From ba8d7ca67d856a16efb25d5f174cde4886504eee Mon Sep 17 00:00:00 2001
From: medusa <newton214@gmail.com>
Date: Tue, 1 Jul 2025 12:58:26 +0000
Subject: [PATCH] Add tech_docs/linux/awk.md

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+# **The Ultimate AWK Guide: From Basics to Advanced Data Wrangling**
+
+## **Table of Contents**
+1. [AWK Fundamentals](#1-awk-fundamentals)
+2. [Patterns & Actions](#2-patterns--actions)
+3. [Built-in Variables](#3-built-in-variables)
+4. [Arrays & Data Structures](#4-arrays--data-structures)
+5. [Control Flow](#5-control-flow)
+6. [Functions & Math](#6-functions--math)
+7. [Advanced Text Processing](#7-advanced-text-processing)
+8. [Real-World Recipes](#8-real-world-recipes)
+9. [Performance & Limitations](#9-performance--limitations)
+
+---
+
+## **1. AWK Fundamentals**
+
+### **Core Syntax**
+```bash
+awk [OPTIONS] 'PATTERN {ACTION}' file.txt
+```
+
+### **Basic Structure**
+```awk
+BEGIN { /* pre-processing */ }
+PATTERN { /* line processing */ }
+END { /* post-processing */ }
+```
+
+### **Common Flags**
+```bash
+awk -F: '{print $1}' /etc/passwd  # Set field separator
+awk -v var=value '...'            # Pass variables
+awk -f script.awk file.txt        # Use script file
+```
+
+---
+
+## **2. Patterns & Actions**
+
+### **Pattern Types**
+```awk
+/error/ {print}                  # Regex match
+$3 > 100 {print $1}              # Field comparison
+NR == 1 {print}                  # Line number
+BEGINFILE {print "Processing:", FILENAME}  # Per-file
+```
+
+### **Special Patterns**
+```awk
+BEGIN {FS=":"; OFS="\t"}         # Set input/output separators
+END {print "Total lines:", NR}    # Final processing
+```
+
+---
+
+## **3. Built-in Variables**
+
+| Variable | Description |
+|----------|-------------|
+| `NR` | Current record number |
+| `NF` | Number of fields |
+| `FS` | Field separator (default: whitespace) |
+| `OFS` | Output field separator |
+| `FILENAME` | Current file name |
+| `FNR` | Record number per file |
+
+### **Example**
+```awk
+awk '{print NR, NF, $0}' file.txt  # Show line stats
+```
+
+---
+
+## **4. Arrays & Data Structures**
+
+### **Associative Arrays**
+```awk
+{count[$1]++}                     # Count occurrences
+END {for (key in count) print key, count[key]}
+```
+
+### **Multi-Dimensional Arrays**
+```awk
+{array[$1,$2] = $3}               # Fake multi-dim
+```
+
+### **Array Functions**
+```awk
+split(string, array, separator)
+asort(array)                      # Sort by value
+asorti(array)                     # Sort by index
+```
+
+---
+
+## **5. Control Flow**
+
+### **Conditionals**
+```awk
+{if ($3 > 100) print "High:", $1
+ else print "Low:", $1}
+```
+
+### **Loops**
+```awk
+for (i=1; i<=NF; i++) {print $i}  # Fields
+for (key in array) {print key}     # Array keys
+```
+
+### **Switch/Case**
+```awk
+switch($1) {
+  case "foo": print "Found foo"; break
+  case /^bar/: print "Starts with bar"; break
+  default: print "Other"
+}
+```
+
+---
+
+## **6. Functions & Math**
+
+### **Built-in Functions**
+```awk
+length($0)                        # String length
+sub(/old/, "new", $1)             # In-field substitution
+system("date")                    # Run shell command
+```
+
+### **Math Operations**
+```awk
+{sum += $3; sumsq += ($3)^2}
+END {print "Mean:", sum/NR, "Std Dev:", sqrt(sumsq/NR - (sum/NR)^2)}
+```
+
+### **User Functions**
+```awk
+function double(x) {return x*2}
+{d = double($1); print d}
+```
+
+---
+
+## **7. Advanced Text Processing**
+
+### **Field Manipulation**
+```awk
+{$1 = toupper($1); $NF = $NF "%"}  # Modify fields
+```
+
+### **Multi-Line Records**
+```bash
+awk -v RS="" '{print $1}'          # Paragraph mode
+```
+
+### **CSV Processing**
+```bash
+awk -FPAT='([^,]+)|("[^"]+")' '{print $2}' data.csv
+```
+
+---
+
+## **8. Real-World Recipes**
+
+### **Log Analysis**
+```awk
+# Top 10 frequent IPs in access.log
+awk '{ip[$1]++} END {for (i in ip) print ip[i], i}' access.log | sort -nr | head
+```
+
+### **Data Transformation**
+```awk
+# Convert TSV to CSV
+BEGIN {FS="\t"; OFS=","} {$1=$1; print}
+```
+
+### **Column Statistics**
+```awk
+# Compute column averages
+NR>1 {for(i=1; i<=NF; i++) sum[i]+=$i}
+END {for(i=1; i<=NF; i++) print "Col", i, "avg:", sum[i]/(NR-1)}
+```
+
+### **JSON Generation**
+```awk
+BEGIN {print "["; FS=","}
+{printf "  {\"name\":\"%s\",\"value\":%s}%s\n", $1, $2, (NR==FNR?",":"")}
+END {print "]"}
+```
+
+---
+
+## **9. Performance & Limitations**
+
+### **Optimization Tips**
+```bash
+LC_ALL=C awk ...                  # 2-3x speedup for ASCII
+mawk (faster alternative to gawk) # For large datasets
+```
+
+### **When Not to Use AWK**
+- Binary data processing
+- Complex nested data structures (use `jq` for JSON)
+- Multi-gigabyte files (consider `split + parallel`)
+
+### **AWK vs Alternatives**
+| Task | Best Tool |
+|------|-----------|
+| Columnar data | AWK |
+| JSON/XML | `jq`/`xq` |
+| Complex stats | R/Python |
+| Multi-file joins | SQLite |
+
+---
+
+## **Pro Techniques**
+
+### **Self-Contained Scripts**
+```bash
+#!/usr/bin/awk -f
+BEGIN {print "Starting processing"}
+/pattern/ {count++}
+END {print "Found", count, "matches"}
+```
+
+### **Two-File Processing**
+```awk
+# Join two files on first field
+NR==FNR {a[$1]=$2; next} 
+$1 in a {print $0, a[$1]}
+```
+
+### **Bit Manipulation**
+```awk
+function is_set(x,bit) {return and(x, lshift(1, bit-1))}
+```
+
+---
+
+## **Further Learning**
+- **Books**: "Effective AWK Programming" (GNU AWK manual)
+- **Cheat Sheets**: [awkcheatsheet.com](https://awkcheatsheet.com)
+- **Practice**: [exercism.org/tracks/awk](https://exercism.org/tracks/awk)
+
+**Need an AWK solution?** Describe your data format and desired transformation!
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