If users have access to the files on your server, but you don’t want they to be able to execute commands, you can limit access to  sftp only.

Add a user to your system as normal with a password and then run the following command:

usermod -s /usr/lib/sftp-server username

Then change add the following to /etc/shells to make it a valid shell:

echo '/usr/lib/sftp-server' >> /etc/shells

Now this user can only run the sftp server as shell

Enjoy