$SHELL

echo $SHELL
/bin/ksh

echo $SHELL
/bin/zsh

exec $SHELL

Related

$PATH
$HOME

See also

• Variables: Environment variables, env, env-cmd, .env/, SHELL, PS1, HOME, $?, PATH, HISTFILE, PWD, LD_LIBRARY_PATH, printenv, getenv, getenv, direnv, AWS cli environmental variables, envsubst, env:, env_file (docker-compose)