ksh PS1 Environment Variable

The PS1 environment variable contains the primary prompt string. The value of $PS1 is expanded before printing, so that

PS1='$PWD> '

puts the current working directory in your prompt. A ! is replaced with the current history number. Use !! to get an explicit exclamation mark. The default value for $PS1 is ($0) (that is, the pathname by which dbx was invoked enclosed in parentheses).