How to set a conditional newline in PS1?

Question

I am trying to set PS1 so that it prints out something just right after login, but preceded with a newline later.

Suppose export PS1=\"\\h:\\W \\u

Answer 1

Running with dogbane's answer, you can make PROMPT_COMMAND "self-destruct", preventing the need to run a function after every command.

In your .bashrc or .bash_profile file, do

export PS1='\h:\W \u\$ '
reset_prompt () {
  PS1='\n\h:\W \u\$ '
}
PROMPT_COMMAND='(( PROMPT_CTR-- < 0 )) && { 
  unset PROMPT_COMMAND PROMPT_CTR
  reset_prompt
}'

When the file is processed, PS1 initially does not display a new-line before the prompt. However, PROMPT_CTR is immediately decremented to -1 (it is implicitly 0 before) before the prompt is shown the first time. After the first command, PROMPT_COMMAND clears itself and the counter before resetting the prompt to include the new-line. Subsequently, no PROMPT_COMMAND will execute.

Of course, there is a happy medium, where instead of PROMPT_COMMAND clearing itself, it just resets to a more ordinary function. Something like

export PS1='\h:\W \u\$ '
normal_prompt_cmd () {
   ...
}
reset_prompt () {
  PS1='\n\h:\W \u\$ '
}
PROMPT_COMMAND='(( PROMPT_CTR-- < 0 )) && {
   PROMPT_COMMAND=normal_prompt_cmd
   reset_prompt
   unset PROMPT_CTR
  }'