I'm implementing a small shell in C and I want to use Ctrl-Up and Ctrl-Down. Are there ASCII codes for Ctrl-Up, Ctrl-Down and Ctrl-Shift+C? I have searched everywhere for them and I couldn't find them.
As #MateoConLechuga mentioned, what you're looking for simply doesn't exist. What actually happens when you press Ctrl-Up is that the terminal sends a special sequence of characters starting with the ESC character. For example, on my terminal, Ctrl-Up sends ESC[1;5A.
What you need to to is use something like the ncurses and/or the termcap libraries to deal with things like terminal input in a terminal-independent manner.
Unfortunately for you, this is probably way more work that you were hoping for. Writing a "small" shell is non-trivial.
Related
I'm very new to coding in general so please excuse any stupid things I say.
I am trying to print (in c) a rather long text using printf() but since it can't all fit on the screen it jumps to the end of the text and the beginning is not visible unless you scroll up. Is there an easy way to have it print the long text but stay at the beginning and allow the user to scroll down as they read before they put in the next command?
On Unix (including Linux and Mac), there are command line programs built in called more and less that does exactly what you describe. more is a program that simply waits for the user to press enter or space before showing the next page of output. less is slightly improved in that it allows vi editor keystrokes (such as j and k) to scroll up and down in the output.
more is also available on the Windows command line as well. You might even be able to find a version of less for Windows as well.
c:\users\selbie> your_program.exe | more
$> ./your_program | less
As to how to do this programmatically, that's a bit more difficult as it would involve measuring the console width and implementing your own scroll buffers. There might be open-source libraries that provide this functionality, but the console environment already has a solution for apps that produce long output.
Not really, though you may find a reasonable and simple solution is to print only a certain number of lines (say 30), then prompt the user to press Enter before display more lines.
You can even find out the current size of the terminal. That's platform specific; for Linux it's explained here: How to get terminal window width?
Not in a standard way, no.
Your output stream in C is just a stream of characters -- scrolling is handled by your terminal.
Depending on your terminal, it may be possible to control scrolling by outputting special characters, like ANSI escape codes. The ncurses library provides a portable way to manipulate terminals.
However, if you just want a more convenient way to look through your output (or really any command output), #selbie's answer is the best: use more or less. This will avoid any extra complexity in your program.
If you type ps aux into your terminal and make the window really small, the output of the command will not wrap and the format is still very clear.
When I use printf and output my 5 or 6 strings, sometimes the length of my output exceeds that of the terminal window and the strings wrap to the next line which totally screws up the format. How can I write my program such that the output continues to the edge of the window but no further?
I've tried searching for an answer to this question but I'm having trouble narrowing it down and thus my search results never have anything to do with it so it seems.
Thanks!
There are functions that can let you know information about the terminal window, and some others that will allow you to manipulate it. Look up the "ncurses" or the "termcap" library.
A simple approach for solving your problem will be to get the terminal window size (specially the width), and then format your output accordingly.
There are two possible answers to fix your problem.
Turn off line wrapping in your terminal emulator(if it supports it).
Look into the Curses library. Applications like top or vim use the Curses library for screen formatting.
You can find, or at least guess, the width of the terminal using methods that other answers describe. That's only part of the problem however -- the tricky bit is formatting the output to fit the console. I don't believe there's any alternative to reading the text word by word, and moving the output to the next line when a word would overflow the width. You'll need to implement a method to detect where the white-space is, allowing for the fact that there could be multiple white spaces in a row. You'll need to decide how to handle line-breaking white-space, like CR/LF, if you have any. You'll need to decide whether you can break a word on punctuation (e.g, a hyphen). My approach is to use a simple finite-state machine, where the states are "At start of line", "in a word", "in whitespace", etc., and the characters (or, rather character classes) encountered are the events that change the state.
A particular complication when working in C is that there is little-to-no built-in support for multi-byte characters. That's fine for text which you are certain will only ever be in English, and use only the ASCII punctuation symbols, but with any kind of internationalization you need to be more careful. I've found that it's easiest to convert the text into some wide format, perhaps UTF-32, and then work with arrays of 32-bit integers to represent the characters. If your text is UTF-8, there are various tricks you can use to avoid having to do this conversion, but they are a bit ugly.
I have some code I could share, but I don't claim it is production quality, or even comprehensible. This simple-seeming problem is actually far more complicated than first impressions suggest. It's easy to do badly, but difficult to do well.
I want to autocomplete on a command line application a but like in bash you can use tab key which will complete the command. But getchar() seems to wait until a newline char is received before it starts reading any characters.
scanf seems to work the same way.
Is there any way I can scan characters one at a time no matter if they are whitespace or control characters?
I want to be able to read char by char as entered building up a command and then as soon as tab char received I will attempt to lookup how to complete and print full command in my application.
Don't reinvent the wheel.
Use what other projects use to build command lines: Libraries that implement that job for you.
Many CLI prompts depend on GNU readline, which I think is fine, if a bit cluttered and heavy. Also, it's GPL, so depends on whether you like that or not.
I'd look into linenoise. It's very lightweight, and if you decide you really want to implement user interface yourself rather than including one or two files, OK, do that, but look at the rather concise reference implementation that seems to be. Caveat: haven't used it myself, so far. The API is pretty simple, though, as can be seen in their example.
A popular alternative is libedit/editline.
You need the GNU Readline Library. Use the rl_bind_key() function to add a filename-completion processing function whenever the users presses a key ('\t', in your case).
You are in for a world of hurt if you try to roll your own readline style function.
(If you are on Windows.)
For a class project we have to implement a basic Linux shell. As it stands now I have everything working to fully implement basic functionality except for the "UP" key previous command scrolling feature.
How might something like this be implemented? I realize it may be as simple as retaining an array of char* to the input strings, but how do we capture "UP" key button presses?
Once we have the above implemented, how can you write to stdout without making it permanent? That is, when you press "UP" again then it erases what was previously written with another command.
A practical way to implement the up key would be to use GNU readline library (and its history sub-library). BTW, some shells actually do use GNU readline (it is under GPL license). And you'll get line edition also. And you could implement completion with the tab key, etc.
There are other ways, like using ncurses or termcap etc... Or emitting ANSI escape codes on a raw terminal. See also the tty demystified & termios(3)...
BTW, most Linux shells are free software, so you could study their source code.
If you are using non-buffered character-by-character input directly from STDIN (which means, among other things, control characters such as CTRL+C are not handled automatically), then you will be reading byte-by-byte. Arrow keys, unlike ASCII symbols, put multiple bytes on STDIN because they are excape sequences. These bytes differ from system to system. The easiest way to determine the escape sequence on your system is to execute the cat command with no arguments, then hit the arrow keys. Something like ^[[A will be displayed, you will need to convert that sequence from ASCII to hex bytes.
Once you've done that, you can read the bytes one at a time with get_char(), just like you're probably already doing in your input function anyway.
I'm developing a CLI program, in C, for my systems class project, and it needs to display incoming text while maintaining a command prompt. Left alone, the incoming text will saw through whatever one tries to type. In other applications I've seen the incoming text print above(or below) the prompt itself. Is there any way to implement this in ANSI escapes? ncurses seems like overkill.
You can print \r to erase the prompt: It will return the cursor to the beginning of the current line. You can then print your output followed by some spaces to clear out any remaining input characters, newline, and reprint the prompt.
With ANSI sequences or terminal-specific libraries you can do even more, but this I think is all you can do reliably using only ASCII. Apart from printing 242 blank lines to redraw the whole screen, of course.
Edit: Sorry, I didn't answer the ANSI part properly. With cursor movement control codes and printing space over existing characters, you can pretty much do anything, and there are some convenience actions to help you, such as "delete line". But keep in mind that Windows doesn't play nice w/ ANSI post XP, and neither are other systems guaranteed to.
For one thing, if you want to maintain a prompt, while printing, you can not use things like scanf. You have to intercept keyboard events or use a non waiting method to get input. Then you can get the terminal number of lines (n) and print the last n-1 lines of your output, and then a prompt.
my2c