How to display current path in command prompt in linux's sh (not bash)?

Command substitutions in double quotes " get expanded immediately. That is not what you want for your prompt. Single quotes ' will preserve the substitutions in $PS1 which then get only expanded when displaying the prompt. Hence this should work:

export PS1='$(whoami)@$(hostname):$(pwd)'

If you want the usual dollar sign and a space at the end of your prompt, simply add $ at the end (no escaping necessary): export PS1='$(whoami)@$(hostname):$(pwd)$ '

sh-4.2$ export PS1="\u@\h:\w>"
jenny@serenity:~>cd /usr/local
jenny@serenity:/usr/local>

This command works for me.

export PS1="\u@\h: \W:$"

Where
\u = username
\h = hostname
\W Name of present folder (not full path)