Execute remote SSH command and evaluate this command on remote side - linux

Question

Let's consider a following code:
CMD=echo $(hostname --fqdn) >> /tmp/hostname_fqdn
ssh some_user#10.9.11.4 -i ~/.ssh/id_rsa $CMD
And now, on remote side created file /tmp/hostname_fqdn contains hostname of client side instead of hostname of remote side. Is it possible to evaluate part of command (hostname --fqdn) on remote side? How to do it?

Answer 1

ssh some_user#10.9.11.4 -i ~/.ssh/id_rsa hostname --fqdn >> /tmp/hostname_fqdn
or, if the CMD may change at runtime:
CMD="hostname --fqdn" && ssh cloud.luminrobotics.com $CMD >> /tmp/xxx
You cannot, however, keep the redirection of the output (>> filename) be part of the command variable, because the command will be executed on the remote host.
PS: If what you want to do with the output changes at runtime as well, then you need to use a pipe and a separate command variable, e.g.,:
CMD_REMOTE="hostname --fqdn"
CMD_LOCAL="tee /tmp/hostname_fqdn"
ssh cloud.luminrobotics.com $CMD | $CMD_LOCAL

Answer 2

First,
CMD=echo $(hostname --fqdn) >> /tmp/hostname_fqdn
will likely do nothing like what you expect.
CMD=echo will be pasred as setting echo as the value of $CMD, and then the return from the hostname subshell we be executed, and likely fail, creating an empty file /tmp/hostname_fqdn
ssh on the other hand is pretty flexible. You could use
ssh some_user#10.9.11.4 -i ~/.ssh/id_rsa 'hostname --fqdn >> /tmp/hostname_fqdn'
if you want the remote hostname saved to a file on the remote host, or
ssh some_user#10.9.11.4 -i ~/.ssh/id_rsa 'hostname --fqdn' >> /tmp/hostname_fqdn
if you want the remote hostname on the local server, or
hostname --fqdn | ssh some_user#10.9.11.4 -i ~/.ssh/id_rsa 'cat >> /tmp/hostname_fqdn'
if you want the local hostname on the remote server...
You have options. :)