Shell Tools Skill

Master jq, xargs, GNU parallel, and advanced pipelines

Learning Objectives

After completing this skill, you will be able to:

• Process JSON with jq
• Use xargs for argument handling
• Parallelize tasks with GNU parallel
• Build efficient data pipelines
• Use utility commands effectively

Prerequisites

• Strong Bash fundamentals
• Text processing basics
• Understanding of pipes

Core Concepts

1. jq Essentials

# Basic queries
jq '.' file.json              # Pretty print
jq '.key' file.json           # Get key
jq '.array[0]' file.json      # First element
jq '.nested.key' file.json    # Nested

# Filtering
jq '.[] | select(.active)'    # Filter
jq '.[] | select(.count > 10)'

# Transform
jq '.[] | {id, name}'         # Select fields
jq 'map(.price * .qty)'       # Calculate
jq -r '.[] | @csv'            # To CSV

# From variables
jq -n --arg x "$VAR" '{value: $x}'

2. Xargs

# Basic usage
echo "a b c" | xargs echo

# Safe with spaces
find . -print0 | xargs -0 rm

# Limit arguments
cat list | xargs -n 1 process
cat list | xargs -n 10 process

# Parallel
cat list | xargs -P 4 -n 1 process

# Placeholder
cat urls | xargs -I {} curl {}

3. GNU Parallel

# Basic
parallel echo ::: a b c

# From file
parallel process :::: list.txt

# With options
parallel -j 4 process ::: *.txt
parallel --progress process ::: *.txt

# Complex
parallel -j 4 --delay 0.5 \
    'curl -s {} | jq .name' :::: urls.txt

4. Pipeline Utilities

# Sort and unique
sort file.txt
sort -n file.txt          # Numeric
sort -u file.txt          # Unique
sort file | uniq -c       # Count

# Cut and paste
cut -d',' -f1,3 file.csv
paste file1.txt file2.txt

# Transform
tr 'a-z' 'A-Z' < file
tr -d '\r' < dos.txt > unix.txt

Common Patterns

API Data Pipeline

curl -s 'https://api.example.com/users' |
    jq -r '.[] | select(.active) | [.id, .email] | @csv' |
    sort -t',' -k2 |
    head -20

Parallel Processing

# Compress all logs in parallel
find . -name "*.log" |
    parallel -j 4 gzip

# Batch API calls with rate limit
cat ids.txt |
    parallel -j 5 --delay 0.2 \
        'curl -s "https://api.example.com/item/{}"'

Data Transformation

# JSON to formatted output
cat data.json |
    jq -r '.items[] | "\(.id)\t\(.name)\t\(.price)"' |
    column -t

Anti-Patterns

Don't	Do	Why
Parse JSON with grep	Use jq	Proper parsing
Sequential when parallel	Use parallel	Speed
cat | xargs	xargs < file	Efficiency

Practice Exercises

1. JSON Processor: Transform API response
2. Batch Processor: Parallel file processing
3. Log Analyzer: Complex log pipeline
4. Data Migrator: Transform and load data

Troubleshooting

Common Errors

Error	Cause	Fix
jq: error	Invalid JSON	Validate with jq .
xargs: arg too long	Too many args	Use -n
parallel: not found	Not installed	apt install parallel

Debug Techniques

# Validate JSON
jq '.' < input.json

# Debug pipeline
command1 | tee /dev/stderr | command2

# Test jq filter
echo '{"a":1}' | jq '.a'

Performance Tips

# Faster sorting
LC_ALL=C sort file.txt

# Parallel for CPU-bound
parallel -j $(nproc) process ::: *.txt

# Stream large files
jq -c '.[]' large.json | while read -r line; do
    # process line
done

Resources

• jq Manual
• GNU Parallel Tutorial
• jq Playground