Devops

Cron works. It’s also from 1975. systemd timers offer logging integration, dependency handling, and more flexible scheduling. Here’s how to use them. Why Timers Over Cron? Logging: Output goes to journald automatically Dependencies: Wait for network, mounts, or other services Flexibility: Calendar events, monotonic timers, randomized delays Visibility: systemctl list-timers shows everything Consistency: Same management as other systemd units Basic Structure A timer needs two files: A .timer unit (the schedule) A .service unit (the job) Place them in /etc/systemd/system/ (system-wide) or ~/.config/systemd/user/ (user). ...

curl is the universal HTTP client. Every developer should know it well—for debugging, testing, and automation. Basic Requests 1 2 3 4 5 6 7 8 9 10 11 # GET (default) curl https://api.example.com/users # POST with data curl -X POST https://api.example.com/users \ -d '{"name":"alice"}' # With headers curl -H "Content-Type: application/json" \ -H "Authorization: Bearer token123" \ https://api.example.com/users JSON APIs 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 # POST JSON (sets Content-Type automatically) curl -X POST https://api.example.com/users \ --json '{"name":"alice","email":"alice@example.com"}' # Or explicitly curl -X POST https://api.example.com/users \ -H "Content-Type: application/json" \ -d '{"name":"alice"}' # Pretty print response curl -s https://api.example.com/users | jq . # From file curl -X POST https://api.example.com/users \ -H "Content-Type: application/json" \ -d @payload.json HTTP Methods 1 2 3 4 5 6 7 curl -X GET https://api.example.com/users/1 curl -X POST https://api.example.com/users -d '...' curl -X PUT https://api.example.com/users/1 -d '...' curl -X PATCH https://api.example.com/users/1 -d '...' curl -X DELETE https://api.example.com/users/1 curl -X HEAD https://api.example.com/users # Headers only curl -X OPTIONS https://api.example.com/users # CORS preflight Authentication 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 # Bearer token curl -H "Authorization: Bearer eyJhbGc..." \ https://api.example.com/me # Basic auth curl -u username:password https://api.example.com/users # Or curl -H "Authorization: Basic $(echo -n 'user:pass' | base64)" \ https://api.example.com/users # API key in header curl -H "X-API-Key: secret123" https://api.example.com/data # API key in query curl "https://api.example.com/data?api_key=secret123" Response Inspection 1 2 3 4 5 6 7 8 9 10 11 12 13 # Show headers curl -I https://api.example.com/users # HEAD request curl -i https://api.example.com/users # Include headers with body # Verbose (see full request/response) curl -v https://api.example.com/users # Just status code curl -s -o /dev/null -w "%{http_code}" https://api.example.com/users # Multiple stats curl -s -o /dev/null -w "code: %{http_code}\ntime: %{time_total}s\nsize: %{size_download} bytes\n" \ https://api.example.com/users File Uploads 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 # Form upload curl -X POST https://api.example.com/upload \ -F "file=@document.pdf" # Multiple files curl -X POST https://api.example.com/upload \ -F "file1=@doc1.pdf" \ -F "file2=@doc2.pdf" # With additional form fields curl -X POST https://api.example.com/upload \ -F "file=@photo.jpg" \ -F "description=Profile photo" \ -F "public=true" # Binary data curl -X POST https://api.example.com/upload \ -H "Content-Type: application/octet-stream" \ --data-binary @file.bin Handling Redirects 1 2 3 4 5 6 7 8 # Follow redirects curl -L https://example.com/shortened-url # Show redirect chain curl -L -v https://example.com/shortened-url 2>&1 | grep "< location" # Limit redirects curl -L --max-redirs 3 https://example.com/url Timeouts and Retries 1 2 3 4 5 6 7 8 9 10 11 # Connection timeout (seconds) curl --connect-timeout 5 https://api.example.com/ # Max time for entire operation curl --max-time 30 https://api.example.com/slow-endpoint # Retry on failure curl --retry 3 --retry-delay 2 https://api.example.com/ # Retry on specific errors curl --retry 3 --retry-all-errors https://api.example.com/ Saving Output 1 2 3 4 5 6 7 8 9 10 11 # Save to file curl -o response.json https://api.example.com/users # Use remote filename curl -O https://example.com/file.zip # Save headers separately curl -D headers.txt -o body.json https://api.example.com/users # Append to file curl https://api.example.com/users >> all_responses.json Cookie Handling 1 2 3 4 5 6 7 8 9 10 11 12 # Send cookies curl -b "session=abc123; token=xyz" https://api.example.com/ # Save cookies to file curl -c cookies.txt https://api.example.com/login \ -d "user=admin&pass=secret" # Use saved cookies curl -b cookies.txt https://api.example.com/dashboard # Both save and send curl -b cookies.txt -c cookies.txt https://api.example.com/ Testing Webhooks 1 2 3 4 5 6 7 8 9 10 11 12 # Simulate GitHub webhook curl -X POST http://localhost:3000/webhook \ -H "Content-Type: application/json" \ -H "X-GitHub-Event: push" \ -H "X-Hub-Signature-256: sha256=..." \ -d @github-payload.json # Stripe webhook curl -X POST http://localhost:3000/stripe/webhook \ -H "Content-Type: application/json" \ -H "Stripe-Signature: t=...,v1=..." \ -d @stripe-event.json SSL/TLS Options 1 2 3 4 5 6 7 8 # Skip certificate verification (development only!) curl -k https://self-signed.example.com/ # Use specific CA bundle curl --cacert /path/to/ca-bundle.crt https://api.example.com/ # Client certificate curl --cert client.crt --key client.key https://mtls.example.com/ Proxy Configuration 1 2 3 4 5 6 7 8 # HTTP proxy curl -x http://proxy:8080 https://api.example.com/ # SOCKS5 proxy curl --socks5 localhost:1080 https://api.example.com/ # No proxy for specific hosts curl --noproxy "localhost,*.internal" https://api.example.com/ Performance Testing 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 # Time breakdown curl -s -o /dev/null -w "\ namelookup: %{time_namelookup}s\n\ connect: %{time_connect}s\n\ appconnect: %{time_appconnect}s\n\ pretransfer: %{time_pretransfer}s\n\ redirect: %{time_redirect}s\n\ starttransfer: %{time_starttransfer}s\n\ total: %{time_total}s\n" \ https://api.example.com/ # Loop for load testing (basic) for i in {1..100}; do curl -s -o /dev/null -w "%{http_code} %{time_total}\n" \ https://api.example.com/ done Config Files Create ~/.curlrc for defaults: ...

JSON is everywhere. APIs return it, configs use it, logs contain it. jq is the Swiss Army knife for processing it all from the command line. Basic Selection 1 2 3 4 5 6 7 8 9 10 11 12 13 # Pretty print echo '{"name":"alice","age":30}' | jq . # Extract a field echo '{"name":"alice","age":30}' | jq '.name' # Output: "alice" # Raw output (no quotes) echo '{"name":"alice","age":30}' | jq -r '.name' # Output: alice # Nested fields echo '{"user":{"name":"alice"}}' | jq '.user.name' Working with Arrays 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 # Get all elements echo '[1,2,3]' | jq '.[]' # Output: # 1 # 2 # 3 # Get specific index echo '["a","b","c"]' | jq '.[1]' # Output: "b" # Slice echo '[1,2,3,4,5]' | jq '.[2:4]' # Output: [3,4] # First/last echo '[1,2,3]' | jq 'first' # 1 echo '[1,2,3]' | jq 'last' # 3 Filtering 1 2 3 4 5 6 7 8 9 10 11 12 13 # Select objects matching condition echo '[{"name":"alice","age":30},{"name":"bob","age":25}]' | \ jq '.[] | select(.age > 27)' # Output: {"name":"alice","age":30} # Multiple conditions jq '.[] | select(.status == "active" and .role == "admin")' # Contains jq '.[] | select(.tags | contains(["important"]))' # Regex matching jq '.[] | select(.email | test("@company\\.com$"))' Transforming Data 1 2 3 4 5 6 7 8 9 10 11 12 # Create new object echo '{"first":"Alice","last":"Smith"}' | \ jq '{fullName: (.first + " " + .last)}' # Output: {"fullName":"Alice Smith"} # Map over array echo '[1,2,3]' | jq 'map(. * 2)' # Output: [2,4,6] # Transform array of objects echo '[{"name":"alice"},{"name":"bob"}]' | \ jq 'map({user: .name, active: true})' API Response Processing 1 2 3 4 5 6 7 8 9 10 11 12 # Extract data from GitHub API curl -s https://api.github.com/users/torvalds/repos | \ jq '.[] | {name, stars: .stargazers_count, language}' | \ jq -s 'sort_by(.stars) | reverse | .[0:5]' # Get just names curl -s https://api.github.com/users/torvalds/repos | \ jq -r '.[].name' # Count items curl -s https://api.github.com/users/torvalds/repos | \ jq 'length' Aggregation 1 2 3 4 5 6 7 8 9 10 11 12 13 14 # Sum echo '[{"value":10},{"value":20},{"value":30}]' | \ jq '[.[].value] | add' # Output: 60 # Average jq '[.[].value] | add / length' # Group by echo '[{"type":"a","n":1},{"type":"b","n":2},{"type":"a","n":3}]' | \ jq 'group_by(.type) | map({type: .[0].type, total: [.[].n] | add})' # Count by field jq 'group_by(.status) | map({status: .[0].status, count: length})' Building Output 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 # Concatenate to string echo '{"host":"db","port":5432}' | \ jq -r '"\(.host):\(.port)"' # Output: db:5432 # Create CSV echo '[{"name":"alice","age":30},{"name":"bob","age":25}]' | \ jq -r '.[] | [.name, .age] | @csv' # Output: # "alice",30 # "bob",25 # Create TSV jq -r '.[] | [.name, .age] | @tsv' # URI encode jq -r '@uri' # Base64 jq -r '@base64' Conditional Logic 1 2 3 4 5 6 7 8 9 10 11 # If/then/else echo '{"score":85}' | \ jq 'if .score >= 90 then "A" elif .score >= 80 then "B" else "C" end' # Alternative operator (default values) echo '{"name":"alice"}' | jq '.age // 0' # Output: 0 # Try (suppress errors) echo '{"a":1}' | jq '.b.c.d // "missing"' # Output: "missing" Modifying JSON 1 2 3 4 5 6 7 8 9 10 11 12 13 14 # Update field echo '{"name":"alice","age":30}' | jq '.age = 31' # Add field echo '{"name":"alice"}' | jq '. + {active: true}' # Delete field echo '{"name":"alice","temp":123}' | jq 'del(.temp)' # Update nested echo '{"user":{"name":"alice"}}' | jq '.user.name = "bob"' # Recursive update jq '.. | objects | .timestamp |= (. // now)' Multiple Files 1 2 3 4 5 6 7 8 9 # Combine objects from files jq -s '.[0] * .[1]' defaults.json overrides.json # Process files independently jq -r '.name' file1.json file2.json # Slurp into array jq -s '.' file1.json file2.json # Output: [{...}, {...}] Stream Processing For large files, use streaming: ...

If you’re still typing ssh -i ~/.ssh/mykey.pem ec2-user@ec2-54-123-45-67.compute-1.amazonaws.com, you’re working too hard. SSH config transforms verbose commands into simple ssh prod invocations. The Basics Create or edit ~/.ssh/config: H o s t H U I p o s d r s e e o t r n d N t a e i m c t e 2 y - F 5 u i 4 s l . e e 1 r 2 ~ 3 / . . 4 s 5 s . h 6 / 7 p r o d - k e y . p e m Now ssh prod connects with the right key and user. No more remembering IP addresses. ...

Makefiles are ancient. They’re also incredibly useful for modern development. Here’s how to use them as your project’s command center. Why Make in 2026? Every project has commands you run repeatedly: Start development servers Run tests Build containers Deploy to environments Format and lint code You could remember them all. Or document them in a README that gets stale. Or put them in a Makefile where they’re executable documentation. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 .PHONY: dev test build deploy dev: docker-compose up -d npm run dev test: pytest -v build: docker build -t myapp:latest . deploy: kubectl apply -f k8s/ Now make dev starts everything. New team member? Run make help. ...

Environment variables are the 12-factor way to configure applications. But “just use env vars” glosses over real complexity. Here’s how to do it well. The Basics Done Right Type Coercion Environment variables are always strings. Handle conversion explicitly: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 import os def get_env_bool(key: str, default: bool = False) -> bool: value = os.getenv(key, "").lower() if value in ("true", "1", "yes", "on"): return True if value in ("false", "0", "no", "off"): return False return default def get_env_int(key: str, default: int = 0) -> int: try: return int(os.getenv(key, default)) except ValueError: return default # Usage DEBUG = get_env_bool("DEBUG", False) PORT = get_env_int("PORT", 8080) WORKERS = get_env_int("WORKERS", 4) Required vs Optional Fail fast on missing required config: ...

Your container says it’s healthy. Your users say the app is broken. Sound familiar? Basic health checks only tell you if a process is running. Here’s how to build checks that catch real problems. Beyond “Is It Alive?” Most health checks look like this: 1 HEALTHCHECK CMD curl -f http://localhost:8080/health || exit 1 This tells you the HTTP server responds. It doesn’t tell you: Can the app reach the database? Is the cache connected? Are critical background workers running? Is the disk filling up? Layered Health Checks Implement three levels: ...

top works. htop works better. It’s colorful, interactive, and actually pleasant to use. Here’s how to get the most from it. Installation 1 2 3 4 5 6 7 8 # Debian/Ubuntu sudo apt install htop # RHEL/CentOS/Fedora sudo dnf install htop # macOS brew install htop The Interface Launch with htop. You’ll see: Top section: CPU bars (one per core) Memory and swap usage Tasks, load average, uptime Process list: ...

Code review catches problems. CI catches problems. But the fastest feedback loop? Catching problems before you even commit. Git hooks run scripts at key points in your workflow. Use them to lint, test, and validate — automatically. Where Hooks Live 1 2 3 4 5 6 ls .git/hooks/ # applypatch-msg.sample pre-commit.sample # commit-msg.sample pre-push.sample # post-update.sample pre-rebase.sample # pre-applypatch.sample prepare-commit-msg.sample # pre-merge-commit.sample update.sample Remove .sample to activate. Hooks must be executable (chmod +x). ...

Your application is running in production. You need to restart it for a config change. Do you: A) kill -9 and hope for the best B) Send a signal it can handle gracefully If you picked A, you’ve probably lost data. Let’s fix that. The Essential Signals Signal Number Default Action Use Case SIGTERM 15 Terminate Graceful shutdown request SIGINT 2 Terminate Ctrl+C, interactive stop SIGHUP 1 Terminate Config reload (by convention) SIGKILL 9 Terminate Force kill (cannot be caught) SIGUSR1/2 10/12 Terminate Application-defined SIGCHLD 17 Ignore Child process state change SIGTERM is the polite ask. “Please shut down when convenient.” SIGKILL is the eviction notice. No cleanup, no saving state, immediate death. ...