I tried redis's DUMP command, redirect to file (or pipe), but RESTORE report this error:
$ redis-cli dump test > /tmp/test.dump
$ cat /tmp/test.dump | redis-cli -x restore test1 0
(error) ERR DUMP payload version or checksum are wrong
$ redis-cli dump test | redis-cli -x restore test1 0
(error) ERR DUMP payload version or checksum are wrong
I am aware that MIGRATE can do this online, but MIGRATE also delete that key from original server, and I don't want my redis expose to public internet.
There are some third-party options, redis-rdb-tools for example, but after all, how exactly do DUMP and RESTORE work?
The dump/restore commands are not really designed to be used from the command line, because the serialization format is binary (it is the same one used for RDB dumps). It makes it inconvenient because the shell tends to interpret those characters (even when the "printable" format is used).
Here is the "printable" format:
$ redis-cli lpush test 1 2 3 4 5
(integer) 5
$ redis-cli dump test
"\n\x15\x15\x00\x00\x00\x12\x00\x00\x00\x05\x00\x00\xf6\x02\xf5\x02\xf4\x02\xf3\x02\xf2\xff\x06\x00\x1c\x8a\xda\x0e}\xcb\xe1."
The "printable" format cannot be used as input for the -x option which really expects the actual data. This is a misleading behavior of redis-cli.
However, there is an easy way to get the raw format:
$ redis-cli --raw dump test | hexdump -C
00000000 0a 15 15 00 00 00 12 00 00 00 05 00 00 f6 02 f5 |................|
00000010 02 f4 02 f3 02 f2 ff 06 00 1c 8a da 0e 7d cb e1 |.............}..|
00000020 2e 0a |..|
Now, it is not possible to directly pipe the result of a --raw dump in a -x restore, because the last character is wrong. Compare the output of the --raw and printable dump. You will notice the --raw option adds an extra \n at the end. The raw option is not 100% raw ;-)
This extra character needs to be removed before the data can be processed by the -x option. Finally, the correct command (on a GNU/Linux system) to pipe the output of a dump in a restore is:
$ redis-cli --raw dump test | head -c-1 | redis-cli -x restore test1 0
OK
This is not pretty. I expect most people would rely of a perl/python/ruby script rather than the shell to do such tasks.