Error logging patterns in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Understanding error redirection in Bash

What will be the output and error behavior when running this script?

#!/bin/bash
ls /nonexistentfile > output.log 2> error.log

Bash Scripting

ls /nonexistentfile > output.log 2> error.log

A Error message appears on screen; output.log and error.log are both empty

B Error message appears on screen; output.log contains error message; error.log is empty

C No output on screen; output.log is empty; error.log contains the error message

D No output on screen; output.log contains error message; error.log is empty

💡 Hint

Remember that '2>' redirects standard error and '>' redirects standard output.

The command redirects standard output to output.log and standard error to error.log. Since the file does not exist, ls produces an error message sent to standard error, so error.log contains the message, output.log is empty, and nothing appears on screen.

💻 Command Output

intermediate

Effect of combined output and error redirection

What will be the content of combined.log after running this command?

grep 'pattern' file.txt &> combined.log

Assume file.txt does not exist.

Bash Scripting

grep 'pattern' file.txt &> combined.log

A combined.log contains the error message about missing file

B Error message appears on screen; combined.log is empty

C combined.log is empty

D combined.log contains the output of grep if file.txt existed

💡 Hint

The '&>' operator redirects both stdout and stderr to the same file.

Since file.txt does not exist, grep outputs an error message to standard error. The '&>' operator redirects both standard output and standard error to combined.log, so the error message is saved there.

❓ Troubleshoot

advanced

Diagnosing silent failures in a script

A script runs a command that fails, but no error message appears on the terminal or in the log file. Which of these is the most likely cause?

A The command's error output is redirected to /dev/null

B The command's output is redirected to a file

C The script uses 'set -e' to stop on errors

D The command is run with sudo

💡 Hint

Think about where error messages go if redirected to a special device.

Redirecting error output to /dev/null discards error messages, causing silent failures with no visible error messages on screen or in logs.

✅ Best Practice

advanced

Best practice for logging errors in a Bash script

Which option shows the best way to log errors to a file while keeping normal output visible on the terminal?

A command > output.log 2>&1

B command 2> error.log

C command &> all.log

D command > output.log 2> error.log

💡 Hint

You want errors in a file but normal output on screen.

Redirecting only standard error to a file (2> error.log) keeps normal output on the terminal and logs errors separately, which is a good practice for error logging.

🔀 Workflow

expert

Order of commands to capture and handle errors in a deployment script

Arrange these steps in the correct order to ensure errors are logged and the script exits on failure:
1. Redirect errors to a log file
2. Run deployment command
3. Check command exit status
4. Exit script if error detected

A 1,3,2,4

B 2,3,1,4

C 2,1,3,4

D 1,2,3,4

💡 Hint

Think about setting up logging before running commands and then checking results.

First, set up error redirection so errors are captured. Then run the command. Next, check if it succeeded by examining the exit status. Finally, exit the script if an error occurred.

Practice

1. What does the 2> operator do in a bash script?

easy

A. Redirects error messages to a file

B. Redirects standard output to a file

C. Appends output to a file

D. Runs a command in the background

Solution

1. Step 1: Understand bash output streams

   Bash has two main output streams: standard output (1) and standard error (2).

2. Step 2: Identify the meaning of 2>

   The operator 2> redirects the standard error stream to a file or location.

3. Final Answer:

   Redirects error messages to a file -> Option A

4. Quick Check:

   2> means error redirect [OK]

Hint: Remember 2 is error stream, so 2> redirects errors [OK]

Common Mistakes:

• Confusing 2> with > which redirects standard output
• Thinking 2> appends instead of overwriting
• Assuming 2> runs commands in background

2. Which of the following is the correct syntax to redirect only error messages (overwriting the file) to a file named error.log?

easy

A. command > error.log

B. command 2>> error.log

C. command &> error.log

D. command 2> error.log

Solution

1. Step 1: Understand redirection operators

   2> redirects standard error and overwrites the file; 2>> appends errors.

2. Step 2: Match requirement to syntax

   We want to redirect only errors and overwrite the file, so 2> error.log is correct.

3. Final Answer:

   command 2> error.log -> Option D

4. Quick Check:

   Use 2> for error overwrite [OK]

Hint: Use 2> to overwrite error file, 2>> to append [OK]

Common Mistakes:

• Using > which redirects standard output, not errors
• Using &> which redirects both output and errors
• Confusing append (>>) with overwrite (>)

3. What will be the output of the following command if script.sh prints "Success" to standard output and "Error occurred" to standard error?

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

medium

A. "Success" is saved in output.log, "Error occurred" in error.log

B. Both messages saved in output.log

C. Both messages appear on the terminal

D. "Error occurred" saved in output.log, "Success" in error.log

Solution

1. Step 1: Analyze redirection operators

   > output.log redirects standard output to output.log, 2> error.log redirects standard error to error.log.

2. Step 2: Match outputs to files

   "Success" goes to standard output, so saved in output.log; "Error occurred" goes to standard error, saved in error.log.

3. Final Answer:

   "Success" is saved in output.log, "Error occurred" in error.log -> Option A

4. Quick Check:

   Separate output and error files [OK]

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

Common Mistakes:

• Assuming both outputs go to same file
• Confusing 2> with >
• Expecting output on terminal despite redirection

4. Identify the error in this command intended to redirect errors to errors.log:

command 2>2 errors.log

medium

A. Should use &> instead of 2>

B. Missing space between command and 2>

C. Extra '2' causes errors to be redirected to file named '2' instead of 'errors.log'

D. No error, command is correct

Solution

1. Step 1: Examine the redirection syntax

   The correct syntax to redirect errors is 2> filename. Here, 2>2 errors.log redirects stderr to file 2, treating errors.log as a command argument.

2. Step 2: Identify the cause of error

   The extra '2' makes the filename 2 instead of errors.log.

3. Final Answer:

   Extra '2' causes errors to be redirected to file named '2' instead of 'errors.log' -> Option C

4. Quick Check:

   2> must be followed by correct filename [OK]

Hint: Check redirection target filename after 2> [OK]

Common Mistakes:

• Adding extra numbers or symbols in filename position
• Confusing &> with 2>
• Missing spaces causing parsing errors

5. You want to run a script and save both standard output and error messages into separate files, but also keep the error messages appended (not overwritten). Which command achieves this?

hard

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

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

C. ./script.sh &> output.log

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

Solution

1. Step 1: Understand output and error redirection

   > overwrites standard output, 2>> appends standard error to a file.

2. Step 2: Match requirement to command

   We want output overwritten in output.log and errors appended in error.log, so ./script.sh > output.log 2>> error.log fits.

3. Final Answer:

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

4. Quick Check:

   Use 2>> to append errors [OK]

Hint: Use 2>> to append errors, > to overwrite output [OK]

Common Mistakes:

• Using 2> which overwrites error file
• Using &> which merges output and errors
• Appending output but overwriting errors