Day 3:Shell Scripting

🚦 1. Analyzing Node Health

A core DevOps script often checks CPU, memory, disk usage, and running processes to ensure “node health.” A sample snippet:

bashCopyEdit#!/bin/bash
set -x
set -e
set -o pipefail

# Check disk space
df -h | grep '/$'

# Check node processes
ps -ef | grep my_app

• set -x echoes each command before execution—great for transparency in logs DEV CommunityStack Overflow.

• set -e halts on any failed command; combined with pipefail, even a failure in a pipeline is caught How-To GeekGitHub.

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🔍 2. set -x, -e, -o pipefail

• set -x: Enables debug mode, revealing commands and arguments via stderr before execution—perfect for tracing PullRequest+1GitHub+1.

• set -e: Abort script when any command exits with a non-zero status (error) Stack Overflow+4PullRequest+4GitHub+4.

• set -o pipefail: Ensures the script dies if any stage in a pipeline fails, not just the final one—critical for robust workflows GitHub.

Combined best-practice shebang:

bashCopyEdit#!/bin/bash
set -euo pipefail

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⚙️ 3. Process Listing: ps -ef and grep

bashCopyEditps -ef | grep my_service | grep -v grep

• ps -ef lists all processes in full format.

• Use grep to filter. Add grep -v grep or other mechanisms to avoid matching your own filter process Unix & Linux Stack Exchange+2golinuxcloud.com+2Devhints.io cheatsheets+2Stack Overflow+1DigitalOcean+1.

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🔗 4. Piping: |

The pipe | streams the output of one command into another:

bashCopyEditps -ef | grep httpd | awk '{print $2}'

Efficiently chain text tools for powerful data extraction.

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🌐 5. Downloading & Fetching: curl & wget

• curl -o image.png URL downloads a remote image to your local disk GitHub+14Unix & Linux Stack Exchange+14Hostinger+14.

• Alternatively:

bashCopyEditcurl URL > image.png

• wget URL does the same, with -O filename to customize names Stack OverflowAsk Ubuntu.

Use wget -i urls.txt -P downloads/ to download multiple URLs from a list dougie.io.

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🔦 6. Searching Files: find

Useful to locate files by name, type, or properties:

bashCopyEditfind /var/log -type f -name "*.log" -mtime -7

Useful in maintenance scripts to clean up or report recent logs.

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🛡️ 7. Privilege Switching: sudo and su

• Prefer launching whole scripts with sudo myscript.sh, rather than peppering sudo inside the script Ask Ubuntu.

• For user-switching mid-script: su user -c "command"—but only works in password-less or root-run environments; sudo -u username cmd is more robust Stack Overflow.

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🔁 8. Control Flow: for Loops & if-else

For-loop example:

bashCopyEditfor file in /var/log/*.log; do
  echo "Analyzing $file"
  grep -q "ERROR" "$file" && echo "Errors found in $file"
done

If-else example:

bashCopyEditif curl -s http://localhost/health | grep -q "OK"; then
  echo "Node is healthy"
else
  echo "⚠️ Node health check failed"
  exit 1
fi

This combines HTTP checking, piping, grep filtering, and flow control seamlessly.

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Some Commands Cheat Sheets

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🧭 Wrapping Up

These commands are the backbone of many DevOps scripts—used in monitoring, automation, maintenance, and deployment pipelines.