File descriptors and redirection in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this redirection command?

Consider the following bash command:

echo 'Hello' 1>&2

What will be the output when you run this command in a terminal?

Bash Scripting

echo 'Hello' 1>&2

A Hello printed to standard error (screen)

B Hello printed to standard output (screen)

C No output, command runs silently

D Syntax error due to incorrect redirection

💡 Hint

Remember that 1 is standard output and 2 is standard error.

The command redirects standard output (file descriptor 1) to standard error (file descriptor 2). So 'Hello' is printed to standard error, which usually appears on the screen but is a different stream than standard output.

💻 Command Output

intermediate

What happens when redirecting both stdout and stderr to a file?

Given this command:

ls /nonexistent /tmp > output.txt 2>&1

What will be the content of output.txt after running it?

Bash Scripting

ls /nonexistent /tmp > output.txt 2>&1

A Only the listing of /tmp directory is saved in output.txt

B Both the error message and the listing of /tmp are saved in output.txt

C Only the error message about /nonexistent is saved in output.txt

D output.txt remains empty

💡 Hint

Think about what 2>&1 means after redirecting stdout.

The command first redirects stdout (1) to output.txt, then redirects stderr (2) to wherever stdout is currently going (output.txt). So both outputs go into output.txt.

📝 Syntax

advanced

Which command correctly appends stderr to a file without affecting stdout?

You want to append only the error messages (stderr) of a command to a file named errors.log, while keeping the standard output visible on the screen. Which command does this correctly?

A command >> errors.log 2>&1

B command > errors.log 2>> errors.log

C command 2> errors.log >> output.log

D command 2>> errors.log

💡 Hint

Appending means using >> and only redirect stderr (2).

Option D appends stderr (2) to errors.log and leaves stdout untouched, so stdout goes to the screen. Option D redirects both stdout and stderr to errors.log. Option D redirects stderr to errors.log but overwrites it, and stdout to output.log. Option D overwrites errors.log with stdout and appends stderr, mixing outputs.

🔧 Debug

advanced

Why does this redirection command produce unexpected output?

Look at this command:

command 2>&1 > output.txt

It produces unexpected output or error. Why?

A Because output.txt does not exist

B Because the order of redirections is correct and should not fail

C Because stderr is redirected to stdout before stdout is redirected to the file

D Because 2>&1 is invalid syntax

💡 Hint

Order of redirections matters in bash.

In bash, redirections are processed left to right. Here, stderr (2) is redirected to where stdout (1) currently points (usually the screen). Then stdout is redirected to output.txt. So stderr still points to the screen, not the file. This causes unexpected output but no syntax error.

🚀 Application

expert

How to redirect stdout to a file and stderr to another file simultaneously?

You want to run a command so that its standard output goes to out.log and its standard error goes to err.log. Which command achieves this correctly?

A command > out.log 2>&1 > err.log

B command 2> err.log > out.log

C command > out.log 2> err.log

D command > out.log 2>&1

💡 Hint

Use separate redirections for stdout and stderr.

Option C redirects stdout to out.log and stderr to err.log separately. Option B also works but order does not matter here. Option A redirects stderr to stdout and then stdout to err.log, mixing outputs. Option D redirects stderr to stdout, so both go to out.log.

Practice

1. Which file descriptor number represents the standard error stream in bash?

easy

A. 3

B. 1

C. 2

D. 0

Solution

1. Step 1: Recall standard file descriptors

   Standard input is 0, standard output is 1, and standard error is 2.

2. Step 2: Identify the error stream descriptor

   The error stream is always file descriptor 2 in bash.

3. Final Answer:

   2 -> Option C

4. Quick Check:

   Standard error = 2 [OK]

Hint: Remember: 0=input, 1=output, 2=error [OK]

Common Mistakes:

• Confusing standard output (1) with error (2)
• Thinking input (0) is error
• Using non-standard descriptor numbers

2. Which of the following is the correct syntax to redirect standard output to a file named output.txt in bash?

easy

A. command >> output.txt 2>&1

B. command < output.txt

C. command 2> output.txt

D. command > output.txt

Solution

1. Step 1: Understand output redirection syntax

   Using > redirects standard output (file descriptor 1) to a file, overwriting it.

2. Step 2: Match syntax to redirect output only

   command > output.txt uses > correctly to redirect output to output.txt.

3. Final Answer:

   command > output.txt -> Option D

4. Quick Check:

   Output redirection uses > [OK]

Hint: Use > for output, < for input redirection [OK]

Common Mistakes:

• Using < instead of > for output
• Redirecting error (2>) instead of output
• Appending (>>) when overwrite is needed

3. What will be the output of the following command if file.txt does not exist?

cat file.txt > output.txt 2>&1

medium

A. Redirects both output and error messages to output.txt

B. Creates an empty output.txt file with no error message

C. Outputs the error message to the terminal and creates output.txt

D. Throws a syntax error and does nothing

Solution

1. Step 1: Understand redirection 2>&1

   This redirects standard error (2) to the same place as standard output (1).

2. Step 2: Analyze command behavior when file missing

   cat tries to read file.txt, fails, and sends error to stderr, which is redirected to output.txt along with stdout.

3. Final Answer:

   Redirects both output and error messages to output.txt -> Option A

4. Quick Check:

   2>&1 merges error into output [OK]

Hint: 2>&1 sends errors to the same place as output [OK]

Common Mistakes:

• Thinking error shows on terminal
• Assuming output.txt stays empty
• Confusing 2>&1 with separate redirections

4. Identify the error in this command that attempts to redirect only standard error to error.log:

ls -l > error.log 2>

medium

A. Output redirection > should be 1>

B. Missing filename after 2> for error redirection

C. Should use 2>&1 instead of 2>

D. Command syntax is correct

Solution

1. Step 1: Check error redirection syntax

   Redirection 2> must be followed by a filename to redirect standard error.

2. Step 2: Identify missing filename

   The command ends with 2> but no file is specified, causing a syntax error.

3. Final Answer:

   Missing filename after 2> for error redirection -> Option B

4. Quick Check:

   2> needs a target file [OK]

Hint: Always specify a file after 2> for error redirection [OK]

Common Mistakes:

• Leaving out filename after 2>
• Confusing 2> with 2>&1
• Thinking > needs to be 1>

5. You want to run a script that outputs normal messages to output.log and error messages to error.log. Which command correctly achieves this?

hard

A. ./script.sh > output.log 2> error.log

B. ./script.sh 2>&1 > output.log error.log

C. ./script.sh > output.log 2>&1 error.log

D. ./script.sh > output.log &> error.log

Solution

1. Step 1: Understand separate redirection for output and error

   To send stdout and stderr to different files, use > for stdout and 2> for stderr separately.

2. Step 2: Analyze each option

   ./script.sh > output.log 2> error.log correctly redirects stdout to output.log and stderr to error.log. Others mix or misuse redirection syntax.

3. Final Answer:

   ./script.sh > output.log 2> error.log -> Option A

4. Quick Check:

   Separate > and 2> for output and error [OK]

Hint: Use > for output, 2> for errors to separate files [OK]

Common Mistakes:

• Using &> redirects both to one file
• Mixing order of redirections incorrectly
• Omitting file names after redirection operators