You can do so by adding the following parameter to your config file (/etc/ssh/sshd_config).

 ForceCommand

         Forces the execution of the command specified by ForceCommand, ignoring any command supplied by the client and ~/.ssh/rc if present.  The command is invoked by using the user's login shell

         with the -c option.  This applies to shell, command, or subsystem execution.  It is most useful inside a Match block.  The command originally supplied by the client is available in the

         SSH_ORIGINAL_COMMAND environment variable.  Specifying a command of “internal-sftp” will force the use of an in-process sftp server that requires no support files when used with

         ChrootDirectory.

An other option is to use the .ssh/rc files on a per user basis.

To use the ForceCommand method you just add ForceCommand /usr/bin/ownscript at the bottom of the file /etc/ssh/sshd_config (on the server).

The script looks like this:

#!/bin/bash

#Script file for ssh

#

#put your commands here

echo "test" > /tmp/test.txt

#

#exit by calling a shell to open for the ssh session

/bin/bash

Don't forget to chmod the script sudo chmod +x /usr/bin/ownscript