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From: Planet Debian
Subject: Pablo Lorenzzoni -- Bash prompts: the essential
To: ticker-planetdebian[bei]lists.infodrom[punkt]org
Date: Thu, 6 Nov 2008 10:26:55 +0100 (CET)

http://www.nardol.org/2008/11/5/bash-prompts-the-essential

Pablo Lorenzzoni -- Bash prompts: the essential

Bash is probably the most common command-line shell in the
GNU/Linux world. Although a lot of people use alternate shells
(such as Zsh), Bash is still shipped with most mainstream distros
as the default. Once you have a lot of different remote machines,
all running Bash as the shell, it becomes increasingly difficult
to pay attention to the prompt, and typing "reboot" in a machine
different from the one you wanted becomes more likely. I deal with
that problem by changing Bash prompts

First of all, the basics: Bash prompts are just environment
variables with special characters you can set and export. Bash has
four of these variables: PS1 to PS4, but usually only the first
two matters (actually, just PS1 for a reference on the others,
check the manpage). The most common PS1 string is: spectra@home:~$
echo $PS1 \u@\h:\w\$ spectra@home:~$

This has 4 special characters, escaped with a backslash: \u
informing us the username; \h informing us the hostname; \w,
informing us the working directory; and \$, which gives us the $
in the end of the prompt (more on this later).

So, essentially, one can change that string to anything else
spectra@home:~$ PS1="my_shell_prompt\$ " my_shell_prompt$

Pretty easy. You can check a complete reference of the special
characters at the section PROMPTING of bash manpage, but the most
useful IMHO are the following:

- \d the date

- \t the time (24-hour format)

- \W the basename of the current working directory

- \! the history number of this command

- \# the command number of this command

- \$ shows # if the UID is 0 (is we are root), or $ for all the
rest

Also, as part of the prompt string, one can use ANSI Colors
enclosed as non-printing characters (that is between \[ and \]).
ANSI sequences always begin with an 'ESC[' and end with an 'm'.
(Yes Really arbitrary but that's the way it is). ESC can be
represented as \e Here is a list of the most common colors in ANSI
sequences:

- Black: 0;30

- Red: 0;31

- Green: 0;32

- Brown: 0;33

- Blue: 0;34

- Purple: 0;35

- Cyan: 0;36

- Light Gray: 0;37

Now, notice that there are two numbers separated by a semi-colon
the first is always 0 (zero) in the colors I pointed above, but it
actually refers to an ANSI attribute called Select Graphic
Rendition You can use 0 (zero) to normal colors, 1 for bold, 2 to
faint, etc. So \e[0;30m refers to BLACK, \e[1;30m refers to DARK
GREY. The Wikipedia[1] has a good article on these escape
sequences.

Once you're satisfied with something printed in a color, to go
back to the default (to reset), you issue the \e[0m escape
sequence.

So, back to my problem Each different machine gets a different
color for the hostname. On 'hospital' machine, for instance, my
PS1 looks like: spectra@hospital:~$
PS1="\[\e[1;33m\]\u\[\e[0m\]@\[\e[0;35m\]\h\[\e[0m\]:\[\e[0;32m\]\
w\[\\$ " spectra@hospital:~$

With \e[0;35m (Purple) for the hostname. On 'home' machine, it may
be \e[0;34m (Blue)... On 'server', it may be \e[0;36m (Cyan), and
so on After a while, you get used to the color and end up linking
the color to the machine so that typing 'reboot' on a machine with
the wrong color gets harder than before.

To make the changes permanent, put export PS1 in one of the config
script of bash (.bashrc, .bash_profile, etc). On some systems,
/etc/environment holds lots of environment variables definitions.

I just scratched the surface That's just what works for me The
Bash-Prompt-HOWTO[2] has some interesting examples, and I actually
have a friend who uses more esoterical stuff, such as fancybash[3]
or bashish[4], but I'll leave this up to you

03:58pm[5] by spectra

Links:
1. http://en.wikipedia.org/wiki/ANSIescapecode
2. http://tldp.org/HOWTO/Bash-Prompt-HOWTO/c816.html
3. http://home.ircnet.de/cru/fancybash/
4. http://bashish.sourceforge.net/
5. http://www.nardol.org/2008/11/5/bash-prompts-the-essential

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