# curl Quick Reference

*HTTP requests, headers, auth, forms, debugging*

> Source: curl Documentation (curl.se/docs) · MIT

## Basic Usage

### Simple Requests

```
curl https://example.com          # GET request
curl -o file.html https://url     # save to file
curl -O https://url/file.tar.gz   # save with remote name
curl -L https://url               # follow redirects
```

### Common Flags

| Command | Description |
|---------|-------------|
| `-s` | Silent mode (no progress) |
| `-S` | Show errors in silent mode |
| `-f` | Fail silently on HTTP errors |
| `-L` | Follow redirects |
| `-o file` | Write output to file |
| `-O` | Save with remote filename |
| `-C -` | Resume interrupted download |
| `--max-time 30` | Timeout after 30 seconds |

## HTTP Methods

### GET & HEAD

```
curl https://api.example.com/users
curl -I https://example.com       # HEAD (headers only)
curl -i https://example.com       # include response headers
```

### POST

```
curl -X POST https://api.example.com/users \
  -H "Content-Type: application/json" \
  -d '{"name":"Jo","email":"jo@ex.com"}'
```

### PUT & PATCH & DELETE

```
curl -X PUT https://api.example.com/users/1 \
  -d '{"name":"Updated"}'
curl -X PATCH https://api.example.com/users/1 \
  -d '{"email":"new@ex.com"}'
curl -X DELETE https://api.example.com/users/1
```

## Headers

### Setting Headers

```
curl -H "Content-Type: application/json" URL
curl -H "Accept: text/html" URL
curl -H "X-Custom: value" URL
curl -H "Header1: v1" -H "Header2: v2" URL
```

### Response Headers

| Command | Description |
|---------|-------------|
| `-i` | Include response headers in output |
| `-I` | Fetch headers only (HEAD) |
| `-D file` | Dump response headers to file |
| `-w '%{http_code}'` | Print HTTP status code |

## Authentication

### Basic & Token Auth

```
curl -u user:pass https://api.example.com
curl -H "Authorization: Bearer TOKEN" URL
curl -u user:pass --digest URL
curl --negotiate -u : URL   # Kerberos/SPNEGO
```

### Auth Methods

| Command | Description |
|---------|-------------|
| `-u user:pass` | Basic authentication |
| `--digest` | HTTP Digest auth |
| `--negotiate` | Kerberos/SPNEGO auth |
| `--ntlm` | NTLM authentication |
| `-n` | Use ~/.netrc credentials |

## Data & Forms

### Sending Data

```
curl -d "key=val&key2=val2" URL   # form urlencoded
curl -d @data.json URL            # data from file
curl --data-raw '{"raw":"json"}' URL
curl --data-urlencode "q=hello world" URL
```

### File Uploads

```
curl -F "file=@photo.jpg" URL
curl -F "file=@doc.pdf;type=application/pdf" URL
curl -F "field=value" -F "file=@img.png" URL
```

### Multipart vs URL-Encoded

| Command | Description |
|---------|-------------|
| `-d` | application/x-www-form-urlencoded |
| `-F` | multipart/form-data |
| `--json` | Shorthand: sets Content-Type + Accept to JSON |
| `-T file` | Upload file via PUT |

## SSL/TLS

### Certificate Options

```
curl --cacert ca.pem URL          # custom CA bundle
curl --cert client.pem URL        # client certificate
curl --cert client.pem --key key.pem URL
curl -k URL                       # skip TLS verify (dev only)
```

### TLS Flags

| Command | Description |
|---------|-------------|
| `-k / --insecure` | Skip TLS certificate verification |
| `--cacert file` | Use custom CA certificate |
| `--cert file` | Client certificate |
| `--key file` | Client private key |
| `--tlsv1.2` | Force minimum TLS 1.2 |
| `--tlsv1.3` | Force minimum TLS 1.3 |

## Output & Debugging

### Verbose & Trace

```
curl -v URL                       # verbose output
curl --trace dump.txt URL         # full trace to file
curl --trace-ascii - URL          # trace to stdout
curl -w "\n%{http_code}\n" URL    # custom output format
```

### Write-Out Variables

| Command | Description |
|---------|-------------|
| `%{http_code}` | HTTP response status code |
| `%{time_total}` | Total time in seconds |
| `%{time_connect}` | Time to establish connection |
| `%{size_download}` | Downloaded bytes |
| `%{speed_download}` | Average download speed |
| `%{redirect_url}` | Redirect URL (if any) |
| `%{ssl_verify_result}` | SSL verification result (0 = OK) |

### Write-Out Example

```
curl -s -o /dev/null -w \
  "code: %{http_code}\ntime: %{time_total}s\n" \
  https://example.com
```

## Common Patterns

### API Workflow

```
# GET JSON and pipe to jq
curl -s https://api.example.com/data | jq '.items[]'
# POST JSON with auth
curl -s -H "Authorization: Bearer $TOKEN" \
  --json '{"key":"val"}' https://api.example.com
```

### Download Patterns

```
# Download with progress bar
curl -# -O https://releases.example.com/v2.tar.gz
# Resume interrupted download
curl -C - -O https://releases.example.com/v2.tar.gz
# Download multiple files
curl -O https://url/file1 -O https://url/file2
```

### Scripting Helpers

```
# Check if URL is reachable
curl -sf -o /dev/null https://example.com && echo OK
# Save cookies and reuse
curl -c cookies.txt -b cookies.txt URL
# Rate-limit request
curl --limit-rate 100k URL
```

## Proxy & Network

### Proxy Settings

```
curl -x http://proxy:8080 URL
curl -x socks5://proxy:1080 URL
curl --proxy-user user:pass -x http://proxy:8080 URL
curl --noproxy "*.local,localhost" URL
```

### DNS & Resolve

| Command | Description |
|---------|-------------|
| `--resolve host:port:addr` | Force DNS resolution to addr |
| `--dns-servers 8.8.8.8` | Use custom DNS server |
| `--interface eth0` | Use specific network interface |
| `-4 / -6` | Force IPv4 / IPv6 |

## Config & Advanced

### Config Files

```
# ~/.curlrc — default options
--silent
--location
--max-time 30

# Use config file explicitly
curl -K myconfig.txt URL
```

### Useful Flags

| Command | Description |
|---------|-------------|
| `--retry 3` | Retry on transient errors |
| `--retry-delay 2` | Delay between retries (seconds) |
| `--compressed` | Request and decompress gzip/br |
| `--limit-rate 100k` | Throttle transfer speed |
| `-Z` | Parallel transfers (curl 7.66+) |
| `--create-dirs` | Create path directories for -o |