Using the curses library to view IoT data
Old School
![© Lead Image © Andrey Kiselev, 123RF.com © Lead Image © Andrey Kiselev, 123RF.com](/var/linux_magazin/storage/images/issues/2020/232/curses/123rf_37876152_steampunk-research-library_andreykiselev_resized.png/762832-1-eng-US/123rf_37876152_SteamPunk-Research-Library_AndreyKiselev_resized.png_medium.png)
© Lead Image © Andrey Kiselev, 123RF.com
When you need some quick graphical output, the old school curses library can save you some time and effort.
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).
Listing 1
curses_text.py
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()
![](/var/linux_magazin/storage/images/issues/2020/232/curses/figure-1/762835-1-eng-US/Figure-1_large.png)
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).
Listing 2
c1.c
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.
Listing 3
bigtxt.py
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()
![](/var/linux_magazin/storage/images/issues/2020/232/curses/figure-2/762838-1-eng-US/Figure-2_large.png)
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