When ssh'ing, how can I set an environment variable on the server that changes from session to session?

Question

When I ssh into a server, how can I pass an environment variable from the client to the server? This environment variable changes between different invocations of ssh so I don't want to overwrite $HOME/.ssh2/environment every time I do an ssh call. How can I do this?

Answer 1

Of course, you can set the environment variable inside the command, however you'll have to be careful about quoting: remember that your shell is going to parse your local command line, and then the remote shell will have a go on the string it receives.

If you want a variable to get the same value on the server that it has on the client, try the SendEnv option:

ssh -o SendEnv=MYVAR server.example.com mycommand

This requires support from the server, though. With OpenSSH, the variable name has to be authorized in /etc/sshd_config.

If the server only allows certain specific variable names, you can work around that; for example a common setup allows LC_* through, and you can do the following:

ssh -o SendEnv=LC_MYVAR server.example.com 'MYVAR=$LC_MYVAR; unset LC_MYVAR; export MYVAR; mycommand'

If even LC_* is not an option, you can pass information in the TERM environment variable, which is always copied (there may be a length limit however). You'll still have to make sure that the remote shell doesn't restrict the TERM variable to designate a known terminal type. Pass the -t option to ssh if you're not starting a remote interactive shell.

env TERM="extra information:$TERM" ssh -t server.example.com 'MYVAR=${TERM%:*}; TERM=${TERM##*:}; export MYVAR; mycommand'

Another possibility is to define the variable directly in the command:

ssh -t server.example.com 'export MYVAR="extra information"; mycommand'

Thus, if passing a local variable:

ssh -t server.example.com 'export MYVAR='"'$LOCALVAR'"'; mycommand'

However, beware of quoting issues: the value of the variable will be interpolated directly into the shell snippet executed on the remote side. The last example above assumes that $LOCALVAR does not contain any single quotes (').

Answer 2

If you can administrate the target host you can configure sshd to allow passing your local environment variables along to the target host.

From the sshd_config man page:

 PermitUserEnvironment
     Specifies whether ~/.ssh/environment and environment= options in
     ~/.ssh/authorized_keys are processed by sshd.  The default is
     "no".  Enabling environment processing may enable users to bypass
     access restrictions in some configurations using mechanisms such
     as LD_PRELOAD.

sshd configuration typically lives at /etc/ssh/sshd_config

Answer 3

So, on your client, you have some environment variable, and you want that to be available to the remote command? I don't think there's a way to have ssh magically pass it along, but you can probably do something like this. Instead of using, say:

ssh remote.host my_command

You can do this:

ssh remote.host env ENV_VAR=$ENV_VAR my_command

Answer 4

On your local client, in your ~/.ssh/config you can add SetEnv, e.g.

Host myhost
  SetEnv FOO=bar

^{Note: Check man ssh_config.}

Then on the server, make sure to allow client to pass certain environment variables in your /etc/ssh/sshd_config config file:

AcceptEnv LANG LC_* FOO BAR*

^{Note: Check man sshd_config.}

Answer 5

@emptyset's response (which didn't work for me) led me to this answer:

You can add this command to your ~/.ssh/authorized_keys file:

command="/usr/bin/env VARIABLE=<something> $SHELL" ssh-rsa <key>

export VARIABLE=<something> was immediately exiting, and the SSH connection was closed (locking me out of the server), whereas /usr/bin/env ... $SHELL will run your default shell with a modified environment.

Answer 6

You could try invoking a custom command, assuming you have password-less ssh login setup. On the server, edit your ~/.ssh/authorized_keys entry that corresponds to the key from you client:

command="export VARIABLE=<something>" ssh-rsa <key>

Look at this link in the section Forced Command for a little more detail.

Answer 7

I was making a custom build of OpenSSH for a device with a cramfs in the home directory and /etc (Cram FS is read-only) so ~/.ssh/environment would not work not without rebuilding the entire FS and these were field deployed devices(Embedded Systems Hence the use of CRAMFS). You can specify in the sshd_config the location of the authroized_keys file but for some reason the environment= only work for environment variables in the ~/.ssh/authroized_keys. Editing the /etc/profile wasn't an option and I had to load ssh in a non-standard directory. In session.c after the child_set_env(..."MAIL"...) just add the enviroment variables you need(This is a hack I know...) but just incase someone needs some hardcoded envs for a session if you are compiling from source you can do this. TGI-FLOSS

Answer 8

just one simple command :

ssh -t your_host_or_ip 'export some_var_name=whatever_you_want; bash'