Control the terminal, the right way

18 minutes read Keywords: terminal, console

Nowadays, there are plenty of terminal emulators in the wild. Each one has a specific way to handle controls. How many colours does it support? How to control the style of a character? How to control more than style, like the cursor or the window? In this article, we are going to explain and show in action the right ways to control your terminal with a portable and an easy to maintain API. We are going to talk about stat, tput, terminfo, hoa/ console… but do not be afraid, it's easy and fun!

Introduction

Terminals. They are the ancient interfaces, still not old fashioned yet. They are fast, efficient, work remotely with a low bandwidth, secured and very simple to use.

A terminal is a canvas composed of columns and lines. Only one character fits at a position. According to the terminal, we have some features enabled; for instance, a character might be stylized with a colour, a decoration, a weight etc. Let's consider the former. A colour belongs to a palette, which contains either 2, 8, 256 or more colours. One may wonder:

Well, this article is going to explain how a terminal works and how we interact with it. We are going to talk about terminal capabilities, terminal information (stored in database) and Hoa\Console, a PHP library that provides advanced terminal controls.

The basis of a terminal

A terminal, or a console, is an interface that allows to interact with the computer. This interface is textual. Like a graphical interface, there are inputs: The keyboard and the mouse, and ouputs: The screen or a file (a real file, a socket, a FIFO, something else…).

There is a ton of terminals. The most famous ones are:

Whatever the terminal you use, inputs are handled by programs (or processus) and outputs are produced by these latters. We said outputs can be the screen or a file. Actually, everything is a file, so the screen is also a file. However, the user is able to use redirections to choose where the ouputs must go.

Let's consider the echo program that prints all its options/arguments on its output. Thus, in the following example, foobar is printed on the screen:

$ echo 'foobar'

And in the following example, foobar is redirected to a file called log:

$ echo 'foobar' > log

We are also able to redirect the output to another program, like wc that counts stuff:

$ echo 'foobar' | wc -c
7

Now we know there are 7 characters in foobar… no! echo automatically adds a new-line (\n) after each line; so:

$ echo -n 'foobar' | wc -c
6

This is more correct!

Detecting type of pipes

Inputs and outputs are called pipes. Yes, trivial, this is nothing more than basic pipes!

There are 3 standard pipes:

If the output is attached to the screen, we say this is a “direct output”. Why is it important? Because if we stylize a text, this is only for the screen, not for a file. A file should receive regular text, not all the decorations and styles.

Hopefully, the Hoa\Console\Console class provides the isDirect, isPipe and isRedirection static methods to know whether the pipe is respectively direct, a pipe or a redirection (damn naming…!). Thus, let Type.php be the following program:

<?php

echo 'is direct:      ';
var_dump(Hoa\Console\Console::isDirect(STDOUT));

echo 'is pipe:        ';
var_dump(Hoa\Console\Console::isPipe(STDOUT));

echo 'is redirection: ';
var_dump(Hoa\Console\Console::isRedirection(STDOUT));

Now, let's test our program:

$ php Type.php
is direct:      bool(true)
is pipe:        bool(false)
is redirection: bool(false)

$ php Type.php | xargs -I@ echo @
is direct:      bool(false)
is pipe:        bool(true)
is redirection: bool(false)

$ php Type.php > /tmp/foo; cat !!$
is direct:      bool(false)
is pipe:        bool(false)
is redirection: bool(true)

The first execution is very classic. STDOUT, the standard output, is direct. The second execution redirects the output to another program, then STDOUT is of kind pipe. Finally, the last execution redirects the output to a file called /tmp/foo, so STDOUT is a redirection.

How does it work? We use fstat to read the mode of the file. The underlying fstat implementation is defined in C, so let's take a look at the documentation of fstat(2). stat is a C structure that looks like:

struct stat {
    dev_t    st_dev;              /* device inode resides on             */
    ino_t    st_ino;              /* inode's number                      */
    mode_t   st_mode;             /* inode protection mode               */
    nlink_t  st_nlink;            /* number of hard links to the file    */
    uid_t    st_uid;              /* user-id of owner                    */
    gid_t    st_gid;              /* group-id of owner                   */
    dev_t    st_rdev;             /* device type, for special file inode */
    struct timespec st_atimespec; /* time of last access                 */
    struct timespec st_mtimespec; /* time of last data modification      */
    struct timespec st_ctimespec; /* time of last file status change     */
    off_t    st_size;             /* file size, in bytes                 */
    quad_t   st_blocks;           /* blocks allocated for file           */
    u_long   st_blksize;          /* optimal file sys I/O ops blocksize  */
    u_long   st_flags;            /* user defined flags for file         */
    u_long   st_gen;              /* file generation number              */
}

The value of mode returned by the PHP fstat function is equal to st_mode in this structure. And st_mode has the following bits:

#define S_IFMT   0170000 /* type of file mask                */
#define S_IFIFO  0010000 /* named pipe (fifo)                */
#define S_IFCHR  0020000 /* character special                */
#define S_IFDIR  0040000 /* directory                        */
#define S_IFBLK  0060000 /* block special                    */
#define S_IFREG  0100000 /* regular                          */
#define S_IFLNK  0120000 /* symbolic link                    */
#define S_IFSOCK 0140000 /* socket                           */
#define S_IFWHT  0160000 /* whiteout                         */
#define S_ISUID  0004000 /* set user id on execution         */
#define S_ISGID  0002000 /* set group id on execution        */
#define S_ISVTX  0001000 /* save swapped text even after use */
#define S_IRWXU  0000700 /* RWX mask for owner               */
#define S_IRUSR  0000400 /* read permission, owner           */
#define S_IWUSR  0000200 /* write permission, owner          */
#define S_IXUSR  0000100 /* execute/search permission, owner */
#define S_IRWXG  0000070 /* RWX mask for group               */
#define S_IRGRP  0000040 /* read permission, group           */
#define S_IWGRP  0000020 /* write permission, group          */
#define S_IXGRP  0000010 /* execute/search permission, group */
#define S_IRWXO  0000007 /* RWX mask for other               */
#define S_IROTH  0000004 /* read permission, other           */
#define S_IWOTH  0000002 /* write permission, other          */
#define S_IXOTH  0000001 /* execute/search permission, other */

Awesome, we have everything we need! We mask mode with S_IFMT to get the file data. Then we just have to check whether it is a named pipe S_IFIFO, a character special S_IFCHR etc. Concretly:

I encourage you to read the implementation of the Hoa\Console\Console::getMode method.

So yes, this is useful to enable styles on text but also to define the default verbosity level. For instance, if a program outputs the result of a computation with some explanations around, the highest verbosity level would output everything (the result and the explanations) while the lowest level would output only the result. Let's try with the toUpperCase.php program:

<?php

$verbose = Hoa\Console\Console::isDirect(STDOUT);
$string  = $argv[1];
$result  = (new Hoa\String\String($string))->toUpperCase();

if(true === $verbose) {
    echo $string, ' becomes ', $result, ' in upper case!', "\n";
} else {
    echo $result, "\n";
}

Then, let's execute this program:

$ php toUpperCase.php 'Hello world!'
Hello world! becomes HELLO WORLD! in upper case!

And now, let's execute this program with a pipe:

$ php toUpperCase.php 'Hello world!' | xargs -I@ echo @
HELLO WORLD!

Useful and very simple, isn't it?

Terminal capabilities

We can control the terminal with the inputs, like the keyboard, but we can also control the outputs. How? With the text itself. Actually, an output does not contain only the text but it includes control functions. It's like HTML: Around a text, you can have an element, specifying that the text is a link. It's exactly the same for terminals! To specify that a text must be in red, we must add a control function around it.

Hopefully, these control functions have been standardized in the ECMA-48 document: Control Functions for Coded Character Set. However, not all terminals implement all this standard, and for historical reasons, some terminals use slightly different control functions. Moreover, some information do not belong to this standard (because this is out of its scope), like: How many colours does the terminal support? or does the terminal support the meta key?

Consequently, each terminal has a list of capabilities. This list is splitted in 3 categories:

For instance:

All the capabilities can be found in the documentation of terminfo(5) or in the documentation of xcurses. I encourage you to follow these links and see how rich the terminal capabilities are!

Terminal information

Terminal capabilities are stored as information in databases. Where are these databases located? In files with a binary format. Favorite locations are:

Inside these directories, we have a tree of the form: _xx_/_name_, where _xx_ is the ASCII value in hexadecimal of the first letter of the terminal name _name_, or _n_/_name_ where _n_ is the first letter of the terminal name. The terminal name is stored in the TERM environment variable. For instance, on my computer:

$ echo $TERM
xterm-256color
$ file /usr/share/terminfo/78/xterm-256color
/usr/share/terminfo/78/xterm-256color: Compiled terminfo entry

We can use the Hoa\Console\Tput class to retrieve these information. The getTerminfo static method allows to get the path of the terminal information file. The getTerm static method allows to get the terminal name. Finally, the whole class allows to parse a terminal information database (it will use the file returned by getTerminfo by default). For instance:

<?php

$tput = new Hoa\Console\Tput();
var_dump($tput->count('max_colors'));

/**
 * Will output:
 *     int(256)
 */

On my computer, with xterm-256color, I have 256 colours, as expected. If we parse the information of xterm and not xterm-256color, we will have:

$tput = new Hoa\Console\Tput(Hoa\Console\Tput::getTerminfo('xterm'));
var_dump($tput->count('max_colors'));

/**
 * Will output:
 *     int(8)
 */

The power in your hand: Control the cursor

Let's summarize. We are able to parse and know all the terminal capabilities of a specific terminal (including the one of the current user). If we would like a powerful terminal API, we need to control the basis, like the cursor.

Remember. We said that the terminal is a canvas of columns and lines. The cursor is like a pen. We can move it and write something. We are going to (partly) see how the Hoa\Console\Cursor class works.

I like to move it!

The moveTo static method allows to move the cursor to an absolute position. For example:

<?php

Hoa\Console\Cursor::moveTo($x, $y);

The control function we use is cursor_address. So all we need to do is to use the Hoa\Console\Tput class and call the get method on it to get the value of this string capability. This is a parameterized one: On xterm-256color, its value is e[%i%p1%d;%p2%dH. We replace the parameters by $x and $y and we output the result. That's all! We are able to move the cursor on an absolute position on all terminals! This is the right way to do.

We use the same strategy for the move static method that moves the cursor relatively to its current position. For example:

<?php

Hoa\Console\Cursor::move('right up');

We split the steps and for each step we read the appropriated string capability using the Hoa\Console\Tput class. For right, we read the parm_right_cursor string capability, for up, we read parm_up_cursor etc. Note that parm_right_cursor is different of cursor_right: The first one is used to move the cursor a certain number of times while the second one is used to move the cursor only one time. With performances in mind, we should use the first one if we have to move the cursor several times.

The getPosition static method returns the position of the cursor. This way to interact is a little bit different. We must write a control function on the output, and then, the terminal replies on the input. See the implementation by yourself.

<?php

print_r(Hoa\Console\Cursor::getPosition());

/**
 * Will output:
 *     Array
 *     (
 *         [x] => 7
 *         [y] => 42
 *     )
 */

In the same way, we have the save and restore static methods that save the current position of the cursor and restore it. This is very useful. We use the save_cursor and restore_cursor string capabilities.

Also, the clear static method splits some parts to clear. For each part (direction or way), we read from Hoa\Console\Tput the appropriated string capabilities: clear_screen to clear all the screen, clr_eol to clear everything on the right of the cursor, clr_eos to clear everything bellow the cursor etc.

<?php

Hoa\Console\Cursor::clear('left');

See what we learnt in action:

<?php

echo 'Foobar', "\n",
     'Foobar', "\n",
     'Foobar', "\n",
     'Foobar', "\n",
     'Foobar', "\n";

           Hoa\Console\Cursor::save();
sleep(1);  Hoa\Console\Cursor::move('LEFT');
sleep(1);  Hoa\Console\Cursor::move('↑');
sleep(1);  Hoa\Console\Cursor::move('↑');
sleep(1);  Hoa\Console\Cursor::move('↑');
sleep(1);  Hoa\Console\Cursor::clear('↔');
sleep(1);  echo 'Hahaha!';
sleep(1);  Hoa\Console\Cursor::restore();

echo "\n", 'Bye!', "\n";

The result is presented in the following figure.

Moving a cursor in the terminal

Saving, moving, clearing and restoring the cursor with Hoa\Console.

The resulting API is portable, clean, simple to read and very easy to maintain! This is the right way to do.

To get more information, please read the documentation.

Colours and decorations

Now: Colours. This is mainly the reason why I decided to write this article. We see the same and the same libraries, again and again, doing only colours in the terminal, but unfortunately not in the right way 😞.

A terminal has a palette of colours. Each colour is indexed by an integer, from 0 to potentially +∞ . The size of the palette is described by the max_colors number capability. Usually, a palette contains 1, 2, 8, 256 or 16 million colours.

xterm-256color palette

The xterm-256color palette (source).

So first thing to do is to check whether we have more than 1 colour. If not, we must not colorize the given text. Next, if we have less than 256 colours, we have to convert the style into a palette containing 8 colours. Same with less than 16 million colours, we have to convert into 256 colours.

Moreover, we can define the style of the foreground or of the background with respectively the set_a_foreground and set_a_background string capabilities. Finally, in addition to colours, we can define other decorations like bold, underline, blink or even inverse the foreground and the background colours.

One thing to remember is: With this capability, we only define the style at a given “pixel” and it will apply on the following text. In this case, it is not exactly like HTML where we have a beginning and an end. Here we only have a beginning. Let's try!

<?php

Hoa\Console\Cursor::colorize('underlined foreground(yellow) background(#932e2e)');
echo 'foo';
Hoa\Console\Cursor::colorize('!underlined background(normal)');
echo 'bar', "\n";

The API is pretty simple: We start to underline the text, we set the foreground to yellow and we set the background to #932e2e  . Then we output something. We continue with cancelling the underline decoration in addition to resetting the background. Finally we output something else. Here is the result:

A styled text in the terminal

Fun with Hoa\Console\Cursor::colorize.

What do we observe? My terminal does not support more than 256 colours. Thus, #932e2e is automatically converted into the closest colour in my actual palette! This is the right way to do.

For fun, you can change the colours in the palette with the Hoa\Console\Cursor::changeColor static method. You can also change the style of the cursor, like ▋, _ or |.

To get more information, please read the documentation.

The power in your hand: Readline

A more complete usage of Hoa\Console\Cursor and even Hoa\Console\Window is the Hoa\Console\Readline class that is a powerful readline. More than autocompleters, history, key bindings etc., it has an advanced use of cursors. See this in action:

Play with autocompleters

An autocompletion menu, made with Hoa\Console\Cursor and Hoa\Console\Window.

We use Hoa\Console\Cursor to move the cursor or change the colours and Hoa\Console\Window to get the dimensions of the window, scroll some text in it etc. I encourage you to read the implementation.

To get more information, please read the documentation.

The power in your hand: Sound 🎵

Yes, even sound is defined by terminal capabilities. The famous bip is given by the bell string capability. You would like to make a bip? Easy:

<?php

$tput = new Hoa\Console\Tput();
echo $tput->get('bell');

That's it!

Bonus: Window

As a bonus, a quick demo of Hoa\Console\Window because it's fun.

The video shows the execution of the following code:

<?php

Hoa\Console\Window::setSize(80, 35);
var_dump(Hoa\Console\Window::getPosition());

foreach(
    [
        [100, 100], [150, 150], [200, 100], [200, 80],
        [200,  60], [200, 100]
    ]
    as list($x, $y)
) {
    sleep(1);  Hoa\Console\Window::moveTo($x, $y);
}

sleep(2);  Hoa\Console\Window::minimize();
sleep(2);  Hoa\Console\Window::restore();
sleep(2);  Hoa\Console\Window::lower();
sleep(2);  Hoa\Console\Window::raise();

We resize the window, we get its position, we move the window on the screen, we minimize and restore it, and finally we put it behind all other windows just before raising it.

To get more information, please read the documentation.

Conclusion

In this article, we saw how to control the terminal by: Firstly, detecting the type of pipes, and secondly, reading and using the terminal capabilities. We know where these capabilities are stored and we saw few of them in action.

This approach ensures your code will be portable, easy to maintain and easy to use. The portability is very important because, like browsers and user devices, we have a lot of terminal emulators released in the wild. We have to care about them.

I encourage you to take a look at the Hoa\Console library and to contribute to make it even more awesome 😄.