HTTP and curl

curl -sSv https://api.example.com/health 2>&1 | grep -vE '^\{|^\}'   # handshake, headers, status
curl -o /dev/null -sw 'code=%{http_code} dns=%{time_namelookup} conn=%{time_connect} tls=%{time_appconnect} ttfb=%{time_starttransfer} total=%{time_total}\n' https://api.example.com/health
curl --resolve api.example.com:443:10.0.0.5 -sv https://api.example.com/health   # bypass DNS, keep SNI and Host

curl -X POST https://api.example.com/items \
  -H 'content-type: application/json' \
  -d '{"name":"widget","qty":4}'

curl -X POST https://api.example.com/items -d @payload.json -H 'content-type: application/json'
curl -F 'file=@report.csv' -F 'note=monthly' https://api.example.com/upload   # multipart
curl -G https://api.example.com/search --data-urlencode 'q=name with spaces' --data-urlencode 'limit=10'
curl -X PATCH -H 'content-type: application/merge-patch+json' -d '{"qty":5}' https://api.example.com/items/1
curl -T ./artifact.tgz https://uploads.example.com/path/    # PUT a file

cat > curl-format.txt <<'EOF'
     dns  %{time_namelookup}s
 connect  %{time_connect}s
     tls  %{time_appconnect}s
 waiting  %{time_starttransfer}s
   total  %{time_total}s
    size  %{size_download} bytes
    code  %{http_code}
EOF

curl -o /dev/null -s -w '@curl-format.txt' https://api.example.com/health

# Ten samples, to separate a slow server from a slow tail
for i in $(seq 10); do curl -o /dev/null -sw '%{time_total}\n' https://api.example.com/health; done | sort -n

curl -vI https://example.com 2>&1 | grep -E 'SSL connection|subject|issuer|expire'
curl --tlsv1.3 --tls-max 1.3 -sI https://example.com    # force a version
curl --cacert /path/ca.pem https://internal.example.com
curl --cert client.pem --key client.key https://mtls.example.com
openssl s_client -connect example.com:443 -servername example.com </dev/null 2>/dev/null | openssl x509 -noout -dates -subject -issuer

curl -x http://proxy.example.com:3128 https://api.example.com
export http_proxy=http://proxy:3128 https_proxy=http://proxy:3128 no_proxy=.internal,localhost,10.0.0.0/8
curl --proxy-user user:pass -x http://proxy:3128 URL
curl -x socks5h://localhost:1080 URL          # h = resolve DNS at the proxy
curl -sv https://api.example.com 2>&1 | grep -i 'CONNECT\|proxy'

if ! body=$(curl -fsS --max-time 10 --retry 3 --retry-connrefused https://api.example.com/health); then
  printf 'health check failed\n' >&2
  exit 1
fi

code=$(curl -o /tmp/body -D /tmp/headers -sw '%{http_code}' -X POST -d @payload.json -H 'content-type: application/json' https://api.example.com/items)
case $code in
  20*) ;;
  429) sleep "$(awk 'tolower($1) ~ /^retry-after:/ {print $2}' /tmp/headers | tr -d '\r')" ;;
  *)   printf 'unexpected %s: %s\n' "$code" "$(cat /tmp/body)" >&2; exit 1 ;;
esac

# Status code only
curl -o /dev/null -sw '%{http_code}\n' https://example.com

# Follow redirects and show each hop
curl -sIL https://example.com | awk '/^HTTP|^location/'

# Final URL after redirects
curl -o /dev/null -sw '%{url_effective}\n' -L https://example.com

# Response headers only, sorted
curl -sD - -o /dev/null https://example.com | sort

# Poll until a service is healthy
until curl -fsS --max-time 2 http://localhost:8080/health >/dev/null; do sleep 1; done

# Hammer an endpoint 50 times in parallel and summarise codes
seq 50 | xargs -P10 -I{} curl -o /dev/null -sw '%{http_code}\n' https://api.example.com/ | sort | uniq -c

# Compare two environments' responses
diff <(curl -s https://staging.example.com/api/config | jq -S .) <(curl -s https://prod.example.com/api/config | jq -S .)

# Download with resume
curl -C - -O https://example.com/big.iso

# Send a signed webhook payload
body='{"event":"test"}'; sig=$(printf '%s' "$body" | openssl dgst -sha256 -hmac "$SECRET" -hex | awk '{print $2}'); curl -X POST -H "x-signature: sha256=$sig" -d "$body" https://api.example.com/hook

# Check which HTTP version and ALPN was negotiated
curl -so /dev/null -w '%{http_version} %{scheme}\n' https://example.com

# Request with a fixed Host header against an IP (vhost testing)
curl -H 'Host: api.example.com' http://10.0.0.5/health

# Measure the slowest of N endpoints
for u in "${urls[@]}"; do printf '%s %s\n' "$(curl -o /dev/null -sw '%{time_total}' "$u")" "$u"; done | sort -rn | head