File descriptors and redirection in Bash Scripting - Mini Project: Build & Apply

File Descriptors and Redirection in Bash

📖 Scenario: You are working on a small bash script to manage output messages. You want to separate normal messages from error messages by using file descriptors and redirection.

🎯 Goal: Build a bash script that writes a normal message to standard output and an error message to standard error using file descriptors and redirection.

📋 What You'll Learn

Create a file descriptor for error messages

Redirect error messages to a separate file

Write a normal message to standard output

Write an error message to the error file using the file descriptor

Display the contents of both output and error files

💡 Why This Matters

🌍 Real World

Separating normal output and error messages helps in debugging scripts and logging important information separately.

💼 Career

Understanding file descriptors and redirection is essential for writing robust shell scripts used in system administration and automation tasks.

1

Create output and error files

Create two empty files called output.txt and error.txt using the touch command.

Bash Scripting

# Create the files output.txt and error.txt
# Your code here

Hint

Use the touch command to create empty files.

2

Redirect file descriptor 3 to error.txt

Add a command to redirect file descriptor 3 to write to the file error.txt.

Bash Scripting

touch output.txt error.txt
# Redirect file descriptor 3 to error.txt
# Your code here

Hint

Use exec 3>error.txt to open file descriptor 3 for writing to error.txt.

3

Write messages using redirection

Write the message "This is a normal message." to output.txt using standard output redirection, and write the message "This is an error message." to error.txt using file descriptor 3.

Bash Scripting

touch output.txt error.txt
exec 3>error.txt
# Write normal message to output.txt
# Write error message to error.txt using fd 3
# Your code here

Hint

Use echo "message" > output.txt for normal output and echo "message" >&3 to write to file descriptor 3.

4

Display contents of output and error files

Print the contents of output.txt and error.txt using the cat command to show the messages stored in each file.

Bash Scripting

touch output.txt error.txt
exec 3>error.txt
 echo "This is a normal message." > output.txt
 echo "This is an error message." >&3
# Print contents of output.txt and error.txt
# Your code here

Hint

Use cat output.txt and cat error.txt to display the contents of the files.

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