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 allows these
prompt strings to be customized by inserting a number of back-
slash-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" )
[...]
\\ 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
The command number and the history number are usually different:
the history number of a command is its position in the history
list, which may include commands restored from the history file
(see HISTORY below), while the command number is the position in
the sequence of commands executed during the current shell
session. After the string is decoded, it is expanded via parame-
ter expansion, command substitution, arithmetic expansion, and
quote removal, subject to the value of the promptvars shell option
(see the description of the shopt command under SHELL BUILTIN COM-
MANDS below).
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