What protocols does cURL support? How to use FTP/SFTP for file transfer? - 面试题

cURL is a multi-protocol data transfer tool supporting over 20 protocols. Understanding these protocol capabilities is important for choosing the appropriate transfer method.

Protocols Supported by cURL

Protocol	Purpose	Example URL
HTTP/HTTPS	Web requests	`http://example.com`
FTP/FTPS	File transfer	`ftp://ftp.example.com`
SFTP	Secure file transfer	`sftp://user@host.com`
SCP	Secure copy	`scp://user@host.com/file`
TFTP	Simple file transfer	`tftp://host.com/file`
LDAP/LDAPS	Directory services	`ldap://ldap.example.com`
SMTP/SMTPS	Email sending	`smtp://mail.example.com`
POP3/POP3S	Email receiving	`pop3://mail.example.com`
IMAP/IMAPS	Email access	`imap://mail.example.com`
RTSP/RTMP	Streaming media	`rtsp://stream.example.com`
FILE	Local files	`file:///path/to/file`
GOPHER	Early internet protocol	`gopher://gopher.example.com`
DICT	Dictionary protocol	`dict://dict.org/d:word`
TELNET	Remote login	`telnet://host.com`

FTP File Transfer

FTP Download

bash
# Anonymous FTP download
curl ftp://ftp.example.com/pub/file.txt -o file.txt

# Authenticated FTP download
curl -u "username:password" \
     ftp://ftp.example.com/remote/file.txt \
     -o file.txt

# Download directory listing
curl ftp://ftp.example.com/pub/

# Passive mode FTP
curl --ftp-skip-pasv-ip \
     ftp://ftp.example.com/file.txt

FTP Upload

bash
# Upload file to FTP
curl -u "username:password" \
     -T local-file.txt \
     ftp://ftp.example.com/remote/path/

# Upload with specific filename
curl -u "username:password" \
     -T local-file.txt \
     ftp://ftp.example.com/remote/renamed-file.txt

# Create directory and upload
curl -u "username:password" \
     --ftp-create-dirs \
     -T file.txt \
     ftp://ftp.example.com/new-folder/file.txt

Advanced FTP Operations

bash
# Delete remote file
curl -u "username:password" \
     -Q "DELE remote-file.txt" \
     ftp://ftp.example.com/

# Rename file
curl -u "username:password" \
     -Q "RNFR old-name.txt" \
     -Q "RNTO new-name.txt" \
     ftp://ftp.example.com/

# List detailed file info
curl -u "username:password" \
     ftp://ftp.example.com/ -l

# Use FTPS (FTP over SSL)
curl -u "username:password" \
     ftps://ftp.example.com/file.txt

# Active mode FTP
curl --ftp-port - \
     -u "username:password" \
     ftp://ftp.example.com/file.txt

SFTP File Transfer

SFTP Download

bash
# Basic SFTP download
curl -u "username:password" \
     sftp://sftp.example.com/remote/file.txt \
     -o file.txt

# Use key authentication (recommended)
curl -u "username:" \
     --key ~/.ssh/id_rsa \
     --pubkey ~/.ssh/id_rsa.pub \
     sftp://sftp.example.com/remote/file.txt \
     -o file.txt

# Specify port
curl -u "username:password" \
     sftp://sftp.example.com:2222/remote/file.txt

# Download directory (requires tar)
ssh user@sftp.example.com "tar czf - /remote/folder" | tar xzf -

SFTP Upload

bash
# Upload file to SFTP
curl -u "username:password" \
     -T local-file.txt \
     sftp://sftp.example.com/remote/path/

# Use key for upload
curl -u "username:" \
     --key ~/.ssh/id_rsa \
     -T local-file.txt \
     sftp://sftp.example.com/remote/

# Upload multiple files
curl -u "username:password" \
     -T "file1.txt" \
     -T "file2.txt" \
     sftp://sftp.example.com/remote/

SCP File Transfer

bash
# SCP download
curl -u "username:password" \
     scp://host.example.com/remote/file.txt \
     -o file.txt

# SCP upload
curl -u "username:password" \
     -T local-file.txt \
     scp://host.example.com/remote/

# Use key
curl -u "username:" \
     --key ~/.ssh/id_rsa \
     scp://host.example.com/remote/file.txt

Protocol Comparison

Feature	FTP	FTPS	SFTP	SCP
Encryption	❌	✅	✅	✅
Port	21	21/990	22	22
Authentication	Password	Password/Cert	Password/Key	Password/Key
Firewall Friendly	❌	⚠️	✅	✅
Recommended For	Internal	Enterprise	General	Simple transfer

Email Protocol Examples

bash
# SMTP send email
curl -v smtp://smtp.gmail.com:587 \
  --mail-from "sender@example.com" \
  --mail-rcpt "recipient@example.com" \
  --upload-file email.txt \
  --user "username:password"

# POP3 read email
curl -u "username:password" \
     pop3://pop.gmail.com

# IMAP list mailboxes
curl -u "username:password" \
     imap://imap.gmail.com

Utility Script

bash
#!/bin/bash
# FTP/SFTP automatic backup script

SOURCE="/local/path/to/backup"
DEST="sftp://backup.example.com/backups/"
USER="backup_user"
KEY="~/.ssh/backup_key"
DATE=$(date +%Y%m%d)
ARCHIVE="backup-${DATE}.tar.gz"

# Create archive
tar czf "/tmp/${ARCHIVE}" -C "$(dirname $SOURCE)" "$(basename $SOURCE)"

# Upload to SFTP
curl -u "${USER}:" \
     --key "$KEY" \
     -T "/tmp/${ARCHIVE}" \
     "${DEST}${ARCHIVE}"

# Clean up local temp file
rm "/tmp/${ARCHIVE}"

echo "Backup completed: ${ARCHIVE}"

Common Issues

bash
# Issue 1: FTP passive mode fails
# Solution: Use --ftp-skip-pasv-ip
curl --ftp-skip-pasv-ip ftp://ftp.example.com/file

# Issue 2: SFTP key permission error
# Solution: Ensure private key permissions are correct
chmod 600 ~/.ssh/id_rsa

# Issue 3: SSL certificate verification fails
# Solution: Specify CA or skip verification (testing only)
curl --cacert /path/to/ca.crt ftps://ftp.example.com/file
curl -k ftps://ftp.example.com/file  # Not secure

# Issue 4: Filename contains spaces
# Solution: Use URL encoding or quotes
curl -u "user:pass" "ftp://ftp.example.com/file%20name.txt"