Welcome to the capstone lesson of Linux Command Line for DevOps! In this comprehensive guide, you will master the Unix streaming model (STDIN, STDOUT, STDERR), input/output redirection, production-grade Bash scripting with strict safety modes (set -euo pipefail), and persistent terminal sessions with tmux.
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1. Standard Streams & File Descriptors
Every process started by the Linux kernel automatically opens three standard file descriptors (FD):
| Stream Name | File Descriptor | Default Source / Destination | Redirection Symbol |
|---|---|---|---|
Standard Input (stdin) | 0 | Keyboard or piped input stream | < |
Standard Output (stdout) | 1 | Terminal screen display (Standard messages) | >, >>, 1> |
Standard Error (stderr) | 2 | Terminal screen display (Error/diagnostic messages) | 2>, 2>> |
2. Redirection Mastery: >, >>, 2>, 2>&1 & /dev/null
bash
# 1. Overwrite STDOUT to a file (> operator) — ERASED previous content!
echo "APP_PORT=8080" > .env
# 2. Append STDOUT to a file (>> operator) — Adds to the bottom without erasing!
echo "LOG_LEVEL=debug" >> .env
# 3. Redirect STDERR (Errors only) to a separate file (2>)
./deploy-script.sh 2> errors.log
# 4. Redirect BOTH STDOUT and STDERR to the same log file (Modern syntax: &>)
npm run build &> build.log
# 5. Legacy equivalent of &> (Redirect FD 2 to wherever FD 1 is pointing)
./migrate-database.sh > migration.log 2>&1
# 6. Discard unwanted noisy output to /dev/null (The Linux Bit Bucket)
curl -s https://api.mycompany.com/health > /dev/null 2>&1
# 7. Provide file content as STDIN input (< operator)
mysql -u root -p database_name < schema.sql
The tee Command: Split Streams (View AND Save)
When running a deployment, you often want to watch output live in your terminal while simultaneously logging it to disk:
bash
# Pipes STDOUT to terminal screen AND writes to deploy.log
./deploy.sh | tee deploy.log
# Append to log file instead of overwriting (-a flag)
docker compose up -d | tee -a runtime.log
# Write to a root-protected file using sudo and tee
echo "nameserver 1.1.1.1" | sudo tee /etc/resolv.conf
3. Command Chaining & Control Flow Operators
bash
# 1. && (AND) — Run second command ONLY if the first succeeds (Exit code 0)
# Standard CI/CD pipeline pattern:
npm run lint && npm run test && npm run build
# 2. || (OR) — Run second command ONLY if the first FAILS (Non-zero exit code)
# Great for fallback notifications or emergency alerts:
ping -c 1 10.0.1.50 || echo "🚨 Server unreachable! Sending alert to PagerDuty..."
# 3. ; (Sequential) — Execute commands in order regardless of pass/fail
echo "Starting cleanup..." ; rm -rf /tmp/cache ; echo "Done!"
# 4. () (Subshell) — Execute commands inside an isolated subshell environment
# Temporarily cd into directory without changing parent terminal path:
(cd /opt/app && git pull && npm run build)
pwd # Still in your original directory!
4. Production Bash Scripting: The Unofficial Strict Mode
When writing shell scripts for CI/CD pipelines, Docker entrypoints, or deployment automation, always include strict safety flags at the top of your script:
bash
#!/usr/bin/env bash
# ==============================================================================
# BASH STRICT MODE
# -e : Exit immediately if any command exits with a non-zero status.
# -u : Treat unset variables as an error and exit immediately.
# -o pipefail : If any command in a pipe fails, return that failure code.
# ==============================================================================
set -euo pipefail
# Optional debug mode (prints each command before executing it):
# set -x
# Temporary workspace directory
TEMP_DIR="/tmp/deploy-$$"
mkdir -p "$TEMP_DIR"
# Automated Cleanup Hook using trap
# Ensures temporary files are deleted on normal exit, error, or Ctrl+C!
cleanup() {
echo "🧹 Cleaning up temporary workspace: $TEMP_DIR"
rm -rf "$TEMP_DIR"
}
trap cleanup EXIT INT TERM
# Script logic starts here
echo "🚀 Starting automated deployment..."
ENVIRONMENT="${1:-staging}" # Default to 'staging' if argument $1 is not provided
echo "Deploying to environment: $ENVIRONMENT"
if [[ "$ENVIRONMENT" == "production" ]]; then
echo "🔒 Validating production secrets..."
[[ -z "${DATABASE_URL:-}" ]] && { echo "❌ ERROR: DATABASE_URL is required!"; exit 1; }
fi
echo "✅ Deployment completed successfully!"
5. Terminal Multiplexing with tmux
When executing long-running database migrations, builds, or backups over SSH, an accidental WiFi disconnect or closed laptop lid will kill your shell process immediately.
tmux (Terminal Multiplexer) runs persistent shell sessions in the background on the server:
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Essential tmux Commands
bash
# 1. Start a new named session
tmux new -s deploy-session
# 2. List active running tmux sessions
tmux ls
# Output: deploy-session: 2 windows (created Sun Aug 18 10:00:00 2026)
# 3. Reconnect (attach) to an existing session after logging back into server
tmux attach -t deploy-session
# 4. Kill an obsolete session
tmux kill-session -t deploy-session
tmux Keyboard Shortcuts (Default Prefix: Ctrl + b)
Press Ctrl + b, release, then press:
- d — Detach from session (leaves everything running in background on server!).
- % — Split current window vertically into two side-by-side panes.
- " — Split current window horizontally into top and bottom panes.
- Arrow Keys — Switch between panes.
- c — Create a new window (tab).
- n / p — Next / Previous window.
- x — Close current pane.
- [ — Enter scroll/copy mode (use arrow keys to scroll past logs, press q to exit).
6. Top 10 DevOps Emergency Troubleshooting Recipes
bash
# 1. Find who is eating all disk space:
du -ah /var | sort -rh | head -n 10
# 2. Check live disk partition capacity:
df -hT
# 3. Live check for established external TCP connections:
ss -t state established
# 4. Kill all frozen Node.js processes immediately:
pkill -9 -f node
# 5. Extract distinct IP addresses attempting brute-force SSH logins:
grep "Failed password" /var/log/auth.log | awk '{print $(NF-3)}' | sort | uniq -c | sort -nr
# 6. Test HTTP response code and total latency:
curl -o /dev/null -s -w "HTTP: %{http_code} | Total: %{time_total}s\n" https://api.mycompany.com/health
# 7. Delete all files older than 30 days in a log directory:
find /var/log/custom-apps -type f -name "*.log" -mtime +30 -delete
# 8. Check RAM swap usage:
free -h
# 9. Test remote TCP port without telnet/nc:
timeout 3 bash -c "</dev/tcp/10.0.1.50/5432" && echo "Port is OPEN" || echo "Port is CLOSED"
# 10. Check recent system reboot and login history:
last -n 10
Course 0 Complete! 🎉
Congratulations! You have officially conquered the complete Linux Command Line for DevOps foundational curriculum:
- Lesson 1: Linux kernel architecture, FHS, and cloud/container primitives.
- Lesson 2: Terminal navigation, prompt anatomy, and Readline shortcuts.
- Lesson 3: Filesystem operations, absolute vs relative paths, and live log streaming (
tail -F). - Lesson 4: Text editing (
vim,nano), pattern matching (grep), indexing (find), and stream transformation (sed,awk). - Lesson 5: Linux security model, octal permissions (
chmod 755/600), ownership (chown), andvisudo. - Lesson 6: Process lifecycles, resource auditing (
htop), Linux signals (SIGTERM/SIGKILL), andsystemdservices. - Lesson 7: Package management across distros (
apt,dnf,apk) and secure GPG keyrings. - Lesson 8: Network interface inspection (
ip a), DNS querying (dig), sockets (ss -tulnp), and HTTP diagnostics (curl). - Lesson 9: Asymmetric SSH cryptography (Ed25519), bastion host jumping (
~/.ssh/config), and daemon hardening. - Lesson 10: Unix I/O streams, strict Bash scripting (
set -euo pipefail), andtmuxsession multiplexing.
What's Next in Your Zero-to-Hero Roadmap?
Now that your Linux foundation is rock solid, you are ready to advance through the rest of the DevOps curriculum:
- Course 1: Git & GitHub Fundamentals — Branching strategies, atomic commits, PR code reviews, and Git Flow.
- Course 2: GitHub Actions Automation — CI/CD pipelines, service containers, and automated deployments.
- Course 3: CI/CD Pipeline Architecture — Deployment strategies (Blue-Green, Canary), DORA metrics, and automated rollbacks.
- Course 4: Docker Fundamentals — Minimal multi-stage builds, layer caching, and rootless containerization.
- Course 5: Docker Compose — Multi-service microservice orchestration, volumes, and networking.
- Course 6 & 7: Kubernetes & K3s — Cluster administration, Pods, Deployments, Ingresses, and StatefulSets.
- Course 8: Capstone Project — Full-stack GitOps CI/CD to live Kubernetes!