Old School

Many projects require a lot of time building colorful web pages or custom graphical user interfaces (GUIs). In a number of cases, however, only a simple text interface is required, especially for remote connections into a Raspberry Pi when you just want a quick system update.

In this article, I review a 1980s technology called curses [1], which lets you create text-based applications without requiring X windows or web services. In one example I look at C and Python apps that simulate Raspberry Pi sensor data, and in two examples, I output large text presentations and dynamic bars in Python [2].

Python Curses

The curses module is standard in Python and includes features such as:

• dynamic screen positioning
• ASCII box-drawing characters
• basic color support
• window and pad objects

As a first example, I create an interface with a color background, header, footer, and dynamic text that simulates sensor data to a Raspberry Pi (Figure 1; Listing 1).

01 import curses , time, random
02
03 # create a curses object
04 stdscr = curses.initscr()
05 height, width = stdscr.getmaxyx() # get the window size
06
07 # define two color pairs, 1- header/footer , 2 - dynamic text, 3 - background
08 curses.start_color()
09 curses.init_pair(1, curses.COLOR_RED, curses.COLOR_WHITE)
10 curses.init_pair(2, curses.COLOR_GREEN, curses.COLOR_BLACK)
11 curses.init_pair(3, curses.COLOR_WHITE, curses.COLOR_BLUE)
12
13 # Write a header and footer, first write colored strip, then write text
14 stdscr.bkgd(curses.color_pair(3))
15 stdscr.addstr(0, 0, " " * width,curses.color_pair(1) )
16 stdscr.addstr(height-1, 0, " " * (width - 1),curses.color_pair(1) )
17 stdscr.addstr(0, 0, " Curses Dynamic Text Example" ,curses.color_pair(1) )
18 stdscr.addstr(height-1, 0, " Key Commands : q - to quit " ,curses.color_pair(1) )
19 stdscr.addstr(3, 5, "RASPBERRY PI SIMULATED SENSOR VALUES" ,curses.A_BOLD )
20 stdscr.refresh()
21
22 # Cycle to update text. Enter a 'q' to quit
23 k = 0
24 stdscr.nodelay(1)
25 while (k != ord('q')):
26     # write 10 lines text with a label and then some random numbers
27     for i in range(1,11):
28         stdscr.addstr(4+ i, 5, "Sensor " + str(i) + " : " ,curses.A_BOLD )
29         stdscr.addstr(4+ i, 20, str(random.randint(10,99)) ,curses.color_pair(2) )
30     time.sleep(2)
31     k = stdscr.getch()
32
33 curses.endwin()

Figure 1: Curses dynamic text example.

The first step in a curses app is to define a main screen object (stdscr, line 4). The next step is to enable color and create some color pairs (lines 8-11). Color pair 3 sets the background to blue (line 14). Lines 15-18 use color pairs and the screen size (with height and width, defined in line 5) to add a header and footer strip.

The stdscr.nodelay command allows the program to cycle until the stdscr.getch() call returns a key stroke. The simulated Rasp Pi values refresh every 10 seconds until the q key is captured (line 25), after which, the terminal settings are returned to normal (curses.endwin()) and the program exits.

This simulated Raspberry Pi example only took about 30 lines of code, which is significantly less than if an equivalent web application were used.

C curses Example

For the C example, I used a Raspberry Pi and the previous Python example. Before you begin, you need to install the curses library:

sudo apt-get install libncurses5-dev

The curses syntax is similar between C and Python, but not identical (Listing 2). For example, in Python the addstr command includes a color pair reference. In C this is not supported, so an on/off attribute (attron/attroff) toggles the color pair. However, C does support a formatted string write command, mvprintw (lines 38 and 42).

01 /* c1.c - Basic Curses Example */
02
03 #include <curses.h>
04 #include <stdlib.h>
05 #include <unistd.h>
06
07 int main(void)
08 {
09     int row, col, k;
10 // Create a curses object and define color pairs
11     initscr();
12     getmaxyx(stdscr,row,col);
13     start_color();
14     init_pair(1,COLOR_RED,COLOR_WHITE);
15     init_pair(2,COLOR_GREEN,COLOR_BLACK);
16     init_pair(3,COLOR_WHITE,COLOR_BLUE);
17     curs_set(0);
18     noecho();
19     nodelay(stdscr, TRUE);
20 // Write a header and footer, first write colored strip, then write text
21     bkgd(COLOR_PAIR(3));
22     attron(COLOR_PAIR(1));
23 // Create a top and bottom color strip
24     for (int i = 0; i < col; i++) {
25         mvaddstr(0, i, " ");
26         mvaddstr(row-1, i, " ");
27     }
28     mvaddstr(0, 0, " Curses C Dynamic Text Example");
29     mvaddstr(row-1, 0, " Key Commands: q - to quit");
30     attroff(COLOR_PAIR(1));
31     mvaddstr(2, 5,"RASPBERRY PI SIMULATED SENSOR VALUES" );
32     refresh();
33 // Cycle with new values every 2 seconds until a q key (133) is entered
34     while (k != 113)
35     {
36         attroff(COLOR_PAIR(2));
37         for (int i = 0; i < 10; i++) {
38             mvprintw((4+i), 5, " Sensor %d : ",i);
39         }
40         attron(COLOR_PAIR(2));
41         for (int i = 0; i < 10; i++) {
42             mvprintw((4+i), 20, "%d",rand() %100);
43         }
44         k = getch();
45         sleep(2);
46     }
47     endwin();
48     exit(0);
49 }

To compile and run the program, enter:

gcc -o c1 c1.c -lncurses
./c1

With the exception of the caption in the top line, the C and Python output look identical.

FIGlet for Large Custom Text

To generate large custom text for presentation, you can use the FIGlet library [3], which has an extensive selection of "fonts" created with standard ASCII characters. The FIGlet library is installed in Python with the command:

pip install pyfiglet

An example executed from the Python shell shows how it works:

>>> import pyfiglet
>>> value1 = pyfiglet.figlet_format(         "12.3", font = "starwars" )
>>> print(value1)
 __   ___       ____
/_ | |__ \     |___ \
 | |    ) |      __) |
 | |   / /      |__ <
 | |  / /_   __ ___) |
 |_| |____| (__)____/

By combining curses with FIGlet, you can create some simple Raspberry Pi interfaces. A little bit of trial and error might be required to get a FIGlet font [4] that matches your requirements. I found that the starwars and doom fonts worked well for dynamic text and the small font was good for headings.

Listing 3 shows the code that generates the FIGlet large text in Figure 2. In this example, a get_io() function generates random numbers; for a true Rasp Pi project, this function would return sensor values.

01 import curses, time
02 import pyfiglet, random
03
04 def get_io(): # Get a random value. Tweek later with real data
05     global value1
06     testvalue = str(random.randint(100,1000)/10) + " C"
07     value1 = pyfiglet.figlet_format(testvalue, font = "starwars" )
08
09 # Create a string of text based on the Figlet font object
10 title = pyfiglet.figlet_format("Raspi Data", font = "small" )
11
12 stdscr = curses.initscr() # create a curses object
13 # Create a couple of color definitions
14 curses.start_color()
15 curses.init_pair(1, curses.COLOR_YELLOW, curses.COLOR_BLACK)
16 curses.init_pair(2, curses.COLOR_GREEN, curses.COLOR_BLACK)
17
18 # Write the BIG TITLE text string
19 stdscr.addstr(1,0, title,curses.color_pair(1) )
20 stdscr.addstr(8,0, "Sensor 1: GPIO 7 Temperature Reading" ,curses.A_BOLD)
21
22 # Cycle getting new data, enter a 'q' to quit
23 stdscr.nodelay(1)
24 k = 0
25 while (k != ord('q')):
26     get_io() # get the data values
27     stdscr.addstr(10,0, value1,curses.color_pair(2) )
28     stdscr.refresh()
29     time.sleep(2)
30
31     k = stdscr.getch()
32
33 curses.endwin()

Figure 2: FIGlet large text.

Curses Windows

The examples so far have been based on a main screen object, but the curses library also supports windows. Windows are useful because they support border outlines, and text can be cleared from and written to windows without affecting the main screen object. The syntax to create a curses window object is:

mynewwindow = curses.newwin(  height, width, begin_y, begin_x)

The code for Figure 3,

# define a win1 window object
win1 = curses.newwin(9, 44, 6, 4)
# write text inside the window object
win1.addstr(8,0, "Sensor 1: Temperature Reading", curses.A_BOLD)
value1 = pyfiglet.figlet_format(  "23 C", font = "doom") win1.addstr(0,0,value1,curses.color_pair(2) )

Figure 3: Large text in two curses windows.

produces two curses windows. The major difference in the code is that information within a window is addressed by the window object instead of the background screen object.

Dynamic Bars

Simple progress or indicator bars can be created by a curses window with a border (Listing 4). The bar itself is generated by writing a space character with inverse video (Figure 4).

01 import curses
02
03 bar = ' '   # with reverse video a space will show up
04 value1 = 10  # in real life this needs to be scaled
05
06 stdscr = curses.initscr()
07 curses.curs_set(0) # don't show the cursor
08
09 stdscr.addstr(1,3, "Python Curses Bar")
10 stdscr.refresh()
11 # Define windows
12 win1 = curses.newwin(3, 32, 3, 2)
13 win1.border(0)  # add a border
14 # a horizontal bar 10 characters wide
15 win1.addstr(1, 1, bar * value1,curses.A_REVERSE )
16 win1.refresh()
17
18 # Wait for a key press then exit
19 stdscr.getch()
20 curses.endwin() # restore the terminal to original settings

Figure 4: A simple curses bar.

After I had the basic curses bar working, I was able to use what I learned in the earlier examples to create Raspberry Pi interfaces that included color and large text (Figure 5).

Figure 5: Raspberry Pi sensor data presented as bars.

Summary

If you are looking for a quick and easy way to present data, without the bother of a GUI, curses is a great solution. Curses is supported in a variety of programming languages. I focused my curses work here on C and Python, but I've also had good success in Lua.

I only had one presentation issue and that was with line drawing characters like borders when I used Putty (a Windows-based SSH program). To fix this issue, I changed the Putty Window | Translation setting to use the VSCII character set; then, everything looked good.