I noticed that on my Debian system the default PS1 value for non root users is, as reported by "echo $PS1":
I tried to find out some clear explanation about such a syntax (apart from the meaning of the "usual" \w,\h... codes which I already know), but I did not find anything.
More precisely, I found many documents and examples about PS1 customization, but the above syntax (particularly the ${debian_chroot:+($debian_chroot)} part) is either not used or used but not explained, as it's knowledge was taken for granted.

Can someone explain it to me or tell me where to find some clear explanation?

First let's see the syntax:
this is a shell parameter expansion, that can be explained as "if debian_chroot is null or unset, nothing is substituted, otherwise the expansion of $debian_chroot inside parentheses is substituted". In other words, the content of the variable debian_chroot - embedded in parentheses - is inserted into the prompt only if the variable itself is set and is not null. Otherwise a couple of empty parentheses will be displayed. You can find more details about this kind of substitution in the bash reference manual, here.

Regarding $debian_chroot it should be an alias for the chroot location, as set in /etc/debian_chroot. Not sure about this. Maybe a Debian expert might shed some light.

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Thanks a lot, I finally understood.
That kind of substitution is NOT explained in the bash man pages, is it?

What about the first part of the value?
What is it? I saw it appears when I issue the "echo $PS1" command in a pseudo terminal (after having switched from root to user with "su user") but does not appear in ttys; why?

Finally, as it's not every day to find a... guru :], a last question: how can I, while in man pages (less), search for a word located at the beginning of a line?

Yes, it is. You can find it in man bash, section EXPANSION, subsection Parameter Expansion. The bash man page has almost the same content of the Bash Reference Manual.

Not sure about this. Please, can you elaborate the difference between tty and pts and between user and root?

To search at the very beginning of the line, type
to ignore blank spaces at the beginning:
as for regular expressions. The only difference is that the search is case-insensitive.

Thanks for your help and courtesy.
I made some more tests. The situation seems to be as follows.
If I issue the "echo $PS1" command as "armando" user in a virtual terminal (tty) I get the expected output
If I issue the same command in a pseudo terminal in Gnome (pts/...) I get the "strange" one:
I tried both logging in to Gnome as "armando" and also logging in as root, opening a pts terminal and then "becoming" armando by means of "su armando". No difference.

Sorry for late... I missed your last post!

Indeed it looks weird. The first part does not make sense to me:
since it's not the usual color code. Anyway it looks like the PS1 variable is somehow modified by retaining the old value. Take a look at /etc/profile, /etc/bash.bashrc and eventually to your custom $HOME/.bashrc.

/etc/profile is the first configuration file sourced at login, where PS1 is set for interactive shells as
then in /etc/bash.bashrc the debian_chroot part is appended at the beginning:
After that something checks if you're running in a pseudo-terminal and does a mistake, like:
Please, see if you can find something similar somewhere, then we can try to investigate the reasons and the exact meaning. Cheers!

An update: looking at the Bash Prompt HOWTO, here, it looks like the escape sequence at the beginning is meant to set the terminal title. This is the reason why only pseudo-terminal allocations bring to that apparently weird prompt (actually it was my ignorance).

In the ANSI escape sequences the symbol \[ begins a sequence of non-printing characters. The symbol \] terminates it. So the part in grey:
is a non-printing sequence prepended to the actual prompt specification to set the window title. You can demonstrate it if you change one of the items, for example:
so that you will see the current working directory in place of the username. The only part I cannot explain in details is
the closing square bracket puzzles me. Maybe someone else can shed some light.

Sorry for digging up an older thread but, I believe that this portion of the PS1 value:
is a reset sequence for foreground, background, and boldness color to their default values for the "Window Title" of the pseudo-term. (Most likely specified in /etc/bash.bashrc). I believe it can be read as:

\[
{begin non-print chars}

\e]0;
{"Window Title" color and boldness default}

\u@\h: \w\a\]
{Set term title as - $USER@$HOSTNAME: $PWD#}

This is all kind of hazed speculation seeing as my days 'beautifying' my Xterm are long gone.

Goodluck.

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