal, and wc -l produces no stderr, leaving errors.log empty.

📝 Syntax

advanced

Which command correctly appends stderr to a file?

Choose the command that appends stderr output to errors.log without overwriting it.

A command 2>> errors.log

B command 2> errors.log

C command >> 2 errors.log

D command 2> > errors.log

💡 Hint

Appending means adding to the end of the file.

2>> appends stderr to the file. 2> overwrites. The other options are invalid syntax.

🚀 Application

expert

How to redirect both stdout and stderr to different files simultaneously?

You want to run mycommand and save its standard output to out.log and its error output to err.log. Which command does this correctly?

A mycommand > out.log 2>> err.log

B mycommand 2> err.log > out.log

C mycommand > out.log 2> err.log

D mycommand 2>> err.log > out.log

💡 Hint

Order of redirections matters less here, but appending vs overwriting does.

Both A and B redirect stdout and stderr to separate files correctly. However, the order does not affect the result here. Options C and D append stderr instead of overwriting. The question asks for saving outputs, not appending. So A is the best answer.