What is the name of the $ or # signs that indicates if you're root?

Question

I recently discovered that the # sign indicates that you're root on the shell (at least in a bash shell), and the $ sign indicates that you're not.

What is the name of this sign and does it really have the meaning that I give to it?

Is it only a bash thing?

Example with default bash configuration on Ubuntu:

john@mycomputer /tmp $ echo "I'm a simple user"
mycomputer /tmp # echo "I'm the root user"

Answer 1

There does not appear to be a formal definition of the prompt character indicating your privileges.

This thorough answer to “What is the origin of the UNIX $ (dollar) prompt?” provides lots of historical background, but it does not have a name for this indicator either.

 

In the absence of an official term, I'd call this the “prompt privilege indicator” or the “prompt indicator symbol” or even just “prompt indicator.”

 

All shells have a prompt whose default string ends in an indicator symbol, which (by default) almost always indicates whether you are root. Most shells allow you to change the prompt and most shells have a way to specify the prompt indicator symbol.

Here's what you can find in the various shell docs and standards:

A segment of the bash man page:

PROMPTING
      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 displays PS0 after it
      reads a command but before executing  it.   Bash  allows  these  prompt
      strings  to  be  customized  by inserting a number of backslash-escaped
      special characters that are decoded as follows:
…
             \$     if the effective UID is 0, a #, otherwise a $

From the zsh man pages:

Shell state
    %#       A `#' if the shell is running with privileges,  a  `%'  if  not.
             Equivalent  to `%(!.#.%%)'.  The definition of `privileged', for
             these purposes, is that either the effective user  ID  is  zero,
             or,  if  POSIX.1e  capabilities are supported, that at least one
             capability is raised in  either  the  Effective  or  Inheritable
             capability vectors.

The POSIX standard doesn't even specify what the privileged prompt symbol should be:

PS1

Each time an interactive shell is ready to read a command, the value of this variable shall be subjected to parameter expansion and written to standard error. The default value shall be $ . For users who have specific additional implementation-defined privileges, the default may be another, implementation-defined value.

See also the dash man page (dash is a popular basic POSIX shell, often installed as /bin/sh):

    PS1      The primary prompt string, which defaults to ``$ '', unless you
             are the superuser, in which case it defaults to ``# ''.

POSIX and dash don't have anything dynamic in what you can set $PS1 to. The default prompt is set once and is replaced by any assignments to the PS1 variable, losing the ability to distinguish between root and unprivileged users with a single character. You'd have to write your own code to determine if you're privileged and use its output in your PS1 assignment.