How have both local and remote variable inside an SSH command locally available

Question

This is a variation of the question here: How have both local and remote variable inside an SSH command

How do I get the "ssh-variable" B to my local computer?

 A=3
 ssh host@name "echo $A; B=5; echo $A; echo \$B;"
 echo $A
 echo $B
 echo \$B

returns 3, NOTHING, $B

How can I locally access a value for B?

some explanations based on comments: I am using a bash script to access a remote host to modify some things there, amongst other things create a workspace there. The location of this workspace is what I want to store in a variable and save for later use.

Basically, I have a function to go to the remote host and make the workspace and then another function to use the path to that workspace to do other things there.

I was hoping for a lightweight, slim solution that can be integrated and easily read in a command similar to this:

 ssh host@name "ws_allocate myworkspace; workspace=ws_find myworkspace;"

and then locally use $workspace. This is part of a larger bash script, that should be easy to understand for non-expert bash users (like myself)...

Turns out this is a duplicate of bash—Better way to store variable between runs?

You can save the content of the remote variable in a file, and then recover the file content in your localhost. — aicastell
– aicastell, Commented Oct 18, 2018 at 13:21
Why? Write a local script, scp to upload the local script in the remote server, ssh to execute the script and store result in a /tmp file (in the remote server), scp to download the response, and then process the answer locally. — aicastell
– aicastell, Commented Oct 18, 2018 at 13:27
Think about it: you want a variable defined by a remote shell to be defined in your local shell. That's some significant interprocess communication; it's not going to be simple. — chepner
– chepner, Commented Oct 18, 2018 at 13:27
On the other hand, if your example is accurate and the assignment statement is hard-coded in the argument to ssh, why don't you just define it locally to begin with? — chepner
– chepner, Commented Oct 18, 2018 at 13:28

rje · Accepted Answer · 2018-10-18 13:39:17Z

0

You could do something like this:

X=$(ssh user@host 'echo $X')

This will run echo $X on the server and place the resulting output in a local variable X, which you can later use by saying $X

So to apply this to your example, you could say:

workspace=$(ssh host@name 'ws_allocate myworkspace; ws_find myworkspace;')

Note that the local variable workspace will contain the output from the command that is run remotely. So I'm assuming that in your example only the ws_find myworkspace will print any output and that the call to ws_allocate is silent.

edited Oct 18, 2018 at 13:39

answered Oct 18, 2018 at 13:21

rje

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5 Comments

gugy Over a year ago

C=$(ssh user@host "echo $B"); echo $C empty output

rje Over a year ago

This is because you are using double quotes "echo $B". Try using single quotes: 'echo $B'

gugy Over a year ago

can it bee that B is lost when ssh disconnects and therefore is not available when I connect again to save it in C with echo?

rje Over a year ago

Yes; if you made the variable in a previous ssh session it will not be available in the next session. But in that case I also don't get the point of doing this, please clarify what you are trying to do.

gugy Over a year ago

thanks, I have learnt a lot (again). In my example, this will not work out I am afraid (see my answer below)

gugy · Accepted Answer · 2018-10-18 14:02:59Z

-2

Ok, so the solution that appears to me to be the easiest and that allows the value of $B to be stored on the remote host is to save it in a file. As described and discussed here already: bash—Better way to store variable between runs? :(

ssh ${supercomputer} "
ws_list -a >workspaces.txt;
if grep -q "${experiment}" workspaces.txt; 
then echo 'we have a workspace already'; 
else ws_allocate ${experiment} 30; 
fi; 
ws_find ${experiment} > mypathtoworkspace;"

where supercomputer is the address to the host, experiment the workspace I want to create (or not, if it is there already) and workspace the workspace I want to create for my experiment.

I wanted to use the path to the remote workspace locally, e.g.

 echo "${workspace}"

but this seems to not be possible - therefore I will download the file I am writing on the remote host and read the path from that file to use it locally

If anyone sees a better solution it is welcome!

edited Oct 18, 2018 at 14:02

answered Oct 18, 2018 at 13:51

gugy

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4 Comments

rje Over a year ago

This does not make sense to me. Your question is about getting a remote variable to the local machine, and the answer is to store a local value on the remote machine?

gugy Over a year ago

it is a remote value that is stored on the remote machine. I am not leaving my remote host in this ssh command

rje Over a year ago

I see. Do you agree that this is not what the question is about?

gugy Over a year ago

in the overall context it is, in the context of the answer maybe not

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