What is the difference between execution ssh command followed by remote command with and quotes and the remote command without quotes?
E.g. what is the difference between:
ssh remotehost tar xf archivename.tar
and
ssh remotehost "tar xf archivename.tar"
or what is the difference between:
ssh remotehost <path to script>/myscript.sh param
and
ssh remotehost "<path to script>/myscript.sh param"
The trailing arguments are combined into a single string which is passed to as an argument to the -c option of your login shell on the remote machine. In your first example, there is little difference. However, consider the following:
$ ssh remotehost echo *
Here, * is expanded locally, and the result is passed to the remote machine. On my machine, my working directory consisted of a single entry containing an apostrophe (single quote), so the remote host tried to run a command like
bash -c "echo chepner's folder"
which of course resulted in an error.
$ ssh remotehost "echo *"
Now the remote host gets a literal *, but it is subject to shell expansion before being passed to bash -c. As a result, you output all the files in your home directory on the remote host.
$ ssh remotehost "echo \*"
Finally, the remote shell gets a quoted asterisk in its input, so you get a single literal * as output.
The quotes prevents expansion of special characters.
Imagine if there were any in your command you want to execute remotely (;, &, &&, |, ||, >, >> etc.). They would take effect before actually getting to your remote host, because bash would not know that all of it is just an argument to ssh.
Also, if the path to your script contained more spaces, they'd get lost and script wouldn't be found.
