How can I detect if the shell is controlled from SSH?

Question

I want to detect from a shell script (more specifically .zshrc) if it is controlled through SSH. I tried the HOST variable but it's always the name of the computer which is running the shell. Can I access the hostname where the SSH session is coming from? Comparing the two would solve my problem.

Every time I log in there is a message stating the last login time and host:

Last login: Fri Mar 18 23:07:28 CET 2011 from max on pts/1
Last login: Fri Mar 18 23:11:56 2011 from max

This means the server has this information.

Answer 1

Here are the criteria I use in my ~/.profile:

• If one of the variables SSH_CLIENT or SSH_TTY is defined, it's an ssh session.
• If the login shell's parent process name is sshd, it's an ssh session.

if [ -n "$SSH_CLIENT" ] || [ -n "$SSH_TTY" ]; then
  SESSION_TYPE=remote/ssh
# many other tests omitted
else
  case $(ps -o comm= -p $PPID) in
    sshd|*/sshd) SESSION_TYPE=remote/ssh;;
  esac
fi

(Why would you want to test this in your shell configuration rather than your session startup?)

Answer 2

You should be able to check via the SSH_TTY, SSH_CONNECTION, or SSH_CLIENT variables.

Answer 3

I think Gilles and Cakemox's answers are good, but just for completeness...

Last login: Fri Mar 18 23:07:28 CET 2011 from max on pts/1

comes from pam_lastlog.

You can print pam_lastlog information using the lastlog command, e.g.

$ lastlog -u mikel  
Username         Port     From             Latest
mikel            tty1                      Fri Jan 28 10:58:10 +1100 2011

for a local login, compared to

Username         Port     From             Latest
mikel            pts/9    mikel-laptop     Sat Mar 19 11:11:58 +1100 2011

for an SSH login.

On my system, this works to extract it

$ lastlog -u mikel | sed -ne '2{p;q}' | cut -c 27-42
mikel-laptop 

last and w could be helpful too, for example

$ TTY=$(tty)
$ last -n 1 ${TTY#/dev/} | sed -ne '1{p;q}'
mikel    pts/12       :0.0             Sat Mar 19 11:29   still logged in 

Answer 4

I just had the same problem in Linux, using Bash. I first used the environment variable SSH_CONNECTION, but then realized that it is not set if you su -.

The lastlog solution above didn't work either after su or su -.

Finally, I am using who am i, which shows the remote IP (or the hostname) at the end if it's an SSH connection. It also works after su.

Using Bash regular expressions, this works:

if [[ $(who am i) =~ \([-a-zA-Z0-9\.]+\)$ ]] ; then echo SSH; else echo no; fi

If zsh doesn't support regular expressions, the same can be achieved in many different ways with grep, cut, sed, or whatever.

For the curious, below is what I use this for, in root's .bashrc :

    # We don't allow root login over ssh.
    # To enable root X forwarding if we are logged in over SSH, 
    # use the .Xauthority file of the user who did su

    w=$(who am i)
    if [[ $w =~ \([-a-zA-Z0-9\.]+\)$ ]] ; then
        olduser=${w/ .*/}
        oldhome=$(getent passwd $olduser | cut -d: -f 6)
        [ -f "$oldhome/.Xauthority" ] \
          && export XAUTHORITY=$oldhome/.Xauthority
    fi