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I've written a tiny program in Ansi C on Windows first, and I compiled it on Ubuntu with the built-in GCC now.

The program is simple:

  • read the line from console with scanf().
  • Analyze the string and calculate.

But something weird happens. When I try to move the cursor, it prints four characters:

  • pressing Up prints "^[[A"
  • pressing Dn prints "^[[B"
  • pressing Rt prints "^[[C"
  • pressing Lt prints "^[[D"

  • How can this be avoided?

  • Why does it print these 4 characters instead of moving the cursor?

Drew Dormann
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Kevin Dong
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    Special characters like ^U and Backspace will work because the keyboard driver handles those. But the arrow key behavior you're used to is programmed into the shell. When you write your own program, you have to handle it yourself, or you could install `rlwrap` and run your program as `rlwrap my_prog` and it will do what you expect. The characters you see are the escape sequences generated by those key presses. – lurker Jan 27 '14 at 15:02
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    Those are the traditional ANSI escape sequences which represent those cursor keys. See http://stackoverflow.com/questions/4130048/recognizing-arrow-keys-with-stdin – keshlam Jan 27 '14 at 15:05
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    [This answer](https://unix.stackexchange.com/a/444270/87749) on unix.stackexchange may help, it helped me understand the issue. – A1rPun May 26 '21 at 21:30
  • In Ubuntu if yout just need a posix shell, you can use `set -o posix` or `--posix` to force the default shell into compliance. Dash is not compiled for interactive use. – Ray Foss Jun 10 '21 at 18:08

5 Answers5

40

Because that's what the keyboard actually sends to the PC (more precisely, what the terminal prints for what it actually receives from the keyboard). bash for example gets those values, deciphers them and understands that you want to move around, so it will either move the cursor (in case of left/right) or use its history to fetch previous commands (up/down). So you can't expect your program to magically support arrow keys.

However, reading from standard input from the terminal already supports left/right arrow keys (I believe, but I'm not in Linux right now to test and make sure). So my guess is that there is another issue interfering. One possible cause could be that one of your modifier keys is stuck? Perhaps ALT, CTRL or SUPER?

Shahbaz
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    When I press arrow keys in terminal, it'll move the cursor, but when reading input from buffer with scanf(), it can't. – Kevin Dong Jan 27 '14 at 15:10
  • @KevinDongNaiJia, it could be that `bash` detects the modifier and ignores them. Unfortunately, right now I'm in Windows, so I can't really tell. Just to be sure, you using the normal `gnome-terminal` and have not modified any settings, either of the terminal or of the keyboard (in Ubuntu settings)? – Shahbaz Jan 27 '14 at 15:16
  • Yes, I tested it in normal `gnome-terminal` without any settings. I just used Ubuntu in the second day. ;-) – Kevin Dong Jan 27 '14 at 15:22
  • What is your keyboard's layout? Try changing its layout to "Enlish (US)" and see if the problem persists. – Shahbaz Jan 27 '14 at 15:28
  • Nothing changed when using "English (US)" keyboard layout. And I tried to execute it in tty1~tty6, this problem still happened. – Kevin Dong Jan 27 '14 at 15:36
  • This is really weird, what is your keyboard brand/model? Does this happen in the terminal of Windows if you have one? What if you attach this keyboard to someone else's computer with Ubuntu? Do you still see the same behavior? – Shahbaz Jan 27 '14 at 16:02
  • I use notebook, so the keyboard is built-in. Then I used USB-keyboard (English (US) Layout), but the problem still existed. Last, I tested it in other Ubuntu derivative, **K**ubuntu, and this problem didn't solve. – Kevin Dong Jan 28 '14 at 01:18
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    This very likely some incompatibility between something in your laptop and Ubuntu (you can thank Microsoft's monopoly for that). Try asking on askubuntu.com, perhaps someone there has experienced something like that. Make sure to include the laptop brand and model – Shahbaz Jan 28 '14 at 08:23
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    Actually, the problem seems to be the **mode of the terminal**: The terminal has different modes, like either directly sending each character to the process, or aggregating a line until the user presses enter, or displaying stuff with history scroll-back as opposed to `vim` style full-screen mode. There is a lot of stuff that's going on, and I don't have the time right now to look up the details (which is why I'm not posting an answer myself). Anyway, when you see `^[[A` appear on the screen, that's the escape sequence sent to represent the up arrow, and nothing interpreted it as such. – cmaster - reinstate monica Aug 10 '18 at 11:07
  • The symbols ^[[A are a common problem (exactly these chars) when trying to use the line-edit and the shell mismatches. It is definitely not a problem about stuck keys such as ALT. answer from cmaster looks good. – Blindleistung Nov 18 '20 at 14:21
27

For those who are coming from the osx (mac) try changing the shells to bash

Terminal -> Preferences -> Shells open with -> [select] Command (complete path)

then paste

/bin/bash

Neuron - Freedom for Ukraine
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KhaledMohamedP
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  • I was trying to exec into a docker instance on OSX (using Kitematic). This worked a treat, thank you. The Kitematic preferences let you set the shell to `bash`. – chichilatte Jul 27 '18 at 09:58
22

This might be because the user account is created in shell. You can change it to bash by two ways.

Permament solution is - 

sudo chsh -s /bin/bash ${username}

To get this solution working you will have to logout and login

Temporary solution is everytime when you login into the ubuntu server type bash and hit return.

Sumith08
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0

On MacOS Terminal for me was enough to uncheck "Scroll alternate screen" for the issue to disappear. See screenshot of the preferences below.enter image description here

Ivan Carosati
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i think simple way is we can just do kill %% because this sometimes happen because of background processes.

Milan Parmar
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