curl Tricks Most Developers Don't Know

curl can do way more than just GET requests. Here are the tricks I use constantly that most people don't know about.

Time Any Request in Detail

curl -w "\nDNS: %{time_namelookup}s\nConnect: %{time_connect}s\nTLS: %{time_appconnect}s\nFirst byte: %{time_starttransfer}s\nTotal: %{time_total}s\nSize: %{size_download} bytes\n" -o /dev/null -s https://example.com

This is incredibly useful for debugging slow APIs. You can immediately see if the bottleneck is DNS, TLS handshake, or server processing.

Bypass DNS / Test Specific IPs

# Hit a specific IP but send the right Host header
curl --resolve example.com:443:93.184.216.34 https://example.com

# Test a staging server
curl --resolve api.myapp.com:443:10.0.0.5 https://api.myapp.com/health

Retry with Backoff

# Retry 5 times with exponential backoff
curl --retry 5 --retry-delay 2 --retry-max-time 60 https://api.example.com

# Only retry on specific errors
curl --retry 3 --retry-connrefused https://api.example.com

Auth Patterns

# Bearer token
curl -H "Authorization: Bearer $TOKEN" https://api.example.com/me

# Basic auth
curl -u username:password https://api.example.com

# API key in header
curl -H "X-API-Key: abc123" https://api.example.com

Upload Files

# Upload a file
curl -F "file=@photo.jpg" https://upload.example.com

# Upload with metadata
curl -F "file=@doc.pdf" -F "name=report" -F "public=true" https://api.example.com/upload

# PUT a file
curl -T backup.tar.gz https://storage.example.com/backups/

JSON Workflow

# POST JSON
curl -X POST https://api.example.com/users \
  -H 'Content-Type: application/json' \
  -d '{"name": "test", "email": "test@example.com"}'

# POST JSON from file
curl -X POST https://api.example.com/data \
  -H 'Content-Type: application/json' \
  -d @payload.json

# Pretty print response (pipe to jq)
curl -s https://api.example.com/data | jq .

Download Like a Pro

# Download with progress bar
curl -# -O https://example.com/big-file.iso

# Resume broken download
curl -C - -O https://example.com/big-file.iso

# Download and rename
curl -o myfile.zip https://example.com/download?id=123

# Rate limit download (100KB/s)
curl --limit-rate 100K -O https://example.com/big-file.iso

Bonus: Create a .curlrc

# ~/.curlrc - defaults for every curl command
-w "\n"              # always add newline after output
--connect-timeout 5   # don't wait forever
-s                    # silent by default
-S                    # but show errors

The -w flag for timing and --resolve for DNS bypass are the two tricks that save me the most time debugging production issues.