Part 5: Memory Commands, While Loop, and Vim Command

In this section, we will explore memory management commands, while loops, the Vim text editor, and more. We’ll also create a shell script that ties everything together to monitor disk space.

Important Memory Commands

1. free Command

The free command displays the system’s memory usage, including total, used, and free memory.

Example:

free -h

Output:

               total        used        free      shared  buff/cache   available
Mem:            8.0G        2.1G        4.5G        100M        1.4G        5.5G
Swap:           2.0G        500M        1.5G

2. top Command

The top command provides real-time information about system processes, including memory and CPU usage.

Example:

top

You can sort the processes by memory usage by pressing Shift + M while top is running.

3. df -h Command

The df -h command displays disk space usage in a human-readable format.

Example:

df -h

Output:

Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1        50G   35G   15G  70% /
tmpfs           2.0G     0  2.0G   0% /dev/shm

The awk Command in Linux

The awk command is a powerful text-processing tool that allows you to filter and format output.

Example 1: Extracting Specific Columns

df -h | awk '{print $1, $5}'

Output:

Filesystem Use%
/dev/sda1 70%
tmpfs 0%

Example 2: Filtering Rows

df -h | awk '$5 > 70 {print $1, $5}'

Output:

/dev/sda1 70%

This filters filesystems where disk usage is above 70%.

Example 3: Calculating Total Memory

free -h | awk '/Mem:/ {print "Total Memory: " $2 " Used: " $3 " Free: " $4}'

Output:

Total Memory: 8.0G Used: 2.1G Free: 4.5G

While Loop

The while loop in shell scripting executes a block of code repeatedly while a condition is true.

Example: Display Disk Details Using awk

#!/bin/bash

while read -r line; do
    echo "$line" | awk '{print "Disk: "$1, "Used: "$3}'
done < <(df -h)

This script reads disk details line by line and prints the disk name and memory used.

Vim Command

Vim is a powerful text editor that offers advanced features compared to Nano.

Advantages of Vim Over Nano

1. Modes: Vim offers normal, insert, and visual modes for efficient editing.

2. Customizability: Supports plugins and extensive configuration.

3. Performance: Faster for large files.

Basic Usage of Vim

• Open a file: vim filename

• Insert mode: Press i to edit.

• Save and exit: Press Esc, then :wq.

• Exit without saving: Press Esc, then :q!.

The cut Command

The cut command extracts specific parts of a file or output.

Example in Vim Command

Suppose we have a file data.txt:

Name Age
Alice 25
Bob 30

Using cut to extract the first column:

cut -d' ' -f1 data.txt

Output:

Name
Alice
Bob

Disk Space Warning Script

Here’s a script that monitors disk space and warns when usage exceeds 90%:

#!/bin/bash

# Check disk space and send a warning if usage exceeds 90%
while read -r line; do
    usage=$(echo "$line" | awk '{print $5}' | sed 's/%//')
    if [ "$usage" -gt 90 ]; then
        disk=$(echo "$line" | awk '{print $1}')
        echo "Warning: $disk is ${usage}% full!"
    fi
done < <(df -h | grep '^/dev')

Explanation:

1. Read disk details line by line: Use df -h output filtered for mounted devices.

2. Extract usage percentage: Use awk to get the fifth column and remove % using sed.

3. Check if usage exceeds 90%: Compare the value and print a warning.

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This part of the blog focuses on essential commands and techniques, enabling DevOps engineers to manage memory, monitor disk space, and efficiently edit configuration files. Experiment with these commands to master them!