Linux:/etc/shells or /etc/os-release

 eks@GRDLUSAWSAMUT01:~$ cat /etc/os-release

NAME="Ubuntu"

VERSION="18.04.6 LTS (Bionic Beaver)"

ID=ubuntu

ID_LIKE=debian

PRETTY_NAME="Ubuntu 18.04.6 LTS"

VERSION_ID="18.04"

HOME_URL="https://www.ubuntu.com/"

SUPPORT_URL="https://help.ubuntu.com/"

BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"

PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"

VERSION_CODENAME=bionic

UBUNTU_CODENAME=bionic

eks@GRDLUSAWSAMUT01:~$ cat /etc/shells

# /etc/shells: valid login shells

/bin/sh

/bin/bash

/bin/rbash

/bin/dash

/usr/bin/tmux

/usr/bin/screen

eks@GRDLUSAWSAMUT01:~$