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139 lines
6.8 KiB
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[#]: subject: "Position text on your screen in Linux with ncurses"
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[#]: via: "https://opensource.com/article/21/8/ncurses-linux"
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[#]: author: "Jim Hall https://opensource.com/users/jim-hall"
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[#]: collector: "lujun9972"
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[#]: translator: "perfiffer"
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[#]: reviewer: " "
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[#]: publisher: " "
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[#]: url: " "
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Position text on your screen in Linux with ncurses
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======
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Use ncurses in Linux to place text at specific locations on the screen
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and enable more user-friendly interfaces.
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![Person using a laptop][1]
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Most Linux utilities just scroll text from the bottom of the screen. But what if you wanted to position text on the screen, such as for a game or a data display? That's where **ncurses** comes in.
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**curses** is an old Unix library that supports cursor control on a text terminal screen. The name _curses_ comes from the term _cursor control_. Years later, others wrote an improved version of **curses** to add new features, called _new curses_ or **ncurses**. You can find **ncurses** in every modern Linux distribution, although the development libraries, header files, and documentation may not be installed by default. For example, on Fedora, you will need to install the **ncurses-devel** package with this command:
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```
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`$ sudo dnf install ncurses-devel`
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```
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### Using ncurses in a program
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To directly address the screen, you'll first need to initialize the **ncurses** library. Most programs will do that with these three lines:
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* initscr(); Initialize the screen and the **ncurses** code
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* cbreak(); Disable buffering and make typed input immediately available
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* noecho(); Turn off echo, so user input is not displayed to the screen
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These functions are defined in the **curses.h** header file, which you'll need to include in your program with:
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```
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`#include <curses.h>`
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```
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After initializing the terminal, you're free to use any of the **ncurses** functions, some of which we'll explore in a sample program.
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When you're done with **ncurses** and want to go back to regular terminal mode, use **endwin();** to reset everything. This command resets any screen colors, moves the cursor to the lower-left of the screen, and makes the cursor visible. You usually do this right before exiting the program.
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### Addressing the screen
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The first thing to know about **ncurses** is that screen coordinates are _row,col_, and start in the upper-left at 0,0. **ncurses** defines two global variables to help you identify the screen size: LINES is the number of lines on the screen, and COLS is the number of columns. The bottom-right position is LINES-1,COLS-1.
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For example, if you wanted to move the cursor to line 10 and column 30, you could use the move function with those coordinates:
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```
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`move(10, 30);`
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```
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Any text you display after that will start at that screen location. To display a single character, use the **addch(c)** function with a single character. To display a string, use **addstr(s)** with your string. For formatted output that's similar to **printf**, use **printw(fmt, …)** with the usual options.
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Moving to a screen location and displaying text is such a common thing that **ncurses** provides a shortcut to do both at once. The **mvaddch(row, col, c)** function will display a character at screen location _row,col_. And the **mvaddstr(row, col, s)** function will display a string at that location. For a more direct example, using **mvaddstr(10, 30, "Welcome to ncurses");** in a program will display the text "Welcome to ncurses" starting at row 10 and column 30. And the line **mvaddch(0, 0, '+');** will display a single plus sign in the upper-left corner at row 0 and column 0.
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Drawing text to the terminal screen can have a performance impact on certain systems, especially on older hardware terminals. So **ncurses** lets you "stack up" a bunch of text to display to the screen, then use the **refresh()** function to make all of those changes visible to the user.
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Let's look at a simple example that pulls everything together:
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```
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#include <curses.h>
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int
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main()
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{
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initscr();
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cbreak();
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noecho();
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mvaddch(0, 0, '+');
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mvaddch(LINES - 1, 0, '-');
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mvaddstr(10, 30, "press any key to quit");
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refresh();
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getch();
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endwin();
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}
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```
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The program starts by initializing the terminal, then prints a plus sign in the upper-left corner, a minus in the lower-left corner, and the text "press any key to quit" at row 10 and column 30. The program gets a single character from the keyboard using the getch() function, then uses **endwin()** to reset the terminal before the program exits completely.
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**getch()** is a useful function that you could use for many things. I often use it as a way to pause before I quit the program. And as with most **ncurses** functions, there's also a version of **getch()** called **mvgetch(row, col)** to move to screen position _row,col_ before waiting for a character.
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### Compiling with ncurses
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If you tried to compile that sample program in the usual way, such as `gcc pause.c`, you'll probably get a huge list of errors from the linker. That's because the **ncurses** library is not linked automatically by the GNU C Compiler. Instead, you'll need to load it for linking using the `-l ncurses` command-line option.
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```
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`$ gcc -o pause pause.c -lncurses`
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```
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Running the new program will print a simple "press any key to quit" message that's more or less centered on the screen:
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![centered message in a program window][2]
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Figure 1: A centered "press any key to quit" message in a program.
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### Building better programs with ncurses
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Explore the **ncurses** library functions to learn about other ways to display text to the screen. You can find a list of all **ncurses** functions in the man ncurses manual page. This gives a general overview of **ncurses** and provides a table-like list of the different **ncurses** functions, with a reference to the manual page that has full details. For example, **printw** is described in the _curs_printw(3X)_ manual page, which you can view with:
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```
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`$ man 3x curs_printw`
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```
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or just:
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```
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`$ man curs_printw`
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```
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With **ncurses**, you can create more interesting programs. By printing text at specific locations on the screen, you can create games and advanced utilities to run in the terminal.
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/8/ncurses-linux
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作者：[Jim Hall][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/jim-hall
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/laptop_screen_desk_work_chat_text.png?itok=UXqIDRDD (Person using a laptop)
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[2]: https://opensource.com/sites/default/files/press-key_0.png
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