Colorful output of Terminal characters

Colorful output of Terminal characters

Recently, I have been improving the deployment script of the site. When debugging, there is some intermediate status information that I want to highlight, so I need to modify the output style of the Terminal characters. How to modify it? Look down.

Modify terminal character output style

Bash command	Output preview
echo "hello world"
echo -e "\e[31mHello World\e[0m"
echo -e "\e[31mHello\e[0m World"
echo -e "\e[31;47mHello\e[0m World"

As described in the table above, we change the font color (foreground color) and background color (backcolor) of the output characters. Usually, achieving this effect already meets my needs this time. How is it achieved?

Run help echo in the terminal, we can see the brief instructions for using the echo command, part of which is as follows:

$ help echo
echo: echo [-neE] [arg ...]
    Write arguments to the standard output.

    Display the ARGs, separated by a single space character and followed by a
    newline, on the standard output.

    Options:
      -e enable interpretation of the following backslash escapes
                Allow interpretation of trailing backslash escapes
      -E explicitly suppress interpretation of backslash escapes
                Disable interpretation of following backslash escapes

    `echo' interprets the following backslash-escaped characters:
      \e escape character
                <Esc>
    ...

    Exit Status:
    Returns success unless a write error occurs.

Now we understand, -e is ‘allow interpretation of the backslash escape’, and \e represents .

echo "hello world"
echo -e "\e[31mHello World\e[0m"
echo -e "\e[31mHello\e[0m World"
echo -e "\e[31;47mHello\e[0m World"

Let’s analyze this format again and exclude the directly output characters hello world, such as \e[31m, \e[0m, \e[31;47m This kind of characters are the ones that change the character output style!

So, what are they? CSI!

CSI

CSI (Control Sequence Introducer, control sequence adapter; control sequence guide code).

The CSI sequence consists of ESC [ (also written as \e[ or \033), several (including 0) “parameter bytes”, It consists of several “intermediate bytes” and a “final byte”.

It’s not that complicated, at least you don’t need to remember it. You just need to use \e[ to guide a CSI sequence.

All common sequences simply use the argument as a series of semicolon-separated numbers, such as 1;2;3. Missing numbers are treated as 0 (such as 1;;3, which is equivalent to the middle number being 0, ESC[m > In this case, if there is no parameter, it is equivalent to the parameter being 0).

CSI is a type of ANSI escape sequence, a standard for escape sequences for in-band signals used to control cursor position, color, and other options on video text terminals.

ANSI Escape Sequences – Embed certain byte sequences in the text, most starting with the Esc [ character. The terminal will interpret these byte sequences as corresponding instructions instead of ordinary character encodings.

In CSI, the parameter mode CSI n m (such as \e31m) is called SGR.

SGR

SGR (Select Graphic Rendition), by setting corresponding parameters, changes the character output style, such as font color, background coloring, bold, italics, etc.

Some of its commonly used parameters are as follows:

Code	Function	Remarks	Example
0	Reset/Normal	Turn off all properties.
1	Bold or increase intensity
2	Weakening (reducing strength)	Not widely supported.
3	Italic	Not widely supported. Sometimes viewed as reverse phase display.
4	Underline
5	Slow flashing	Less than 150 times per minute.
6	Flashing quickly	MS-DOS ANSI. SYS; more than 150 per minute; not widely supported .
7	Inverse display	The foreground color and background color are exchanged.
8	Hide	Not widely supported.
9	Strikeout	The characters are clear but marked for deletion. Not widely supported.
10	Main (default) font
21	Turn off bold or double underline	Turn off bold not widely supported; double underline has little support.
22	Normal color or intensity	Neither strong nor weak.
23	Non-italic, non-cusp
24	Turn off underlining	Remove single and double underlines.
25	Turn off flashing
27	Turn off reverse display
28	Turn off hiding
29	Turn off erasing
30–37	Set the foreground color	See the color table below .
38	Set the foreground color	The next parameter is 5;n Or 2;r;g;b, see below.
39	The default foreground color	is implementation-defined (per the standard).
40–47	Set the background color	See the color table below.
48	Set background color	The next parameter is 5;n Or 2;r;g;b, see below.
49	Default background color	Implementation-defined (per standard).

Observe carefully 1-9 and 21-29, they correspond one to one, and the latter is exactly the effect of turning off the former. Let’s focus on the foreground and background colors and see how they are distributed.

The initial specs only came in 8 colours, with only their names given. SGR parameters 30-37 select the foreground color, 40-47 select the background color.

Quite a few terminals implement “bold” (SGR code 1) as a brighter color instead of a different font, thus providing 8 additional foreground colors, but usually not for background colors, although sometimes Reverse display (SGR code 7) allows this. For example: to display black text on a white background use ESC[30;47m, to display red text use ESC[31m, to display bright red text use ESC[1 ;31m. To reset to default colors use ESC[39;49m (not supported by some terminals), to reset all properties use ESC[0m. Later terminals added the ability to directly specify “bright” colors using 90-97 and 100-107.

> 3/4 bit 8 colors - black, red, green, yellow, blue, magenta, cyan, white

As 256-color lookup tables became more common on graphics cards, corresponding escape sequences were added to select from the predefined 256 colors. Anyway, the principle is the same. like:

Bash command	Output preview
echo -e "\e[38;5;183mhello world\e[m"
echo -e "\e[48;5;125mhello world\e[m"

\e[38;5;183m

> 8-bit 256 color mode

If you don’t want to remember so many color numbers, but you happen to be familiar with RGB, then you can also use it (such as \e[38;2;R;G;Bm) to easily set the color. like:

Bash command	Output preview
echo -e "\e[38;2;255;1;1mhello world\e[m"
echo -e "\e[48;2;255;1;1mhello world\e[m"

At this point, you can basically easily achieve the character effects you want to output.

reference

• Introduction & ANSI Escape Codes – IntelliJ IDEA Guide
• ANSI escape code – Wikipedia
• ANSI escape sequences – Wikipedia, the free encyclopedia
• Detailed explanation of the color and format of output in shell bash terminal (super detailed)
• shell echo display color – Zhihu