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Tip: Prompt magic
Enhancing the system prompt
Daniel Robbins
President-CEO, Gentoo Technologies,
Inc.
September 2000
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Why stick with the standard boring shell prompt when
you can easily make it colorful and more informative? In this tip,
Daniel Robbins will show you how to get your shell prompt just the
way you like it, as well as how to dynamically update your X
terminal's title bar.
As Linux/UNIX people, we spend a lot of time working in the
shell, and in many cases, this is what we have staring back at us:
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If you happen to be root, you're entitled to the "prestige"
version of this beautiful prompt:
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These prompts are not exactly pretty. It's no wonder that several
Linux distributions have upgraded their default prompts that add
color and additional information to boot. However, even if you
happen to have a modern distribution that comes with a nice,
colorful prompt, it may not be perfect. Maybe you'd like to add or
change some colors, or add (or remove) information from the prompt
itself. It isn't hard to design your own colorized, tricked-out
prompt from scratch. <A name=1>
Prompt basics
Under bash, you
can set your prompt by changing the value of the PS1 environment
variable, as follows:
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Changes take effect immediately, and can be made permanent by
placing the "export" definition in your ~/.bashrc file. PS1 can
contain any amount of plain text that you'd like:
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$ export PS1="This is my super prompt > "
This is my super prompt >
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While this is, um, interesting, it's not exactly useful to have a
prompt that contains lots of static text. Most custom prompts
contain information like the current username, working directory, or
hostname. These tidbits of information can help you to navigate in
your shell universe. For example, the following prompt will display
your username and hostname:
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$ export PS1="\u@\H > "
drobbins@freebox >
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This prompt is especially handy for people who log in to various
machines under various, differently-named accounts, since it acts as
a reminder of what machine you're actually on and what privileges
you currently have.
In the above example, we told bash to insert the username and
hostname into the prompt by using special backslash-escaped
character sequences that bash replaces with specific values when
they appear in the PS1 variable. We used the sequences "\u" (for
username) and "\H" (for the first part of the hostname). Here's a
complete list of all special sequences that bash recognizes (you can
find this list in the bash man page, in the "PROMPTING" section):
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| Sequence |
Description |
| \a |
The ASCII bell character (you can also type \007) |
| \d |
Date in "Wed Sep 06" format |
| \e |
ASCII escape character (you can also type \033) |
| \h |
First part of hostname (such as "mybox") |
| \H |
Full hostname (such as "mybox.mydomain.com") |
| \j |
The number of processes you've suspended in this shell by
hitting ^Z |
| \l |
The name of the shell's terminal device (such as
"ttyp4") |
| \n |
Newline |
| \r |
Carriage return |
| \s |
The name of the shell executable (such as "bash") |
| \t |
Time in 24-hour format (such as "23:01:01") |
| \T |
Time in 12-hour format (such as "11:01:01") |
| \@ |
Time in 12-hour format with am/pm |
| \u |
Your username |
| \v |
Version of bash (such as 2.04) |
| \V |
Bash version, including patchlevel |
| \w |
Current working directory (such as "/home/drobbins") |
| \W |
The "basename" of the current working directory (such as
"drobbins") |
| \! |
Current command's position in the history buffer |
| \# |
Command number (this will count up at each prompt, as long
as you type something) |
| \$ |
If you are not root, inserts a "$"; if you are root, you
get a "#" |
| \xxx |
Inserts an ASCII character based on three-digit number xxx
(replace unused digits with zeros, such as "\007") |
| \\ |
A backslash |
| \[ |
This sequence should appear before a sequence of
characters that don't move the cursor (like color escape
sequences). This allows bash to calculate word wrapping
correctly. |
| \] |
This sequence should appear after a sequence of
non-printing characters. |
So, there you have all of bash's special backslashed escape
sequences. Play around with them for a bit to get a feel for how
they work. After you've done a little testing, it's time to add some
color. <A name=2>
Colorization
Adding color is
quite easy; the first step is to design a prompt without color.
Then, all we need to do is add special escape sequences that'll be
recognized by the terminal (rather than bash) and cause it to
display certain parts of the text in color. Standard Linux terminals
and X terminals allow you to set the foreground (text) color and the
background color, and also enable "bold" characters if so desired.
We get eight colors to choose from.
Colors are selected by adding special sequences to PS1 --
basically sandwiching numeric values between a "\e[" (escape
open-bracket) and an "m". If we specify more than one numeric code,
we separate each code with a semicolon. Here's an example color
code:
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When we specify a zero as a numeric code, it tells the terminal
to reset foreground, background, and boldness settings to their
default values. You'll want to use this code at the end of your
prompt, so that the text that you type in is not colorized. Now,
let's take a look at the color codes. Check out this screenshot:
Color chart
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To use this chart, find the color you'd like to use, and find the
corresponding foreground (30-37) and background (40-47) numbers. For
example, if you like green on a normal black background, the numbers
are 32 and 40. Then, take your prompt definition and add the
appropriate color codes. This:
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becomes:
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export PS1="\e[32;40m\w> "
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So far, so good, but it's not perfect yet. After bash prints the
working directory, we need to set the color back to normal with a
"\e[0m" sequence:
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export PS1="\e[32;40m\w> \e[0m"
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This definition will give you a nice, green prompt, but we still
need to add a few finishing touches. We don't need to include the
background color setting of 40, since that sets the background to
black which is the default color anyway. Also, the green color is
quite dim; we can fix this by adding a "1" color code, which enables
brighter, bold text. In addition to this change, we need to surround
all non-printing characters with special bash escape sequences, "\["
and "\]". These sequences will tell bash that the enclosed
characters don't take up any space on the line, which will allow
word-wrapping to continue to work properly. Without them, you'll end
up with a nice-looking prompt that will mess up the screen if you
happen to type in a command that approaches the extreme right of the
terminal. Here's our final prompt:
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export PS1="\[\e[32;1m\]\w> \[\e[0m\]"
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Don't be afraid to use several colors in the same prompt, like
so:
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export PS1="\[\e[36;1m\]\u@\[\e[32;1m\]\H> \[\e[0m\]"
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<A name=3>
Xterm fun
I've shown you how to
add information and color to your prompt, but you can do even more.
It's possible to add special codes to your prompt that will cause
the title bar of your X terminal (such as rxvt or aterm) to be
dynamically updated. All you need to do is add the following
sequence to your PS1 prompt:
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Simply replace the substring "titlebar" with the text that you'd
like to have appear in your xterm's title bar, and you're all set!
You don't need to use static text; you can also insert bash escape
sequences into your titlebar. Check out this example, which places
the username, hostname, and current working directory in the
titlebar, as well as defining a short, bright green prompt:
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export PS1="\[\e]2;\u@\H \w\a\e[32;1m\]>\[\e[0m\] "
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This is the particular prompt that I'm using in the colortable
screenshot, above. I love this prompt, because it puts all the
information in the title bar rather than in the terminal where it
limits how much can fit on a line. By the way, make sure you
surround your titlebar sequence with "\[" and "\]", since as far as
the terminal is concerned, this sequence is non-printing. The
problem with putting lots of information in the title bar is that
you will not be able to see info if you are using a non-graphical
terminal, such as the system console. To fix this, you may want to
add something like this to your .bashrc:
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if [ "$TERM" = "linux" ]
then
#we're on the system console or maybe telnetting in
export PS1="\[\e[32;1m\]\u@\H > \[\e[0m\]"
else
#we're not on the console, assume an xterm
export PS1="\[\e]2;\u@\H \w\a\e[32;1m\]>\[\e[0m\] "
fi
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This bash conditional statement will dynamically set your prompt
based on your current terminal settings. For consistency, you'll
want to configure your ~/.bash_profile so that it sources your
~/.bashrc on startup. Make sure the following line is in your
~/.bash_profile:
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This way, you'll get the same prompt setting whether you start a
login or non-login shell.
Well, there you have it. Now, have some fun and whip up some
nifty colorized prompts! <A name=resources>
Resources
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About the author
Residing
in Albuquerque, New Mexico, Daniel Robbins is the President/CEO of
<A href="http://www.gentoo.org/">Gentoo Technologies, Inc.</A>, the
creator of Gentoo Linux, an advanced Linux for the PC, and
the Portage system, a next-generation ports system for Linux.
He has also served as a contributing author for the Macmillan books
Caldera OpenLinux Unleashed, SuSE Linux Unleashed, and
Samba Unleashed. Daniel has been involved with computers in
some fashion since the second grade, when he was first exposed to
the Logo programming language as well as a potentially dangerous
dose of Pac Man. This probably explains why he has since served as a
Lead Graphic Artist at SONY Electronic Publishing/Psygnosis.
Daniel enjoys spending time with his wife, Mary, and his new baby
daughter, Hadassah. You can contact Daniel at <A
href="mailto:drobbins@gentoo.org">drobbins@gentoo.org</A>.
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