curl --parallel --parallel-immediate -k -L -C - -o local_file ftp_file –parallel –parallel-immediate 开启多线程：
–parallel Perform transfers in parallel
–parallel-immediate Do not wait for multiplexing (with –parallel)
–parallel-max Maximum concurrency for parallel transfers
-C – 开启断点续传：
-C, –continue-at <offset> Resumed transfer offset
-o指定输出文件名称：
-o, –output <file> Write to file instead of stdout
失败重新自动提交：
–retry <num> Retry request if transient problems occur
–retry-all-errors Retry all errors (use with –retry)
–retry-connrefused Retry on connection refused (use with –retry)
–retry-delay <seconds> Wait time between retries
–retry-max-time <seconds> Retry only within this period
