コマンド左側の[linuxstudy@xxxx ~]$みたいな文字列の
フォーマットを定義している。
PS1がprimary prompt
PS2がsecondary prompt。2行以上の時のみ。

PS1,PS2値

[linuxstudy@xxxx ~]$ echo $PS1
[\u@\h \W]\$
[linuxstudy@xxxx ~]$ echo $PS2
>

/u, /h,/Wの意味は下記のとおり。※印

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  (uses  the
                     value of the PROMPT_DIRTRIM variable)
              \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).
       ```