Who

who command show who is logged on. https://linux.die.net/man/1/who

who
root     pts/1        2020-04-27 07:25 (192.168.0.172:S.0)
root     pts/2        2020-04-27 07:25 (192.168.0.172:S.1)
ubuntu   pts/3        2020-04-27 09:06 (192.168.0.155)

who -u
root     pts/1        2020-04-27 07:25 02:23        7525 (192.168.0.172:S.0)
root     pts/2        2020-04-27 07:25 02:24        7663 (192.168.0.172:S.1)
ubuntu   pts/3        2020-04-27 09:06   .         20167 (192.168.0.155)

who -b
        system boot  2020-04-27 07:23

who -all
          system boot  2020-04-27 07:23
          run-level 5  2020-04-27 07:23
LOGIN      ttyS0        2020-04-27 07:23              1469 id=tyS0
LOGIN      tty1         2020-04-27 07:23              1479 id=tty1
          pts/0        2020-04-27 07:24              6645 id=ts/0  term=0 exit=0
root     + pts/1        2020-04-27 07:25 02:19        7525 (192.168.0.172:S.0)
root     + pts/2        2020-04-27 07:25 02:20        7663 (192.168.0.172:S.1)
ubuntu   - pts/3        2020-04-27 09:06   .         20167 (192.168.0.155)

See also

• uptime, last, who, pts, tty, journalctl --list-boots, systeminfo , tuptime