How does cURL handle HTTP redirects?

Redirect handling is a common requirement when accessing web resources with cURL. Properly handling redirects ensures requests reach the final destination.

Basic Redirect Handling

bash
# Don't follow redirects (default behavior)
curl http://example.com

# Automatically follow redirects
curl -L http://example.com
curl --location http://example.com

# Show redirect process
curl -L -v http://example.com

Redirect Limit

bash
# Limit maximum redirects (default 50)
curl -L --max-redirs 5 http://example.com

# Unlimited redirects (not recommended)
curl -L --max-redirs -1 http://example.com

POST Request Redirect Behavior

bash
# Default: POST becomes GET after redirect
curl -L -X POST -d "data=test" http://example.com/submit

# Keep POST method (RFC 7231)
curl -L --post301 -X POST -d "data=test" http://example.com/submit

# Keep POST method (non-standard)
curl -L --post302 -X POST -d "data=test" http://example.com/submit
curl -L --post303 -X POST -d "data=test" http://example.com/submit

Redirect Types

Status Code	Type	Default Behavior
301	Permanent Redirect	POST → GET
302	Temporary Redirect	POST → GET
303	See Other	POST → GET
307	Temporary Redirect	Keep Method
308	Permanent Redirect	Keep Method

View Redirect Chain

bash
# Show all redirect URLs
curl -L -v http://example.com 2>&1 | grep -E "(< HTTP|< Location)"

# Output final URL using -w
curl -L -w "Final URL: %{url_effective}\n" -o /dev/null -s http://example.com

# Get redirect count
curl -L -w "Redirects: %{num_redirects}\n" -o /dev/null -s http://example.com

Common Redirect Scenarios

bash
# HTTP to HTTPS redirect
curl -L http://example.com

# www to non-www redirect
curl -L http://www.example.com

# Short link expansion
curl -L -w "Final URL: %{url_effective}\n" -o /dev/null -s https://bit.ly/shorturl

# Redirect after login
curl -L -c cookies.txt -b cookies.txt \
     -X POST \
     -d "username=admin&password=123456" \
     http://example.com/login

Advanced Redirect Control

bash
# Limit redirect protocols
curl -L --proto-redir =https http://example.com

# Keep headers during redirect
curl -L --location-trusted \
     -H "Authorization: Bearer token123" \
     http://example.com/api

# View cookies before and after redirect
curl -L -c cookies.txt -b cookies.txt -v http://example.com 2>&1 | grep -i cookie

# Modify headers during redirect
curl -L -H "Host: newdomain.com" http://example.com

Redirect Debug Script

bash
#!/bin/bash
# Redirect trace script

URL="http://example.com"

echo "========== Redirect Chain Trace =========="
echo "Initial URL: $URL"
echo ""

# Method 1: Use -v to view
curl -L -v "$URL" 2>&1 | grep -E "(< HTTP|< Location)" | head -20

echo ""
echo "========== Final Info =========="

# Get final URL
FINAL_URL=$(curl -L -w "%{url_effective}" -o /dev/null -s "$URL")
echo "Final URL: $FINAL_URL"

# Get redirect count
REDIRECTS=$(curl -L -w "%{num_redirects}" -o /dev/null -s "$URL")
echo "Redirect Count: $REDIRECTS"

# Get final status code
STATUS=$(curl -L -w "%{http_code}" -o /dev/null -s "$URL")
echo "Final Status Code: $STATUS"

Common Issues

bash
# Issue 1: Redirect loop
# Solution: Limit redirect count
curl -L --max-redirs 10 http://example.com

# Issue 2: Lost authentication on cross-domain redirect
# Solution: Use --location-trusted
curl -L --location-trusted -H "Authorization: Bearer token" http://example.com

# Issue 3: POST data lost
# Solution: Use --post302 or --post301
curl -L --post302 -X POST -d "data=test" http://example.com

# Issue 4: HTTPS redirect fails
# Solution: Ensure SSL certificate is valid or use -k
curl -L -k http://example.com

# Issue 5: Redirect to different domain
# Solution: Check and update Host header
curl -L -H "Host: newdomain.com" http://example.com

Practical Examples

bash
# Complete redirect-aware API call
curl -L --max-redirs 5 \
     --post302 \
     -X POST \
     -H "Content-Type: application/json" \
     -H "Authorization: Bearer token123" \
     -d '{"action":"submit"}' \
     -w "\nFinal URL: %{url_effective}\nRedirects: %{num_redirects}\nHTTP Status: %{http_code}\n" \
     http://api.example.com/submit

# Batch short link expansion
for url in "bit.ly/abc" "t.co/xyz"; do
    echo -n "$url -> "
    curl -L -w "%{url_effective}" -o /dev/null -s "https://$url"
    echo ""
done

​