Update tech_docs/linux/jq.md

tech_docs/linux/jq.md

@@ -244,3 +244,113 @@ man jq
 ```
 
 Happy learning! If you have any specific questions or need further assistance with `jq`, let me know!
+
+---
+
+# Learning `jq` for Command-Line JSON Processing
+
+`jq` is a powerful command-line JSON processor that lets you parse, filter, and transform JSON data. Here's a comprehensive guide to get you started:
+
+## Installation
+
+Most Linux distributions and macOS can install it via package managers:
+
+```bash
+# Ubuntu/Debian
+sudo apt install jq
+
+# CentOS/RHEL
+sudo yum install jq
+
+# macOS (using Homebrew)
+brew install jq
+
+# Windows (via Chocolatey)
+choco install jq
+```
+
+## Basic Usage
+
+```bash
+# Basic pretty-printing
+jq '.' file.json
+
+# Read from stdin
+curl -s https://api.example.com/data | jq '.'
+```
+
+## Selecting Data
+
+```bash
+# Get a specific field
+jq '.field' file.json
+
+# Get nested fields
+jq '.parent.child.grandchild' file.json
+
+# Get array elements
+jq '.array[0]' file.json       # First element
+jq '.array[-1]' file.json      # Last element
+jq '.array[2:5]' file.json     # Slice from index 2 to 5
+```
+
+## Common Operations
+
+```bash
+# Get multiple fields
+jq '{name: .name, age: .age}' file.json
+
+# Filter arrays
+jq '.users[] | select(.age > 30)' file.json
+
+# Map operations
+jq '.numbers[] | . * 2' file.json
+
+# Sort
+jq '.users | sort_by(.age)' file.json
+
+# Length/count
+jq '.array | length' file.json
+```
+
+## Advanced Features
+
+```bash
+# String interpolation
+jq '"Hello, \(.name)!"' file.json
+
+# Conditional logic
+jq 'if .age > 18 then "Adult" else "Minor" end' file.json
+
+# Variables
+jq '. as $item | $item.name' file.json
+
+# Custom functions
+jq 'def add(x; y): x + y; add(5; 10)' <<< '{}'
+```
+
+## Practical Examples
+
+```bash
+# Extract all email addresses from JSON
+jq -r '.users[].email' file.json
+
+# Convert JSON to CSV
+jq -r '.users[] | [.name, .email, .age] | @csv' file.json
+
+# Sum all values in an array
+jq '[.numbers[]] | add' file.json
+
+# Find unique values
+jq '.tags[]' file.json | sort | uniq
+
+# Modify JSON structure
+jq '{user: .name, contact: {email: .email, phone: .tel}}' file.json
+```
+
+## Tips
+
+1. Use `-r` for raw output (no quotes around strings)
+2. Combine with `curl` for API responses: `curl -s ... | jq ...`
+3. Use `//` for default values: `jq '.name // "Anonymous"'`
+4. For large files, use `--stream` for iterative parsing