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.