Linux.Desktop.Hacks [Electronic resources] نسخه متنی

Hack 13. Put Your Command Prompt on a Diet

Change your bash prompt to show extra
information without cluttering the area around your command prompt.

This hack saves command-line
prompt space without
sacrificing the informational value that prompt hacks often add to
command-line prompts. Sure, it's great to see the
date, time, uptime, and phase of the moon in your bash prompt, but
most schemes for providing this extra information create a crowded
mess and push more important text off the terminal screen. This hack
uses the tput command to place some of that extra
information in the upper-right corner of the terminal or console,
leaving your prompt neat, clean, and short. Thanks to Giles Orr whose
web site on bash prompts (http://www.tldp.org/HOWTO/Bash-Prompt-HOWTO/)
inspired this example hack.

I'll keep this particular hack simple. You are going
to put the current working directory somewhere on the terminal but
not at the prompt itself. This hack creates a directory path that
always appears in the upper-right corner. Of course, as explained
later, you can modify this location.

The tput command is ideal for this type of trick,
because tput manipulates the location and color of
cursors. Figure 2-1 shows what it might look like
when it is configured as the default prompt.

Figure 2-1. The current working directory in the upper-right corner

This might seem somewhat superfluous when using an X terminal,
because it's easy to put the current working
directory in the titlebar. However, not every terminal works that
way, and there is no titlebar on a virtual console, so it still comes
in really handy, especially when you are working without a graphical
window manager or desktop.

Here is the script, with comments on how it works:

#!/bin/bash
function prompt_command {
# save the current position
tput sc
# backwash is where to position the cursor 
# to write the whole current working directory
# we back up 2 more for the brackets
let backwash=$(tput cols)-$(echo $(pwd) | wc -m)-2
# position the cursor at Y=0, X=calculated length
tput cup 0 ${backwash}
# set foreground color, bold
tput setaf 4 ; tput bold
# wrap the full path in brackets
echo -n "["
# set the color of the current path
tput setaf 6
# show the path
echo -n "$(pwd)"
# set the color of the closing bracket"
tput setaf 4 ; tput bold
# show the closing bracket
echo -n "]"
# return cursor to the saved position
tput rc
}
PROMPT_COMMAND=prompt_command
GREEN="\[$(tput setaf 2 ; tput bold)\]"
WHITE="\[$(tput setaf 7 ; tput bold)\]"
NO_COLOUR="\[$(tput sgr0)\]"
case $TERM in
xterm*|rxvt*)
TITLEBAR='\[\033]0;\u@\h \007\]'
;;
*)
TITLEBAR="
;;
esac
PS1="${TITLEBAR}$GREEN\u@\h $WHITE\$$NO_COLOUR "
PS2='> '
PS4='+ '

Here's how it works. First, the tput
sc command saves the current position of the cursor.

Now let's pick apart the following, somewhat
awkward-looking command from the preceding code:

let backwash=$(tput cols)-$(echo $(pwd) | wc -m)-2

The goal here is to find out where to move the cursor on the X axis
to start printing out the current working directory surrounded by
brackets. The column you want is the width of the terminal minus the
width of the current directory, minus two more columns (for the
brackets).

The tput cols command returns
the number of columns in the current terminal or console. The
pwd command prints the current working directory,
which is echoed and piped through the wc -m
command, which returns the number of characters in the current
working directory. Then the command subtracts that length plus two
more for the brackets. All together, the whole command returns the X
axis location of the cursor where the script should start printing
the current directory in brackets.

The tput cup
<row>
<column> command moves the cursor to
the specified row and column. In this case, we always want row 0 (the
top row), and we calculated the starting location for the column in
the previous command.

Now the script sets up some tput variables to determine how the next
bit of text placed on the screen will look. The
tput setaf 4
command defines the color as dark blue, and tput
bold makes it bold. Then the script echoes the
open bracket to the screen. The script resets the color and prints
the current path, and then resets the color and prints the close
bracket.

Then it returns the cursor back to its saved position with
tput rc. Finally, it sets the
prompt, using variable definitions for tput
sequences to set colors.

You probably won't find setaf in
the manpage for tput. Indeed, Table 2-1 provides some of the rarely documented
features of tput.

The fact that these features are rarely documented implies it is possible they might not work on some systems (such as some terminals or Linux running on machines other than x86es).

tput setab [1-7]

tput setb [1-7]

tput setaf [1-7]

tput setf [1-7]

tput bold

tput dim

tput smul

tput rmul

tput rev

Table 2-2 shows the ANSI escape sequence commands for foregrounds and
backgrounds you can use in place of the colors I've
chosen.

tput setaf 0

tput setab 0

tput bold ; tput setaf 0

tput setaf 1

tput setab 1

tput bold ; tput setaf 1

tput setaf 2

tput setab 2

tput bold ; tput setaf 2

tput setaf 3

tput setab 3

tput bold ; tput setaf 3

tput setaf 4

tput setab 4

tput bold ; tput setaf 4

tput setaf 5

tput setab 5

tput bold ; tput setaf 5

tput setaf 6

tput setab 6

tput bold ; tput setaf 6

tput setaf 7

tput setab 7

tput bold ; tput setaf 7