Logging — Recording Events in Bash Scripts¶

Logging is the process of recording important events, actions, warnings, and errors while a script is running. Good logging helps administrators understand what happened during script execution, troubleshoot failures, monitor automation, and maintain production systems. Every Linux administrator, DevOps engineer, Cloud Architect, Platform Engineer, and Site Reliability Engineer (SRE) should implement proper logging in Bash scripts to improve observability and simplify troubleshooting.

Learning Path¶

Linux Mastery → Module 10: Bash Scripting → Lesson 9

Difficulty: Beginner → Intermediate

Reading Time: 80 Minutes

Course Progress

Course: Linux Mastery

Module: Bash Scripting

Lesson: 9 of 11

What You'll Learn¶

After completing this lesson, you'll be able to:

Understand logging
Write log messages
Add timestamps
Use logging levels
Write logs to files
Log command output
Use the logger command
Apply production logging best practices

Prerequisites¶

Complete:

Modules 1–9
Module 10 Lessons 1–8

Why Learn Logging?¶

Imagine a backup script.

Without logging:

tar -czf backup.tar.gz /data

If the backup fails, there is no information about what happened.

With logging:

echo "$(date) Backup started." >> backup.log

tar -czf backup.tar.gz /data

echo "$(date) Backup completed." >> backup.log

Every important event is recorded.

What is Logging?¶

Logging is the process of recording events while a program executes.

Example:

Script Starts

↓

Operations

↓

Success / Failure

↓

Log File

Logs provide valuable information for troubleshooting and auditing.

Why Logging Matters¶

Logging helps:

Troubleshoot failures
Monitor automation
Audit script execution
Track user actions
Diagnose production issues
Improve system reliability

Writing to a Log File¶

Append text to a log file.

echo "Backup Started" >> backup.log

View the log.

cat backup.log

Adding Timestamps¶

Include the current date and time.

echo "$(date) Backup Started" >> backup.log

Example output:

Tue Aug 18 10:30:15 UTC 2026 Backup Started

Custom Timestamp Format¶

echo "$(date '+%Y-%m-%d %H:%M:%S') Backup Started"

Example:

2026-08-18 10:30:15 Backup Started

Logging Levels¶

Production scripts often classify log messages by severity.

Common levels:

INFO
WARNING
ERROR
DEBUG

Example:

INFO

Application Started

WARNING

Disk Usage High

ERROR

Backup Failed

INFO Messages¶

echo "$(date) [INFO] Backup Started" >> backup.log

WARNING Messages¶

echo "$(date) [WARNING] Disk Usage Above 80%" >> backup.log

ERROR Messages¶

echo "$(date) [ERROR] Backup Failed" >> backup.log

DEBUG Messages¶

Useful during development.

echo "$(date) [DEBUG] Variable VALUE=$VALUE" >> backup.log

Logging Command Output¶

Redirect command output.

ls -l >> backup.log

Capture errors.

ls missing-file 2>> backup.log

Capture both standard output and errors.

ls /tmp >> backup.log 2>&1

Logging with tee¶

Display output and save it simultaneously.

echo "Backup Started" | tee -a backup.log

Using the logger Command¶

Send messages to the system log.

logger "Backup Started"

Example:

logger "Deployment Completed Successfully"

Depending on the Linux distribution, these messages are stored in the system logging service (such as syslog or journald).

Create a Logging Function¶

log() {

    echo "$(date '+%Y-%m-%d %H:%M:%S') [INFO] $1" >> backup.log
}

Use it.

log "Backup Started"

log "Backup Completed"

Common Commands¶

Append log.

echo "Message" >> app.log

Timestamp.

date

Display log.

cat app.log

View live log.

tail -f app.log

System log.

logger "Application Started"

Real Production Examples¶

Application deployment.

echo "$(date) [INFO] Deployment Started" >> deploy.log

Log backup.

tar -czf backup.tar.gz /data >> backup.log 2>&1

Restart service.

systemctl restart nginx >> deploy.log 2>&1

Log monitoring.

tail -f deploy.log

Production Perspective¶

Logging is essential for:

CI/CD pipelines
Backup automation
Monitoring scripts
Deployment tools
Kubernetes automation
Cloud infrastructure
Security auditing
Troubleshooting production incidents

Well-structured logs significantly reduce the time required to diagnose production issues.

Hands-on Lab¶

Task 1¶

Create a log file.

echo "Script Started" >> script.log

Task 2¶

Add timestamps.

echo "$(date) Running Backup" >> script.log

Task 3¶

Log an INFO message.

echo "$(date) [INFO] Backup Started" >> script.log

Task 4¶

Log an ERROR message.

echo "$(date) [ERROR] Backup Failed" >> script.log

Task 5¶

Redirect command output.

ls -l >> script.log

Task 6¶

Redirect errors.

ls missing-file >> script.log 2>&1

Task 7¶

Monitor the log.

tail -f script.log

Task 8¶

Create a logging function.

log() {

    echo "$(date '+%F %T') [INFO] $1" >> script.log
}

log "Script Finished"

Command Deep Dive¶

Command	Purpose	Production Example
`echo`	Write log messages	Script logging
`date`	Add timestamps	Event tracking
`tee`	Display and save output	Interactive logging
`logger`	Write to system logs	System auditing
`tail -f`	Monitor logs	Real-time troubleshooting
`2>&1`	Redirect errors	Complete logging

Common Logging Mistakes¶

Mistake	Solution
No timestamps	Always log time
Logging only errors	Log important events
Overwriting logs	Use `>>` to append
No log levels	Classify messages
Logging sensitive information	Never log passwords or secrets

Production Troubleshooting Scenario¶

Scenario

A nightly backup fails, but no one knows why.

Without logging:

Backup Failed

No additional information is available.

Improved script:

log() {

    echo "$(date '+%F %T') [INFO] $1" >> backup.log
}

log "Backup Started"

tar -czf backup.tar.gz /data >> backup.log 2>&1

log "Backup Completed"

The log now records the complete execution history, making troubleshooting much easier.

Best Practices¶

Include timestamps in every log entry.
Use consistent logging levels.
Log significant events and errors.
Redirect command output to log files when appropriate.
Never log passwords, API keys, or other sensitive information.
Rotate or archive large log files regularly.
Use descriptive and consistent log messages.

Common Mistakes¶

❌ Logging without timestamps.

✅ Avoid this mistake: logging without timestamps.

❌ Overwriting logs instead of appending.

✅ Prefer appending rather than overwriting logs.

❌ Logging sensitive credentials.

✅ Avoid this mistake: logging sensitive credentials.

❌ Writing unclear or inconsistent log messages.

✅ Avoid this mistake: writing unclear or inconsistent log messages.

❌ Ignoring log file growth over time.

✅ Always review log file growth over time.

Interview Questions¶

Beginner¶

What is logging?
Why is logging important?
How do you append text to a log file?
How do you view a log file in real time?

Intermediate¶

What are common logging levels?
What does 2>&1 do?
What is the purpose of the logger command?
Why should timestamps be included in logs?

Architect Level¶

How would you design logging for production automation scripts?
How would you prevent sensitive information from appearing in logs?
What logging practices improve troubleshooting in CI/CD pipelines?

Summary¶

In this lesson, you learned:

Logging fundamentals
Writing log messages
Adding timestamps
Logging levels
Redirecting command output
Using tee
Using the logger command
Production logging best practices

Logging is an essential part of production-quality Bash scripting. Well-designed logs provide visibility into script execution, simplify troubleshooting, support auditing, and help administrators quickly identify and resolve issues in automated systems.

Key Takeaways¶

Log important events during script execution.
Include timestamps in every log entry.
Use logging levels such as INFO, WARNING, and ERROR.
Redirect command output and errors to log files.
Avoid logging sensitive information.
Monitor and manage log files as part of routine system administration.

What's Next?¶

Script Best Practices — Writing Professional Bash Scripts

You'll explore:

Writing clean and readable scripts
Consistent naming conventions
Code organization
Documentation and comments
Input validation
Security considerations
Performance optimization
Production scripting standards

By the end of the lesson, you'll be able to write professional, maintainable, secure, and production-ready Bash scripts that follow industry best practices.