Log rotation script in Bash Scripting - Step-by-Step Execution

Concept Flow - Log rotation script

Start script

↓

Check if log file exists

Yes↓

Check log file size > limit?

No→Exit script

Yes↓

Rename current log file to backup

↓

Create new empty log file

↓

Exit script

The script checks if the log file exists and is too big, then renames it and creates a new empty log file.

Execution Sample

Bash Scripting

#!/bin/bash
LOGFILE="app.log"
MAXSIZE=1000
if [ -f "$LOGFILE" ] && [ $(stat -c%s "$LOGFILE") -gt $MAXSIZE ]; then
  mv "$LOGFILE" "$LOGFILE".1
  touch "$LOGFILE"
fi

This script checks if 'app.log' exists and is larger than 1000 bytes, then renames it to 'app.log.1' and creates a new empty 'app.log'.

Execution Table

Step	Check	Condition Result	Action	Output
1	Does 'app.log' exist?	Yes	Proceed to size check
2	Is size of 'app.log' > 1000 bytes?	Yes	Rename 'app.log' to 'app.log.1'
3	Create new empty 'app.log'	N/A	touch 'app.log'
4	End of script	N/A	Exit

💡 Script ends after creating new empty log file.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
LOGFILE	"app.log"	"app.log"	"app.log"	"app.log"	"app.log"
MAXSIZE	1000	1000	1000	1000	1000
app.log file	exists, size > 1000 bytes	exists, size > 1000 bytes	renamed to app.log.1	new empty app.log created	new empty app.log

Key Moments - 3 Insights

Why do we check if the log file exists before checking its size?

Because checking the size of a non-existent file would cause an error. The execution_table row 1 shows we first confirm the file exists.

What happens if the log file size is not greater than the limit?

The script skips renaming and creating a new file and exits immediately, as shown in execution_table row 2 where condition would be No.

Why do we use 'touch' after renaming the log file?

To create a new empty log file so the application can continue logging without errors. This is shown in execution_table row 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the action taken at step 2?

A Rename 'app.log' to 'app.log.1'

B Create new empty 'app.log'

C Exit script

D Check if log file exists

💡 Hint

Refer to execution_table row 2 under 'Action'

At which step does the script create a new empty log file?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at execution_table row 3 under 'Action'

If the log file does not exist, what will the script do?

A Rename the log file

B Exit without any action

C Create a new empty log file

D Throw an error

💡 Hint

Check execution_table row 1 condition result and flow

Concept Snapshot

Log rotation script in bash:
- Check if log file exists
- Check if file size > limit
- If yes, rename old log file
- Create new empty log file
- Exit script
Use 'stat -c%s' for size and 'mv' to rename.

Full Transcript

This bash log rotation script starts by checking if the log file 'app.log' exists. If it does, it checks whether the file size is greater than the maximum allowed size (1000 bytes). If the file is too large, the script renames the current log file to 'app.log.1' to keep it as a backup. Then it creates a new empty 'app.log' file so the application can continue logging without interruption. If the file does not exist or is not too large, the script exits without making changes. This process helps manage log files by preventing them from growing too large and consuming too much disk space.

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