Code Project: Build an Ncurses UI with Python
Picture the scene: you're logged into a remote server via SSH, or you've installed a new graphics card and you're left staring at the command line. You need to enter a command, but you can't remember the zillion options that go along with it. You're stuck - all you can do is consult the manual pages and pore through pages of waffling technical gobbledygook. We've all been there, and no matter how experienced you are with Linux, sometimes you need to accomplish a job quickly without sifting through masses of reading material.
In this coding project, we're going to solve this problem - and have fun along the way! We'll show you how to write a dialog-based program that gives you options one-by-one so that you don't need to consult the man pages. In this guide, we'll show you how to write a nifty front-end for the useradd utility, a command which (unsurprisingly) lets you add user accounts to your Linux installation. Like many administration tools, useradd requires a long string of options and parameters; we're going to make it much simpler by creating an interactive dialog-driven version called UserMaster.
Now, useradd is a fairly trivial tool and there's an alternative command, adduser, which prompts you step-by-step. But here we'll give you the code and techniques to make your own config tools - so at the end, you'll be able to write user-friendly utilities for starting services, clearing temporary files, or anything else you'd normally do at the shell prompt. Not only will this save you vital time when you're at the command line, but it means you can write config tools for other people who may log into your machines. If you run a server and don't want users trying to guesstimate super-complex commands, you can knock together a quick dialog-based version.
As with our previous code projects (check them out - go on, just right-click the link and select Open Link in New Tab!), this guide assumes a smattering of programming knowledge. Don't worry: you don't need to be a coding whizz, but if you've played around with another language before, you'll be off to a flying start. But even if you've never written a line of code in your life, hopefully this tutorial will show you how programs fit together and what you can do. Enough chit-chat - let's get started with UserMaster...
We'll base our project on Python, a hugely popular programming language that's used in everything from tiny scripting tools to fully-fledged applications. If you've ever installed Fedora or Red Hat Enterprise Linux, you'll have seen a Python application in all its glory: the Anaconda installer is written almost entirely in the language. Python is blissfully easy to read (perhaps the most human- readable computing language around), so we don't need to detail minutiae of the syntax. Indeed, if you're a regular Pythoner you can skip over this bit. But for a quick overview of the language, enter this into a text file and save it in your home directory as test.py:
print "How easy is this?" x = 1 y = 2 z = x + y print "Result of x + y is", z
Now open up a terminal and enter:
The Python interpreter runs and executes our code. It prints a string to the screen, then adds together two variables, and displays the results. You can call variables almost anything you want, providing they don't conflict with Python keywords. So here, x, y and z are fine generic variable names. You don't need to faff around with curly braces either, as Python uses indentation for code blocks, as follows:
def saysmithers(): print "Excellent, Smithers" print "Calling saysmithers function..." saysmithers()
Here, we declare a subroutine (aka function) at the start - it's not executed immediately, but only when we call it. So Python skips over the first chunk of code, as it's a separate function, and begins with the "Calling" print line. Then our code calls the saysmithers() routine which prints the "Excellent" message. Code that belongs to a function or code block is marked with tab indents, as you can see from our examples. You can pass parameters to functions thusly:
def saystuff(mystring): print "You said:", mystring saystuff("Bach rules") saystuff("So does Telemann")
So here, we have a function that just spits out whatever text it receives. Again, it doesn't execute straight away - as it's a standalone function - and code starts at the first non-function line. We call the saystuff function twice, with different text as parameters, and our function simply prints the supplied text to the screen. Super simple!
For our UserMaster tool, we don't want a boring command line interface - after all, that's what the standard user management programs use. Instead, we'll create a friendlier menu and dialog-driven interface using the curses library. If you've never heard of it before, curses is a library of routines that let you create boxes, windows and text-entry panels on text-based displays. It's a play on the word ‘cursor', and on modern Linux systems, its implementation is usually called ncurses.
Essentially, curses bridges the gap between the command line and graphical desktops. In text mode, you can't have zillions of colours and pixel-perfect mouse control, but you can still have more than a string of letters. curses lets you create semi-GUI applications with windows and boxes - if you've ever used the text-based Debian, Ubuntu or Slackware installers, you'll know what we mean.
Here's an example. As before, tap this into a text file and save it into your home directory as test.py. Then execute it with python text.py in a terminal:
import curses myscreen = curses.initscr() myscreen.border(0) myscreen.addstr(12, 25, "Python curses in action!") myscreen.refresh() myscreen.getch() curses.endwin()
Python includes a set of bindings to the curses library - in other words, you can utilise the features of curses in your Python programs, providing you import the curses module as specified in the first line of this code. The second line of code creates a new curses screen object. Python is an object-oriented language, so in this case, we create a new screen workspace (called myscreen) and tell curses to initialise itself (initscr). Now we have control of the terminal window.
Next, we have four lines which tell curses what to do with our new myscreen object. First, we tell curses to draw a border around our screen, which immediately makes it look prettier than a vanilla CLI program. In the following line, we tell curses to add a text string to our myscreen object, specifying the position where it should be printed. Normally, text terminals are 80x25 characters if you're at the raw Linux terminal, or 80x24 if you're in GUI mode and have launched xterm, Konsole or Gnome-Terminal. So we print a line of text 12 characters down and 25 across - the middle of the screen, unless you've resized your terminal of course!
The third line is very important: we need to call refresh on our myscreen object to tell curses that we're ready to go. Hypothetically, curses could refresh the screen every for every command, but that would be very slow with complex programs. So instead, it lets us build up our display bit-by-bit, and then draw the whole lot in one fell swoop. Calling refresh here renders our border and text screen to the screen.
Finally, we have two remaining lines of code: the first, which calls getch on our myscreen object, tells curses to wait for a keypress (get character). This pauses execution until we hit a key. Finally, we call endwin which shuts down curses and returns us to our normal command-line interface. When you run this program, you'll see the results in the first screenshot - very basic, but now we're ready to write UserMaster!
The result of our Python Ncurses code so far - it's not exciting, but it does form the foundation for greater things!
Here's the code for our program. You can download the source code here, but for now, just read over it - you should be able to understand what it's doing. If you've never used Python before, there are a few new concepts in here, but we'll go over them in a moment.
#!/usr/bin/env python from os import system import curses def get_param(prompt_string): screen.clear() screen.border(0) screen.addstr(2, 2, prompt_string) screen.refresh() input = screen.getstr(10, 10, 60) return input def execute_cmd(cmd_string): system("clear") a = system(cmd_string) print "" if a == 0: print "Command executed correctly" else: print "Command terminated with error" raw_input("Press enter") print "" x = 0 while x != ord(‘4'): screen = curses.initscr() screen.clear() screen.border(0) screen.addstr(2, 2, "Please enter a number...") screen.addstr(4, 4, "1 - Add a user") screen.addstr(5, 4, "2 - Restart Apache") screen.addstr(6, 4, "3 - Show disk space") screen.addstr(7, 4, "4 - Exit") screen.refresh() x = screen.getch() if x == ord(‘1'): username = get_param("Enter the username") homedir = get_param("Enter the home directory, eg /home/nate") groups = get_param("Enter comma-separated groups, eg adm,dialout,cdrom") shell = get_param("Enter the shell, eg /bin/bash:") curses.endwin() execute_cmd("useradd -d " + homedir + " -g 1000 -G " + groups + " -m -s " + shell + " " + username) if x == ord(‘2'): curses.endwin() execute_cmd("apachectl restart") if x == ord(‘3'): curses.endwin() execute_cmd("df -h") curses.endwin()
That's quite a bit of code, but the majority of it is spent dealing with the screen, so it's actually not very complicated. This program starts up with a menu offering four choices:
- Add a user account
- Restart the Apache web server
- Display available disk space
It doesn't matter if you don't have Apache installed - this is just to demonstrate how you can expand the program into a more versatile config tool.
The first line lets us run the program without having to specify the Python interpreter by hand (python usermaster.py). More on that in a moment. Then we have two import lines: the first gives us access to Python's system command, which lets us execute external programs (ie binaries you'd find in /usr/bin etc), while the second rolls in curses as described before.
Next we have two functions. The first takes a prompt string parameter, and then asks the user for a string before returning it back. Basically, in human-speak, it says, "Tell me what question to put on the screen, then I'll get some text from the user and send it back." We use this function later in the code to get information for the useradd command.
The second function, meanwhile, executes a command on the system. It takes a string parameter, clears the screen, and then executes the command in this line:
a = system(cmd_string)
Now, what's that a doing there? Well, we want to know whether the command was executed successfully. When we use Python's system routine to run a program, it returns 0 if the program ran successfully, or another number if it encountered an error. Thanks to this, we can print out a message to show whether the command worked - for instance, whether the user-adding process succeeded or failed. Later on, you may expand UserMaster to do other tasks, some of which may not generate any visible output (eg running a cron job), but you'll still know if it worked or messed up.
So, those two functions lay the foundations for our OS. Python will start executing the program at the x = 0 line, which just sets up a variable for the next line, which checks to see if x contains the character 4. Well, at this stage, we haven't got any input from the user yet - but we will in a few steps. We fire up ncurses, clear the screen, set a border and then print some options strings to the screen. Then we have:
x = screen.getch()
The user has a list of commands to choose from; whatever he/she enters is stored in the x variable. So, the following if lines check to see what's in x: if it's one, we're in user-adding mode; if it's 2, we restart Apache; 3, and we show disk space; 4, and we exit. But hang on a minute, where are we checking for 4 there? Well, we're doing that at the start of the loop, in this line:
while x != ord(‘4'):
If the user enters anything other than 1, 2 or 3, control jumps back to the while line, which then stops executing the code block. As mentioned earlier, code blocks in Python are marked with tab indents, so after our program has done checking for the 3 key to be pressed, it goes back to the start of the loop - the while line. It does this because the following (and final) line, the endwin(), is not indented and therefore not part of the while loop code block. So the end of the indents shows where the code block finishes.
No buts, only ifs
Let's look at those if statements. Depending on what the user entered, we run a specified program using our execute_cmd() function. For options 2 and 3, we simply disable curses (so that it doesn't interfere with our program output), and then run the program. execute_cmd() tells us if the program executed correctly, before handing control back to the main code - so we see the menu again.
But it's the first option that's the most interesting. Here, we use our funky get_param() function to display a series of prompts, getting input from the user each time. We ask the user for an account name, default shell and so forth; nothing too taxing. Then we join all these bits of info together and feed them to the system's standard useradd command, with appropriate command-line parameters (for example -d for home directory). Hence this is why the execute_cmd() line here is very long - it joins together all the strings and flags that the command needs in order to work.
Grab usermaster.py, copy it to your home directory, then fire up a terminal and switch to root (sudo bash in Ubuntu or su in others). Enter:
chmod +x usermaster.py mv usermaster.py /usr/sbin/usermaster
Now usermaster is installed into the system path (for administrator commands), so you can run it simply by entering usermaster. Note that you need to run it as root though, as normal users can't create other user accounts! You can now try creating a user via option 1 of the program - don't worry about setting up the exact right groups now.
The finished product: a simple Python Ncurses menu system that automates sysadmin commands.
When you've created a user via our tool, you'll also need to set up a password as follows:
Replace nate with the username that you created. To keep the code listing short, we haven't included this facility in UserMaster, but it's an ideal first mini-project for you: adding a password setting/changing facility! You'd just need to add another option to the menu, get the account name via a get_param() call, then execute passwd on that name.
The exact details of users and groups and how they are related varies quite distinctly from linux distribution to distribution, so you'll want to tailor UserMaster for your own needs. As your normal user account, enter groups in a terminal to see which groups a standard user belongs to, and also see the useradd manual page (man useradd) to learn more about the options we pipe to the command.
In this tutorial we're using some of curses's most common routines - clearing the screen, drawing a border, getting input etc. But it's a lot more capable than that, providing a complete set of window management routines. Yes, it seems futile to have the concept of windows at a text terminal, but it lets you render and remove panels (eg question dialogs) while keeping the text beneath intact. You can even interface with the mouse in X terminals!
Annoyingly, documentation for the Python curses bindings is rather sketchy at present; the official Python documentation only has a short chapter on the library and there's not much else doing the rounds. If you want to do more with curses, your best bet is the Linux Documentation Project's NCURSES Programming How To - it was originally written with C coders in mind, but you'll find many of the same function calls in the Python bindings.
First published in Linux Format magazine