Disk space monitoring script in Bash Scripting - Step-by-Step Execution

Concept Flow - Disk space monitoring script

Start Script

↓

Check disk usage with df

↓

Parse output to get usage %

↓

Compare usage with threshold

↓

Send alert

↓

End Script

The script checks disk usage, compares it to a limit, and sends an alert if usage is too high.

Execution Sample

Bash Scripting

#!/bin/bash
THRESHOLD=80
USAGE=$(df / | tail -1 | awk '{print $5}' | sed 's/%//')
if [ $USAGE -gt $THRESHOLD ]; then
  echo "Disk usage is above threshold!"
fi

This script checks the root disk usage and prints a warning if usage exceeds 80%.

Execution Table

Step	Command/Action	Value/Output	Condition	Branch Taken
1	Set THRESHOLD=80	THRESHOLD=80	N/A	N/A
2	Run df / | tail -1 | awk '{print $5}' | sed 's/%//'	USAGE=85	N/A	N/A
3	Check if USAGE > THRESHOLD	85 > 80	True	Yes branch
4	Print alert message	"Disk usage is above threshold!"	N/A	N/A
5	End script	Script ends	N/A	N/A

💡 Script ends after printing alert because disk usage is above threshold.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
THRESHOLD	unset	80	80	80	80
USAGE	unset	unset	85	85	85

Key Moments - 3 Insights

Why do we remove the % sign from the disk usage output?

The disk usage output includes a % sign (like '85%'), which is not a number. We remove it to compare usage as a number in the condition (see execution_table step 2).

What happens if disk usage is exactly equal to the threshold?

The condition uses 'greater than' (gt), so if usage equals threshold, the alert does NOT trigger (see execution_table step 3).

Why do we use 'tail -1' in the command?

'df' outputs a header line and then data lines. 'tail -1' gets the last line which contains the actual disk usage info (see execution_table step 2).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of USAGE after step 2?

A 80

B 75

C 85

D 90

💡 Hint

Check the 'Value/Output' column in step 2 of the execution_table.

At which step does the script decide to print the alert message?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look for the step where the action is 'Print alert message' in the execution_table.

If the disk usage was 75%, what branch would the script take at step 3?

A No branch (do nothing)

B Yes branch (print alert)

C Error branch

D Loop back to step 2

💡 Hint

Refer to the 'Condition' and 'Branch Taken' columns in step 3 of the execution_table.

Concept Snapshot

Disk space monitoring script syntax:
- Use 'df' to get disk usage.
- Extract usage % and remove '%'.
- Compare usage number with threshold.
- If usage > threshold, send alert.
- Script ends after check.
Simple, effective disk monitoring in bash.

Full Transcript

This script starts by setting a threshold value for disk usage at 80%. It then runs a command to check the current disk usage of the root directory. The output includes a percentage sign, so the script removes it to get a pure number. Next, it compares this number to the threshold. If the usage is greater than 80%, the script prints a warning message. Otherwise, it does nothing and ends. This process helps monitor disk space and alert when usage is too high.

Practice

1. What is the main purpose of a disk space monitoring script in a server environment?

easy

A. To check disk usage and alert when space is low

B. To delete files automatically when disk is full

C. To speed up the disk read/write operations

D. To backup all files to another disk

Solution

1. Step 1: Understand the script's goal

   The script's goal is to monitor disk usage to prevent running out of space.

2. Step 2: Identify the correct purpose

   Among the options, only checking disk usage and alerting matches the monitoring function.

3. Final Answer:

   To check disk usage and alert when space is low -> Option A

4. Quick Check:

   Disk monitoring = alert on low space [OK]

Hint: Monitoring means checking and alerting, not deleting or backing up [OK]

Common Mistakes:

• Confusing monitoring with automatic deletion
• Thinking it speeds up disk operations
• Assuming it performs backups

2. Which of the following is the correct syntax to get the disk usage percentage of the root partition using df and awk in bash?

easy

A. df -h / | awk '{print $5}'

B. df -h / | sed '2q;d'

C. df / | awk 'NR==1 {print $5}'

D. df -h / | awk 'NR==2 {print $5}'

Solution

1. Step 1: Understand df -h / output

   The command shows disk usage for root; the second line (NR==2) has the data.

2. Step 2: Use awk 'NR==2 {print $5}' to get usage percent

   This prints the 5th column (usage %) from the second line, which is correct.

3. Final Answer:

   df -h / | awk 'NR==2 {print $5}' -> Option D

4. Quick Check:

   Second line, fifth column = usage percent [OK]

Hint: Use NR==2 to skip header line in df output [OK]

Common Mistakes:

• Printing all lines instead of second line
• Using NR==1 which is header line
• Using sed incorrectly to get the line

3. Given this script snippet:

usage=$(df / | awk 'NR==2 {print $5}' | sed 's/%//')
if [ "$usage" -gt 80 ]; then
  echo "Warning: Disk usage is above 80%"
else
  echo "Disk usage is normal"
fi

What will be the output if the disk usage is 75%?

medium

A. Disk usage is normal

B. No output

C. Syntax error in script

D. Warning: Disk usage is above 80%

Solution

1. Step 1: Extract usage value

   The script removes % sign and stores 75 in usage variable.

2. Step 2: Compare usage with 80

   75 is not greater than 80, so the else branch runs.

3. Final Answer:

   Disk usage is normal -> Option A

4. Quick Check:

   75 <= 80 means normal usage [OK]

Hint: Compare numeric values after removing % sign [OK]

Common Mistakes:

• Forgetting to remove % sign before comparison
• Using string comparison instead of numeric
• Misreading the if condition

4. Identify the error in this disk space check script snippet:

usage=$(df / | awk 'NR==2 {print $5}')
if [ $usage -gt 90 ]; then
  echo "Disk almost full"
fi

What problem will occur when running this script?

medium

A. No error, script runs fine

B. Syntax error due to missing quotes around $usage

C. df command will fail

D. awk command syntax error

Solution

1. Step 1: Check variable usage in test

   The variable $usage contains a value like '85%'. Without quotes, test may fail if value has spaces or special chars.

2. Step 2: Identify missing % removal and quotes

   Also, % sign is not removed, causing numeric comparison error. Missing quotes cause syntax error.

3. Final Answer:

   Syntax error due to missing quotes around $usage -> Option B

4. Quick Check:

   Always quote variables in test to avoid syntax errors [OK]

Hint: Quote variables and remove % before numeric comparison [OK]

Common Mistakes:

• Not quoting variables in if test
• Not removing % sign before numeric test
• Assuming df or awk commands fail

5. You want to create a bash script that checks all mounted partitions and sends an alert if any partition exceeds 85% usage. Which approach correctly implements this?

hard

A. Use du command on root directory and alert if usage exceeds 85%

B. Run df -h once and alert if total disk usage exceeds 85%

C. Use a loop over df -h output lines, extract usage %, compare with 85, and echo alert if exceeded

D. Check only root partition usage and ignore others

Solution

1. Step 1: Understand requirement to check all partitions

   We must check each mounted partition's usage, not just total or root.

2. Step 2: Loop through df -h output lines and compare usage

   Extract usage percent per line, compare with 85, and alert if exceeded.

3. Final Answer:

   Use a loop over df -h output lines, extract usage %, compare with 85, and echo alert if exceeded -> Option C

4. Quick Check:

   Loop and compare each partition usage [OK]

Hint: Loop through df output lines to check each partition [OK]

Common Mistakes:

• Checking only total or root partition
• Using du instead of df for disk usage
• Not looping through all partitions