October 18, 2012

Ansi Escape Sequences in Shell Scripts

Introduction
List of ANSI Codes
How to Use ANSI Codes in Shell Scripts

1. Introduction

Common terminal emulaters such as xterm, rxvt, PuTTY, and the Default Terminal emulators of the UNIX/Linux desktop environments all support text colour and shape manipulation based on escape sequences. This small article descripes how to make use of this capability in shell scripts (e.g. red or blinking text for warning messages or other colours for status messages).

2. List of ANSI Codes

This is a selective list of ANSI escape sequences and some more special characters useful for shell scripts:

 ::: Text Colours :::

 Black      ESC[30m
 Red        ESC[31m
 Green      ESC[32m
 Yellow     ESC[33m
 Blue       ESC[34m
 Magenta    ESC[35m
 Cyan       ESC[36m
 White      ESC[37m

 Default    ESC[39m


 ::: Background Colours :::

 Black      ESC[40m      
 Red        ESC[41m      
 Green      ESC[42m      
 Yellow     ESC[43m      
 Blue       ESC[44m      
 Magenta    ESC[45m      
 Cyan       ESC[46m      
 White      ESC[47m      

 Default    ESC[49m      


 ::: Text Properties :::

 Bold (preserve colours)............ ESC[1m
 Underlined text.................... ESC[4m
 Blinking text...................... ESC[5m
 Reset everything to default........ ESC[0m


 ::: Miscellaneous :::

 Window Title....................... ESC]0;${TITLE}BEL
 Terminal Bell...................... BEL
 Switch to graphics character set... ^N
 Switch to normal character set..... ^O


 ::: Examples :::

 Bold yellow text on red background. ESC[1;33;41m
 Bold underlined text............... ESC[1;4m


 _____________________________________________                                               
 Legend:
 - ESC = ^[ = \033
 - BEL = ^G = \007
 - ^N = \016
 - ^O = \017

Multiple escape sequences can be combined.

Example: We want to combine bold text shape, yellow text colour and red background.

bold text shape is ESC[1m

yellow text colour is ESC[33m

red background is ESC[41m

The result is ESC[1;33;41m - the escape sequence we use in our examples in the next paragraph.

3. How to Use ANSI Codes in Shell Scripts

Use with »printf«

The codes from the list above can be used with printf on all UNIX systems. To print the line "This is bold yellow text on red background" in bold yellow text on red background with printf type:

$ printf "\033[1;33;41m This is bold yellow text on red background \033[0m\n"

Of course you can use variables here as usual:

$ TEXT="This is bold yellow text on red background"
$ printf "\033[1;33;41m %s \033[0m\n" "$TEXT"

Use with »echo«

On many systems ANSI escape sequences can also be used with echo - depending on system and shell type:

$ echo -e "\033[1;33;41m This is bold yellow text on red background \033[0m\n"

Note the option "-e". Not all versions of echo support this options and would literally print "-e" instead.

Use with other commands

If you want to use ANSI codes with commands not natively supporting ANSI escape sequences it's entirely possible. Since all these sequences are interpreted by the terminal and not by the program you need to replace \033 by the ESCAPE character itself. If you use the vi editor type Ctrl-V ESC in editing mode and you get the ESCAPE character. You will see something like ^[. If you move with the cursor over it you will see it's only one character. If you need the BEL character (\007) you have to use Ctrl-G (in vi type: Ctrl-V Ctrl-G and you see something like ^G.

Examples:

$ BOLDTEXT="^[[1;33;41m This is bold yellow text on red background ^[[0m"
$ echo "$BOLDTEXT"
$
$ TITLE="^[]0; This is the new window title ^G"
$ echo "$TITLE"