Cli

grep, awk, and sed are the foundational text processing tools in Unix. They’re old, they’re cryptic, and they’re incredibly powerful once you learn them. grep: Search and Filter grep searches for patterns in text. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 # Basic search grep "error" logfile.txt # Case insensitive grep -i "error" logfile.txt # Show line numbers grep -n "error" logfile.txt # Count matches grep -c "error" logfile.txt # Invert (lines NOT matching) grep -v "debug" logfile.txt # Recursive search grep -r "TODO" ./src/ # Only filenames grep -l "password" *.conf Regex with grep 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 # Extended regex (-E or egrep) grep -E "error|warning|critical" logfile.txt # Word boundary grep -w "fail" logfile.txt # Matches "fail" not "failure" # Line start/end grep "^Error" logfile.txt # Lines starting with Error grep "done$" logfile.txt # Lines ending with done # Any character grep "user.name" logfile.txt # user1name, username, user_name # Character classes grep "[0-9]" logfile.txt # Lines with digits grep "[A-Za-z]" logfile.txt # Lines with letters Context 1 2 3 4 5 6 7 8 # Lines before match grep -B 3 "error" logfile.txt # Lines after match grep -A 3 "error" logfile.txt # Lines before and after grep -C 3 "error" logfile.txt Real Examples 1 2 3 4 5 6 7 8 9 10 11 # Find IP addresses grep -E "\b[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\b" access.log # Find function definitions grep -n "^def \|^function " *.py *.js # Exclude directories grep -r "config" . --exclude-dir={node_modules,.git} # Find files NOT containing pattern grep -L "copyright" *.py sed: Stream Editor sed transforms text line by line. ...

Every developer ends up using curl. It’s installed everywhere, works with any API, and once you learn the patterns, it’s faster than any GUI tool. 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"}' # Other methods curl -X PUT ... curl -X DELETE ... curl -X PATCH ... Headers 1 2 3 4 5 6 7 8 9 10 # Set content type curl -H "Content-Type: application/json" ... # Auth header curl -H "Authorization: Bearer TOKEN" ... # Multiple headers curl -H "Content-Type: application/json" \ -H "Authorization: Bearer TOKEN" \ -H "X-Custom-Header: value" ... JSON Data 1 2 3 4 5 6 7 8 9 10 11 12 # Inline JSON curl -X POST https://api.example.com/users \ -H "Content-Type: application/json" \ -d '{"name":"Alice","email":"alice@example.com"}' # From file curl -X POST https://api.example.com/users \ -H "Content-Type: application/json" \ -d @data.json # Pretty output (pipe to jq) curl -s https://api.example.com/users | jq Response Info 1 2 3 4 5 6 7 8 9 10 11 # Show response headers curl -i https://api.example.com/users # Only headers (no body) curl -I https://api.example.com/users # HTTP status code only curl -s -o /dev/null -w "%{http_code}" https://api.example.com/users # Timing info curl -w "Time: %{time_total}s\n" -o /dev/null -s https://api.example.com Authentication 1 2 3 4 5 6 7 8 9 10 11 # Basic auth curl -u username:password https://api.example.com # Bearer token curl -H "Authorization: Bearer eyJ..." https://api.example.com # API key in header curl -H "X-API-Key: abc123" https://api.example.com # API key in query curl "https://api.example.com?api_key=abc123" Form Data 1 2 3 4 5 6 7 8 # URL-encoded form curl -X POST https://example.com/login \ -d "username=alice&password=secret" # Multipart form (file upload) curl -X POST https://example.com/upload \ -F "file=@photo.jpg" \ -F "description=My photo" Following Redirects 1 2 3 4 5 # Follow redirects (3xx) curl -L https://example.com/redirect # Show redirect chain curl -L -v https://example.com/redirect 2>&1 | grep "< HTTP\|< Location" SSL/TLS 1 2 3 4 5 6 7 8 # Ignore SSL errors (dev only!) curl -k https://self-signed.example.com # Use specific cert curl --cacert /path/to/ca.crt https://api.example.com # Client cert authentication curl --cert client.crt --key client.key https://api.example.com Timeouts 1 2 3 4 5 6 7 8 # Connection timeout (seconds) curl --connect-timeout 5 https://api.example.com # Max time for entire operation curl --max-time 30 https://api.example.com # Both curl --connect-timeout 5 --max-time 30 https://api.example.com Debugging 1 2 3 4 5 6 7 8 # Verbose output curl -v https://api.example.com # Even more verbose (includes SSL handshake) curl -vv https://api.example.com # Trace everything to file curl --trace trace.log https://api.example.com Saving Output 1 2 3 4 5 6 7 8 # Save to file curl -o response.json https://api.example.com/data # Save with remote filename curl -O https://example.com/file.zip # Silent mode (no progress) curl -s https://api.example.com Cookies 1 2 3 4 5 6 7 8 # Send cookie curl -b "session=abc123" https://example.com # Save cookies to file curl -c cookies.txt https://example.com/login -d "user=alice" # Send cookies from file curl -b cookies.txt https://example.com/dashboard Retry Logic 1 2 3 4 5 6 7 8 # Retry on failure curl --retry 3 https://api.example.com # Retry with delay curl --retry 3 --retry-delay 2 https://api.example.com # Retry on specific errors curl --retry 3 --retry-connrefused https://api.example.com Useful Aliases Add to your .bashrc: ...

APIs return JSON. Config files use JSON. Logs are often JSON. The jq tool lets you slice, filter, and transform JSON from the command line—no scripting required. Basic Syntax 1 2 3 4 5 jq 'filter' input.json # or cat input.json | jq 'filter' # or curl -s api.example.com | jq 'filter' Pretty Print 1 2 3 4 5 6 7 8 9 10 11 # Ugly JSON echo '{"name":"Alice","age":30}' | jq # Output: # { # "name": "Alice", # "age": 30 # } # Compact (remove whitespace) echo '{"name": "Alice"}' | jq -c # {"name":"Alice"} Extract Fields 1 2 3 4 5 6 echo '{"name":"Alice","age":30}' | jq '.name' # "Alice" # Without quotes echo '{"name":"Alice","age":30}' | jq -r '.name' # Alice Nested Objects 1 2 echo '{"user":{"name":"Alice","email":"alice@example.com"}}' | jq '.user.name' # "Alice" Arrays 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 # Get element by index echo '[1,2,3,4,5]' | jq '.[0]' # 1 # Get slice echo '[1,2,3,4,5]' | jq '.[1:3]' # [2, 3] # Get all elements echo '[{"name":"Alice"},{"name":"Bob"}]' | jq '.[].name' # "Alice" # "Bob" # Wrap in array echo '[{"name":"Alice"},{"name":"Bob"}]' | jq '[.[].name]' # ["Alice", "Bob"] Filtering 1 2 3 4 5 6 # Select objects matching condition echo '[{"name":"Alice","age":30},{"name":"Bob","age":25}]' | jq '.[] | select(.age > 26)' # {"name": "Alice", "age": 30} # Multiple conditions jq '.[] | select(.status == "active" and .role == "admin")' Transform Data 1 2 3 4 5 6 7 8 9 10 11 # Create new object echo '{"first":"Alice","last":"Smith"}' | jq '{fullName: "\(.first) \(.last)"}' # {"fullName": "Alice Smith"} # Map over array echo '[1,2,3]' | jq 'map(. * 2)' # [2, 4, 6] # Add field echo '{"name":"Alice"}' | jq '. + {role: "admin"}' # {"name": "Alice", "role": "admin"} Real-World Examples Parse API response: ...

Python is the go-to language for automation scripts. Here’s how to write CLI tools that are reliable and user-friendly. Basic Script Structure 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 #!/usr/bin/env python3 """One-line description of what this script does.""" import argparse import sys def main(): parser = argparse.ArgumentParser(description=__doc__) parser.add_argument('input', help='Input file path') parser.add_argument('-o', '--output', help='Output file path') parser.add_argument('-v', '--verbose', action='store_true') args = parser.parse_args() # Your logic here process(args.input, args.output, args.verbose) if __name__ == '__main__': main() Argument Parsing with argparse Positional Arguments 1 2 3 4 parser.add_argument('filename') # Required parser.add_argument('files', nargs='+') # One or more parser.add_argument('files', nargs='*') # Zero or more parser.add_argument('config', nargs='?') # Optional positional Optional Arguments 1 2 3 4 5 parser.add_argument('-v', '--verbose', action='store_true') parser.add_argument('-q', '--quiet', action='store_false', dest='verbose') parser.add_argument('-n', '--count', type=int, default=10) parser.add_argument('-f', '--format', choices=['json', 'csv', 'table']) parser.add_argument('--config', type=argparse.FileType('r')) Subcommands 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 parser = argparse.ArgumentParser() subparsers = parser.add_subparsers(dest='command', required=True) # 'init' command init_parser = subparsers.add_parser('init', help='Initialize project') init_parser.add_argument('--force', action='store_true') # 'run' command run_parser = subparsers.add_parser('run', help='Run the application') run_parser.add_argument('--port', type=int, default=8080) args = parser.parse_args() if args.command == 'init': do_init(args.force) elif args.command == 'run': do_run(args.port) Error Handling 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 import sys def main(): try: result = process() return 0 except FileNotFoundError as e: print(f"Error: File not found: {e.filename}", file=sys.stderr) return 1 except PermissionError: print("Error: Permission denied", file=sys.stderr) return 1 except KeyboardInterrupt: print("\nInterrupted", file=sys.stderr) return 130 except Exception as e: print(f"Error: {e}", file=sys.stderr) return 1 if __name__ == '__main__': sys.exit(main()) Logging 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 import logging def setup_logging(verbose=False): level = logging.DEBUG if verbose else logging.INFO logging.basicConfig( level=level, format='%(asctime)s - %(levelname)s - %(message)s', datefmt='%Y-%m-%d %H:%M:%S' ) def main(): args = parse_args() setup_logging(args.verbose) logging.info("Starting process") logging.debug("Detailed info here") logging.warning("Something might be wrong") logging.error("Something went wrong") Log to File and Console 1 2 3 4 5 6 7 8 9 10 11 def setup_logging(verbose=False, log_file=None): handlers = [logging.StreamHandler()] if log_file: handlers.append(logging.FileHandler(log_file)) logging.basicConfig( level=logging.DEBUG if verbose else logging.INFO, format='%(asctime)s - %(name)s - %(levelname)s - %(message)s', handlers=handlers ) Progress Indicators Simple Progress 1 2 3 4 5 6 7 8 import sys def process_items(items): total = len(items) for i, item in enumerate(items, 1): process(item) print(f"\rProcessing: {i}/{total}", end='', flush=True) print() # Newline at end With tqdm 1 2 3 4 5 6 7 8 9 10 from tqdm import tqdm for item in tqdm(items, desc="Processing"): process(item) # Or wrap any iterable with tqdm(total=100) as pbar: for i in range(100): do_work() pbar.update(1) Reading Input From File or Stdin 1 2 3 4 5 6 7 8 9 10 import sys def read_input(filepath=None): if filepath: with open(filepath) as f: return f.read() elif not sys.stdin.isatty(): return sys.stdin.read() else: raise ValueError("No input provided") Line by Line 1 2 3 4 5 import fileinput # Reads from files in args or stdin for line in fileinput.input(): process(line.strip()) Output Formatting JSON Output 1 2 3 4 5 6 7 import json def output_json(data, pretty=False): if pretty: print(json.dumps(data, indent=2, default=str)) else: print(json.dumps(data, default=str)) Table Output 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 def print_table(headers, rows): # Calculate column widths widths = [len(h) for h in headers] for row in rows: for i, cell in enumerate(row): widths[i] = max(widths[i], len(str(cell))) # Print header header_line = ' | '.join(h.ljust(widths[i]) for i, h in enumerate(headers)) print(header_line) print('-' * len(header_line)) # Print rows for row in rows: print(' | '.join(str(cell).ljust(widths[i]) for i, cell in enumerate(row))) With tabulate 1 2 3 4 5 6 7 from tabulate import tabulate data = [ ['Alice', 30, 'Engineer'], ['Bob', 25, 'Designer'], ] print(tabulate(data, headers=['Name', 'Age', 'Role'], tablefmt='grid')) Configuration Files YAML Config 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 import yaml from pathlib import Path def load_config(config_path=None): paths = [ config_path, Path.home() / '.myapp.yaml', Path('/etc/myapp/config.yaml'), ] for path in paths: if path and Path(path).exists(): with open(path) as f: return yaml.safe_load(f) return {} # Defaults Environment Variables 1 2 3 4 5 6 7 8 import os def get_config(): return { 'api_key': os.environ.get('API_KEY'), 'debug': os.environ.get('DEBUG', '').lower() in ('true', '1', 'yes'), 'timeout': int(os.environ.get('TIMEOUT', '30')), } Running External Commands 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 import subprocess def run_command(cmd, check=True): """Run command and return output.""" result = subprocess.run( cmd, shell=isinstance(cmd, str), capture_output=True, text=True, check=check ) return result.stdout.strip() # Usage output = run_command(['git', 'status', '--short']) output = run_command('ls -la | head -5') With Timeout 1 2 3 4 5 6 7 8 9 try: result = subprocess.run( ['slow-command'], timeout=30, capture_output=True, text=True ) except subprocess.TimeoutExpired: print("Command timed out") Temporary Files 1 2 3 4 5 6 7 8 9 10 11 12 13 import tempfile from pathlib import Path # Temporary file with tempfile.NamedTemporaryFile(mode='w', suffix='.json', delete=False) as f: f.write('{"data": "value"}') temp_path = f.name # Temporary directory with tempfile.TemporaryDirectory() as tmpdir: work_file = Path(tmpdir) / 'work.txt' work_file.write_text('working...') # Directory deleted when context exits Path Handling 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 from pathlib import Path def process_files(directory): base = Path(directory) # Find files for path in base.glob('**/*.py'): print(f"Processing: {path}") # Path operations print(f" Name: {path.name}") print(f" Stem: {path.stem}") print(f" Suffix: {path.suffix}") print(f" Parent: {path.parent}") # Read/write content = path.read_text() path.with_suffix('.bak').write_text(content) Complete Example 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 #!/usr/bin/env python3 """Process log files and output statistics.""" import argparse import json import logging import sys from collections import Counter from pathlib import Path def setup_logging(verbose): logging.basicConfig( level=logging.DEBUG if verbose else logging.INFO, format='%(levelname)s: %(message)s' ) def parse_args(): parser = argparse.ArgumentParser( description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter ) parser.add_argument( 'logfiles', nargs='+', type=Path, help='Log files to process' ) parser.add_argument( '-o', '--output', type=argparse.FileType('w'), default=sys.stdout, help='Output file (default: stdout)' ) parser.add_argument( '-f', '--format', choices=['json', 'text'], default='text', help='Output format' ) parser.add_argument( '-v', '--verbose', action='store_true', help='Enable verbose output' ) return parser.parse_args() def analyze_logs(logfiles): stats = Counter() for logfile in logfiles: logging.info(f"Processing {logfile}") if not logfile.exists(): logging.warning(f"File not found: {logfile}") continue for line in logfile.read_text().splitlines(): if 'ERROR' in line: stats['errors'] += 1 elif 'WARNING' in line: stats['warnings'] += 1 stats['total'] += 1 return dict(stats) def output_results(stats, output, fmt): if fmt == 'json': json.dump(stats, output, indent=2) output.write('\n') else: for key, value in stats.items(): output.write(f"{key}: {value}\n") def main(): args = parse_args() setup_logging(args.verbose) try: stats = analyze_logs(args.logfiles) output_results(stats, args.output, args.format) return 0 except Exception as e: logging.error(f"Failed: {e}") return 1 if __name__ == '__main__': sys.exit(main()) Usage: ...

awk sits between grep and a full programming language. It’s perfect for columnar data, log files, and quick text transformations. The Basic Pattern 1 awk 'pattern { action }' file If pattern matches, run action. No pattern means every line. No action means print. 1 2 3 4 5 6 7 8 9 10 11 # Print everything awk '{ print }' file.txt # Print lines matching pattern awk '/error/' file.txt # Print second column awk '{ print $2 }' file.txt # Combined: errors, show timestamp and message awk '/error/ { print $1, $4 }' app.log Field Handling awk splits lines into fields by whitespace (default): ...

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: ...

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: ...

You know grep "error" logfile.txt. But grep can do so much more — recursive searches, context lines, inverse matching, and regex patterns that turn hours of manual searching into seconds. The Basics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 # Search for pattern in file grep "error" app.log # Case-insensitive grep -i "error" app.log # Show line numbers grep -n "error" app.log # Count matches grep -c "error" app.log # Only show filenames with matches grep -l "error" *.log Recursive Search 1 2 3 4 5 6 7 8 9 10 11 12 13 14 # Search all files in directory tree grep -r "TODO" ./src # With line numbers grep -rn "TODO" ./src # Include only certain files grep -r --include="*.py" "import os" . # Exclude directories grep -r --exclude-dir=node_modules "console.log" . # Multiple excludes grep -r --exclude-dir={node_modules,.git,dist} "function" . Context Lines When you find a match, you often need surrounding context: ...

You need to change a config value across 50 files. You could open each one, or: 1 sed -i 's/old_value/new_value/g' *.conf Done. sed is the stream editor — it transforms text as it flows through. Master it, and you’ll never manually edit repetitive files again. The Basics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 # Replace first occurrence per line echo "hello hello" | sed 's/hello/hi/' # hi hello # Replace all occurrences (g = global) echo "hello hello" | sed 's/hello/hi/g' # hi hi # Replace in file (print to stdout) sed 's/foo/bar/g' file.txt # Replace in place (-i) sed -i 's/foo/bar/g' file.txt # Backup before in-place edit sed -i.bak 's/foo/bar/g' file.txt The -i flag is powerful and dangerous. Always test without it first. ...

You can grep for lines and cut for columns. But what about “show me the third column of lines containing ERROR, but only if the second column is greater than 100”? That’s awk territory. The Basics awk processes text line by line, splitting each into fields: 1 2 3 4 5 6 7 8 9 # Print second column (space-delimited by default) echo "hello world" | awk '{print $2}' # world # Print first and third columns cat data.txt | awk '{print $1, $3}' # Print entire line awk '{print $0}' file.txt $1, $2, etc. are fields. $0 is the whole line. NF is the number of fields. NR is the line number. ...