$ chsh -s /usr/bin/zsh

$ echo $SHELL
/bin/bash

$ cat /etc/shells
# /etc/shells: valid login shells
/bin/sh
/bin/dash
/bin/bash
/bin/rbash
/bin/zsh
/usr/bin/zsh

$ cat /etc/passwd|grep myuser
myuser:x:1000:1000:myuser,,,:/home/myuser:/usr/bin/zsh