pass variable in ssh

I'm trying to pass a variable to ssh remote but not works. My code is:

#!/bin/bash set -x conexion="[email protected]" parameter="$1" ssh -T $conexion <<'ENDSSH'  clear echo "$parameter" ENDSSH 

I execute:

./ try 

It says me:

parameter: Undefined variable. 

any help please?


Passing variables (environment variables) over ssh is possible but generally restricted.

You need to tell the client to send them. For instance with OpenSSH, that's with:

ssh -o SendEnv=parameter host cmd...

But you also need the server the accept it (AcceptEnv configuration directive with OpenSSH). Accepting any variable is a big security risk so is generally not done by default, though some ssh deployments allow some variables under some namespace (like LC_* in some OpenSSH deployments).

You also need to export the variable before calling ssh, like:

LC_parameter="$parameter" ssh -o SendEnv=LC_parameter host csh << 'END'
echo $LC_parameter:q

Above, we're passing the content of the $parameter bash shell variable as the LC_parameter environment variable to ssh. ssh sends that over to sshd, which if it accepts it, passes it as an environment variable to the login shell of the user which then passes it to that csh command (which can then expand it).

But as mentioned earlier, that won't work unless the administrator of the host machine has added a AcceptEnv LC_parameter or AcceptEnv LC_* (that one sometimes done by default) to the sshd configuration.

The Undefined variable error message in your example suggests the login shell of the remote user is csh or tcsh. It's better to explicitly invoke the shell to avoid surprises (ssh host csh also means a tty is not requested so you don't need -T). Note the $LC_parameter:q syntax, which is the csh way to pass the content of a variable verbatim, not "$LC_parameter" which doesn't work if the variable contains newline characters.

If using LC_* variables is not an option, then alternatively, you can have the client shell (bash in your case) expand the variable. A naive way would be with

ssh host csh << END
echo "$variable"

But that would be dangerous as the content of the variable would be interpreted by the remote shell. If $variable contains `reboot` or "; reboot; : " for instance, that would have bad consequences.

So, you'd need first to make sure the variable is properly quoted in the syntax of the remote shell. Here, I would avoid csh where it's hard to do reliably and use sh/bash/ksh instead.

Use a helper function to do the sh quoting:

shquote() {
  awk -v q=\' -v b='\\' '
      for (i=1; i<ARGC; i++) {
        gsub(q, q b q q, ARGV[i])
       printf "%s ", q ARGV[i] q
      print ""
    }' "[email protected]"

And call ssh as:

ssh host sh << END
parameter=$(shquote "$parameter")
echo "\$parameter"

See how we escape the third $ so the expansion of $parameter is done by the remote shell, not the local one.

Removing quotes from ENDSSH on the 4th line should help.

Courtesy: @Archemar in comments to the question.

WARNING As it was stated by @St├ęphaneChazelas in the comment to this answer, this sollution would cause the $parameter variable in the here doc to be expanded by the local shell, which means the content of the variable would be interpreted as shell code by the remote shell, which means it introduces a command injection vulnerability. So generally it's discouraged to do so.

Category: ssh Time: 2016-07-29 Views: 0
Tags: ssh

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