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Variables

Bash has five prompt strings that can be customized:

• PS0 is displayed after each command, before any output.
• PS1 is the primary prompt which is displayed before each command, thus it is the one most people customize.
• PS2 is the secondary prompt displayed when a command needs more input (e.g. a multi-line command).
• PS3 is not very commonly used. It is the prompt displayed for Bash's select built-in which displays interactive
  menus. Unlike the other prompts, it does not expand Bash escape sequences. Usually you would customize it in
  the script where the select is used rather than in your .bashrc.
• PS4 is also not commonly used. It is displayed when debugging bash scripts to indicate levels of indirection. The first character is repeated to indicate deeper levels.

Escape Sequnces

Things like \w for working directory. Source: man bash

\[...\] # for ANSI stuff like colour -- this tells bash that what's inside doesn't advance the cursor
When  executing  interactively,  bash displays the primary
   prompt PS1 when it is ready to read  a  command,  and  the
   secondary  prompt PS2 when it needs more input to complete
   a command.  Bash allows these prompt strings  to  be  cus­
   tomized by inserting a number of backslash-escaped special
   characters that are decoded as follows:
  \a     an ASCII bell character (07)
  \d     the date  in  "Weekday  Month  Date"  format
 (e.g., "Tue May 26")
  \e     an ASCII escape character (033)
  \h     the hostname up to the first `.'
  \H     the hostname
  \j     the  number of jobs currently managed by the
 shell
  \l     the basename of the shell's terminal  device
 name
  \n     newline
  \r     carriage return
  \s     the  name  of  the shell, the basename of $0
 (the portion following the final slash)
  \t     the current time in 24-hour HH:MM:SS format
  \T     the current time in 12-hour HH:MM:SS format
  \@     the current time in 12-hour am/pm format
  \u     the username of the current user
  \v     the version of bash (e.g., 2.00)
  \V     the release of bash,  version  +  patchlevel
 (e.g., 2.00.0)
  \w     the current working directory
  \W     the  basename  of the current working direc­
 tory
  \!     the history number of this command
  \#     the command number of this command
  \$     if the effective UID is 0, a #, otherwise  a
 $
  \nnn   the  character  corresponding  to  the octal
 number nnn
  \\     a backslash
  \[     begin a sequence of non-printing characters,
 which could be used to embed a terminal con­trol 
 sequence into the prompt
  \]     end a sequence of non-printing characters

Return Code

There's no actual escape sequence for this, so we use parameter expansion.

PS1='($(echo $?))'