Log rotation script in Bash Scripting

Introduction

Log rotation helps keep log files from getting too big and uses less disk space. It also makes it easier to find recent logs.

When your application creates large log files that grow over time.

To keep logs organized by date or size for easier reading and backup.

To automatically delete old logs and save disk space.

When you want to avoid manual cleanup of log files.

To prepare logs for archiving or sending to another system.

Syntax

Bash Scripting

#!/bin/bash

LOG_DIR="/path/to/logs"
MAX_SIZE=10485760  # 10 MB in bytes

for LOG_FILE in "$LOG_DIR"/*.log; do
  if [ -f "$LOG_FILE" ]; then
    FILE_SIZE=$(stat -c%s "$LOG_FILE")
    if [ "$FILE_SIZE" -ge "$MAX_SIZE" ]; then
      mv "$LOG_FILE" "$LOG_FILE.$(date +%Y%m%d%H%M%S)"
      touch "$LOG_FILE"
    fi
  fi
done

Use #!/bin/bash at the top to specify the script runs in bash.

The script checks each .log file size and renames it if too big, then creates a new empty log.

Examples

Set log directory and max size to 5 MB for rotation.

Bash Scripting

# Rotate logs larger than 5 MB
LOG_DIR="/var/log/myapp"
MAX_SIZE=5242880

Rename log file by adding the current date in YYYY-MM-DD format.

Bash Scripting

mv "$LOG_FILE" "$LOG_FILE.$(date +%Y-%m-%d)"

Create a new empty log file after rotating the old one.

Bash Scripting

touch "$LOG_FILE"

Sample Program

This script creates a sample log file, then checks if it is bigger than 1 KB. If yes, it renames the log with a timestamp and creates a new empty log file.

Bash Scripting

#!/bin/bash

LOG_DIR="./logs"
MAX_SIZE=1024  # 1 KB for demo

mkdir -p "$LOG_DIR"

# Create a sample log file with some content
LOG_FILE="$LOG_DIR/sample.log"
echo "This is a test log line." > "$LOG_FILE"
for i in {1..50}; do echo "Line $i" >> "$LOG_FILE"; done

# Log rotation script
for LOG_FILE in "$LOG_DIR"/*.log; do
  if [ -f "$LOG_FILE" ]; then
    FILE_SIZE=$(stat -c%s "$LOG_FILE")
    if [ "$FILE_SIZE" -ge "$MAX_SIZE" ]; then
      mv "$LOG_FILE" "$LOG_FILE.$(date +%Y%m%d%H%M%S)"
      touch "$LOG_FILE"
      echo "Rotated $LOG_FILE"
    else
      echo "$LOG_FILE size is under limit"
    fi
  fi
done

Important Notes

Make sure the script has execute permission: chmod +x script.sh

Adjust MAX_SIZE to your desired log file size limit in bytes.

Use full paths to avoid confusion when running the script from different folders.

Summary

Log rotation scripts help manage log file sizes automatically.

They rename large logs with timestamps and create new empty logs.

This keeps logs organized and prevents disk space issues.

Practice

1. What is the main purpose of a log rotation script in bash?

easy

A. To archive old log files and start new ones to prevent disk space issues

B. To delete all log files permanently

C. To create new log files without removing old ones

D. To convert log files into executable scripts

Solution

1. Step 1: Understand log rotation purpose

   Log rotation scripts archive old logs and create fresh logs to avoid disk space overflow.

2. Step 2: Compare options

   Only To archive old log files and start new ones to prevent disk space issues describes archiving old logs and starting new ones, which matches the purpose.

3. Final Answer:

   To archive old log files and start new ones to prevent disk space issues -> Option A

4. Quick Check:

   Log rotation = archive old + new logs [OK]

Hint: Log rotation means archiving old logs and starting fresh ones [OK]

Common Mistakes:

• Thinking log rotation deletes all logs
• Believing it only creates new logs without archiving
• Confusing log rotation with script conversion

2. Which of the following bash commands correctly appends the current date to a log filename for rotation?

easy

A. mv logfile.log logfile-date.log

B. mv logfile.log logfile-$(date +%Y-%m-%d).log

C. mv logfile.log logfile-$(date +%H:%M:%S).log

D. mv logfile.log logfile-$(date +%Y/%m/%d).log

Solution

1. Step 1: Understand date format for filenames

   Using $(date +%Y-%m-%d) produces a safe filename format like 2024-06-01.

2. Step 2: Check each option

   mv logfile.log logfile-$(date +%Y-%m-%d).log uses dashes which are safe in filenames. mv logfile.log logfile-date.log uses literal 'date', not dynamic. mv logfile.log logfile-$(date +%H:%M:%S).log uses colons which are invalid in filenames on many systems. mv logfile.log logfile-$(date +%Y/%m/%d).log uses slashes which are directory separators, invalid in filenames.

3. Final Answer:

   mv logfile.log logfile-$(date +%Y-%m-%d).log -> Option B

4. Quick Check:

   Date format with dashes = safe filename [OK]

Hint: Use date with dashes, avoid colons or slashes in filenames [OK]

Common Mistakes:

• Using colons or slashes in filenames
• Not using command substitution for date
• Using literal text instead of date command

3. Given this snippet in a log rotation script:

gzip logfile-$(date +%Y-%m-%d).log
ls logfile-*.log.gz | wc -l

What will the wc -l command output if there are 3 compressed log files matching the pattern?

medium

A. Error: No such file or directory

B. 0

C. 1

D. 3

Solution

1. Step 1: Understand the commands

   The gzip command compresses a log file, creating a file ending with .gz. The ls logfile-*.log.gz lists all compressed log files matching the pattern.

2. Step 2: Count matching files

   If there are 3 compressed log files, ls outputs 3 lines. Piping to wc -l counts these lines, so output is 3.

3. Final Answer:

   3 -> Option D

4. Quick Check:

   Count of compressed logs = 3 [OK]

Hint: Count lines from ls output to get number of files [OK]

Common Mistakes:

• Assuming gzip outputs count
• Thinking wc -l counts characters
• Expecting error if files exist

4. Identify the error in this log rotation snippet:

mv logfile.log logfile-$(date +%Y-%m-%d).log
gzip logfile-$(date +%Y-%m-%d).log
rm logfile.log

medium

A. The date format uses slashes which are invalid

B. The gzip command is missing the -d option

C. The rm logfile.log command deletes the original log after moving, causing error

D. The mv command should use cp instead

Solution

1. Step 1: Analyze the commands order

   The mv command renames logfile.log to logfile-date.log, so logfile.log no longer exists after this.

2. Step 2: Check the rm command

   After moving, rm logfile.log tries to delete a file that was already moved, causing an error or no effect.

3. Final Answer:

   The rm logfile.log command deletes the original log after moving, causing error -> Option C

4. Quick Check:

   Deleting moved file causes error [OK]

Hint: Don't delete a file after moving it; it's already renamed [OK]

Common Mistakes:

• Deleting a file after moving it
• Confusing gzip options
• Using invalid date formats

5. You want to write a log rotation script that keeps only the last 5 compressed log backups and deletes older ones. Which bash command snippet correctly deletes older backups?

hard

A. ls -1t logfile-*.log.gz | tail -n +6 | xargs rm -f

B. ls -1t logfile-*.log.gz | head -n +6 | xargs rm -f

C. ls -1 logfile-*.log.gz | tail -n 5 | xargs rm -f

D. ls logfile-*.log.gz | grep -v 'logfile-*.log.gz' | xargs rm -f

Solution

1. Step 1: Understand the command parts

   ls -1t lists files sorted by modification time, newest first. tail -n +6 outputs lines starting from line 6, i.e., files older than the 5 newest. xargs rm -f deletes those files.

2. Step 2: Check other options

   ls -1t logfile-*.log.gz | head -n +6 | xargs rm -f deletes the first 6 newest files (wrong). ls -1 logfile-*.log.gz | tail -n 5 | xargs rm -f deletes last 5 files by name order, not time. ls logfile-*.log.gz | grep -v 'logfile-*.log.gz' | xargs rm -f uses grep incorrectly and does not select older files.

3. Final Answer:

   ls -1t logfile-*.log.gz | tail -n +6 | xargs rm -f -> Option A

4. Quick Check:

   Keep 5 newest, delete older = tail from 6th [OK]

Hint: Use ls -t and tail +6 to delete older files beyond 5 [OK]

Common Mistakes:

• Using head instead of tail to select files
• Deleting newest files instead of oldest
• Not sorting files by modification time