修改linux bash的提示符

PS1    The value of this parameter is expanded (see PROMPTING below) and used as  the  primary
prompt string.  The default value is ``\s-\v\$ ''.
PS2    The  value  of  this parameter is expanded as with PS1 and used as the secondary prompt
string.  The default is ``> ''.
PS3    The value of this parameter is used as the prompt for the  select  command  (see  SHELL
GRAMMAR above).
PS4    The  value  of  this  parameter is expanded as with PS1 and the value is printed before
each command bash displays during an execution trace.  The first character  of  PS4  is
replicated  multiple  times,  as necessary, to indicate multiple levels of indirection.
The default is ``+ ''.



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 backslash-escaped spe-
cial 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")
\D{format}
the format is passed to strftime(3) and the result is inserted into  the  prompt
string;  an  empty format results in a locale-specific time representation.  The
braces are required
\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
\A     the current time in 24-hour HH:MM format
\u     the username of the current user
\v     the version of bash (e.g., 2.00)
\V     the release of bash, version + patch level (e.g., 2.00.0)
\w     the current working directory, with $HOME abbreviated with a tilde
\W     the  basename  of  the  current working directory, with $HOME abbreviated with a
tilde
\!     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
\[     begin a sequence of non-printing characters, which could be used to embed a ter-
minal control sequence into the prompt
\]     end a sequence of non-printing characters

The  command number and the history number are usually different: the history number of a com-
mand 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
parameter 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 COMMANDS below).