How to determine if I'm logged in via SSH?

You could use "w" or "who" command output. When you connect over ssh, they'll show your source IP.

Here is a great answer I found on unix.stackexchange:

• If one of the variables SSH_CLIENT or SSH_TTY is defined, it's an ssh session.
• The login shell's parent process can be checked with ps -o comm= -p $PPID. If it is sshd, it's an ssh session.

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

You could add SSH_* to env_keep in sudoers so that this can be detected while switched to the other user.

I think you want to rethink the way you're thinking of the problem. The question isn't "am I logged in via SSH, because I want to turn off certain commands." It's "am I logged in at the console, because then I will enable certain commands."

If you want to know if you bash shell is directly a child process of sshd (not n>1 layers deep) you can

cat /proc/$PPID/status | head -1 | cut -f2

it should give you sshd or whatever is the parent process name of your current shell.

Yes, as others noted, the info is in the presence of your IP in parentheses in the output of who am i.

You can use Bash regular expressions to detect it:

if [[ $(who am i) =~ \([0-9\.]+\)$ ]]; then echo SSH; else echo no; fi

I've came up with the following, based on tips from others here.

It uses a variable for caching - I am using it in my shell theme.

is_ssh() {
    (( $+SSH_CLIENT )) && return
    if ! (( $+_ZSH_IS_SSH )); then
        # "who am i" displays current user from utmp(5).  This will be empty for
        # a "normal" terminal.  With Konsole, it is ":0" for display :0,
        # for ssh it is the hostname and with tmux sth like "tmux(PID).ID".
        local whoami="$(who am i)"}
        local host="${whoami#*\(*}"
        [[ -n $host && $host != tmux* && $host != :* ]]
        _ZSH_IS_SSH=$?
    fi
    return $_ZSH_IS_SSH
}

Source: is_ssh in https://github.com/blueyed/oh-my-zsh/blob/master/themes/blueyed.zsh-theme#L51-63.

All of the other answers work if you are at the first level of login. But if, once login, you run 'su' or 'sudo' (in my case, to switch to a user account without shell for security reasons, I had to run: sudo su - <userid> -s /bin/bash -l), their solution fail.

Following is a universal solution; using pstree, you check for sshd as a parent.

if pstree -p | egrep --quiet --extended-regexp ".*sshd.*\($$\)"; then
  echo "I am remote."
else
  echo "I am local."
fi

Here is the output of the egrep, when --quiet is removed. It shows the whole hierarchy that matches if one is connected remotely.

   |            |-sshd(18599)---sshd(18603)---bash(18604)---sudo(18823)---bash(18824)-+-egrep(22417)

If you're on a system that uses logind, then loginctl will tell you if the session is remote:

$ loginctl show-session $XDG_SESSION_ID -P Remote
yes

However, there are cases that XDG_SESSION_ID is not defined (including after sudo), so I wrote a script to check more things. It tries loginctl, ssh, remote X display and if all fails, connects to logind via D-Bus and attempts to discover the current session by using the current process PID as a key:

#!/usr/bin/env python3
from os import environ, getpid, system
from sys import argv, stderr

from dbus import SystemBus
from dbus.exceptions import DBusException

DEBUG=0

LOGIND = ('org.freedesktop.login1', '/org/freedesktop/login1')
MANAGER = '%s.Manager' % LOGIND[0]
SESSION = '%s.Session' % LOGIND[0]
SESSION_NOT_FOUND = '%s.NoSessionForPID' % LOGIND[0]
REMOTE = 'Remote'
LOGINCTL = 'loginctl show-session %s -P %s'
PROPERTIES = 'org.freedesktop.DBus.Properties'
DISPLAY = 'DISPLAY'
SSH='SSH_CLIENT'

def debug(msg):
    if DEBUG: print(msg, file=stderr)

def try_loginctl():
    debug('Trying loginctl...')
    try:
        r = system(LOGINCTL % (environ['XDG_SESSION_ID'], REMOTE))
        # exit early if loginctl prints the result by itself
        if r == 0: raise SystemExit
    except KeyError:
        pass

    return False

def try_ssh():
    debug('Trying ssh...')
    return bool(environ.get(SSH))

def try_x():
    debug('Trying X...')
    try:
        display = environ[DISPLAY]
        return ':' in display and len(display.split(':', 1)[0]) > 0
    except KeyError:
        return False
        
def try_dbus():
    debug('Trying dbus...')
    bus = SystemBus()
    logind = bus.get_object(*LOGIND)

    try:
        pid = getpid()
        session_path = logind.GetSessionByPID(pid, dbus_interface=MANAGER)
    except DBusException as e:
        if e.get_dbus_name() == SESSION_NOT_FOUND:
            return False
        else:
            raise

    session = bus.get_object(LOGIND[0], session_path)
    remote = session.Get(SESSION, REMOTE, dbus_interface=PROPERTIES)

    return bool(remote)

def main():
    global DEBUG

    if '-d' in argv or '--debug' in argv:
        DEBUG=1

    remote = try_loginctl() or try_ssh() or try_x() or try_dbus()
    print(remote and 'yes' or 'no')

main()

Look for your shell's parent cmdline and recurse. Maybe something like the following:

#!/usr/bin/env bash

## Find out how I'm logged in
# Tested on RHEL5.5

PD=${1:-$$}
ME=`basename $0`

## Read the shell's PPID
PAR=`ps --no-headers -p $PD -o ppid`

## CMDLINE can contain stuff like the following:
# /sbin/getty-838400tty4 // logged in at a console
# gnome-terminal         // logged in Gnome Terminal
# -bash                  // in a subshell
# su-                    // we became another user using su
# sshd: jc@pts/1         // logged in over ssh
# login                  // logged in terminal or serial device

eval `python - << __EOF__
import re
f = open("/proc/${PAR}/cmdline", 'r')
ln = f.readline()
if re.search(r'^ssh', ln): 
    print "echo Logged in via ssh"
if re.search(r'getty.*?tty', ln):
    print "echo Logged in console"
if re.search("gnome-terminal", ln):
    print "echo Logged in Gnome"
if re.search(r'^login', ln):
    print "echo Logged in console"
if re.search(r'^-?bash', ln) or re.search(r'^su', ln): 
    print "./$ME $PAR"
f.close()
__EOF__
`

Edited to make it actually work :)