lftp problems

[Alexander V. Lukyanov]

1. I cannot specify the protocol (for example, sftp) to use on the command line.
I'm trying to use lftp to issue the 'du' command to the remote host
non-interactively, as follows: "echo "du -sk ." | lftp -u myuser,mypass host".
This works, but does not use the sftp protocol.
2. -c does not behave like -c, although, according to the man page, it should.

It does, but it does not accept extra options. I.e. all arguments after -c
is the command.

lftp -c 'du -sk .' -u myuser,pass myhost
Unknown command `du -sk .'.

You should use this:

lftp -c 'open -u myuser,pass myhost && du -sk .'

3. I can't specify an ssh identity-key on the command line, but only in the
config file

You can specify it using -e option:

lftp -e "set ssl:key-file /root/.ssh/backup_key" -u myuser,pass myhost

4. sftp seems to fail to establish a connection, even though I think I'm doing
echo "open sftp://myhost && du -sk ." | lftp -u myuser

lftp :~> open sftp://myhost
lftp myhost:~> user myuser
Password: <CR>
---- Running connect program (ssh -a -x -s -l myuser myhost sftp)
---> sending a packet, length=5, type=1(INIT), id=0
<--- Permission denied, please try again.
---- Disconnecting
ls: Login failed: Login incorrect

imho, it should be asking me for a password if I have supplied a private key
that is not protected by a passphrase.

It asks for password always when there us a user name. If you authenticate
without password, just hit enter or specify an empty password in the
command line.

BTW, ssh cannot authenticate by lftp's setting ssl:key-file. You should
use this:

set sftp:connect-program "ssh -a -x -i /root/.ssh/backup_key"

HTH.