Growing Pains

When making terminal applications, there are a surprisingly number of portability issues and edge cases. Although Blessed provides an abstraction for the full curses capability database, it is not sufficient to secure you from several considerations shared here.

8 and 16 colors

Where 8 and 16 colors are used, they should be assumed to be the CGA Color Palette. Though there is no terminal standard that proclaims that the CGA colors are used, their values are the best approximations across all common hardware terminals and terminal emulators.

A recent phenomenon of users is to customize their base 16 colors to provide (often, more “washed out”) color schemes. Furthermore, we are only recently getting LCD displays of colorspaces that achieve close approximation to the original video terminals. Some find these values uncomfortably intense: in their original CRT form, their contrast and brightness was lowered by hardware dials, whereas today’s LCD’s typically display well only near full intensity.

Though we may not detect the colorspace of the remote terminal, we can:

  • Trust that a close approximation of the CGA Color Palette for the base 16 colors will be displayed for most users.
  • Trust that users who have made the choice to adjust their palette have made the choice to do so, and are able to re-adjust such palettes as necessary to accommodate different programs (such as through the use of “Themes”).


It has become popular to use dynamic system-wide color palette adjustments in software such as f.lux, which adjust the system-wide “Color Profile” of the entire graphics display depending on the time of day. One might assume that"text") may be completely invisible to such users during the night!

Where is brown, purple, or grey?

There are only 8 color names on a 16-color terminal: The second set of eight colors are “high intensity” versions of the first in direct series.

The colors brown, purple, and grey are not named in the first series, though they are available:

  • brown: yellow is brown, only high-intensity yellow (bright_yellow) is yellow!

  • purple: magenta is purple. In earlier, 4-bit color spaces, there were only black, cyan, magenta, and white of low and high intensity, such as found on common home computers like the ZX Spectrum.

    Additional “colors” were only possible through dithering. The color names cyan and magenta on later graphics adapters are carried over from its predecessors. Although the color cyan remained true in RGB value on 16-color to its predecessor, magenta shifted farther towards blue from red becoming purple (as true red was introduced as one of the new base 8 colors).

  • grey: there are actually three shades of grey (or American spelling, ‘gray’), though the color attribute named ‘grey’ does not exist!

    In ascending order of intensity, the shades of grey are:

    • bold_black: in lieu of the uselessness of an “intense black”, this is color is instead mapped to “dark grey”.
    • white: white is actually mild compared to the true color ‘white’: this is more officially mapped to “common grey”, and is often the default foreground color.
    • bright_white: is pure white (#ffffff).


The default foreground and background should be assumed as white-on-black.

For quite some time, the families of terminals produced by DEC, IBM, and Tektronix dominated the computing world with the default color scheme of green-on-black and less commonly amber-on-black monochrome displays: The inverse was a non-default configuration. The IBM 3270 clients exclusively used green-on-black in both hardware and software emulators, and is likely a driving factor of the default white-on-black appearance of the first IBM Personal Computer.

The less common black-on-white “ink paper” style of emulators is a valid concern for those designing terminal interfaces. The color scheme of black-on-white directly conflicts with the intention of bold is bright, where term.bright_red('ATTENTION!') may become difficult to read, as it appears as pink on white!

History of ink-paper inspired black-on-white

Early home computers with color video adapters, such as the Commodore 64 provided white-on-blue as their basic video terminal configuration. One can only assume such appearances were provided to demonstrate their color capabilities over competitors (such as the Apple ][).

More common, X11’s xterm and the software HyperTerm bundle with MS Windows provided an “ink on paper” black-on-white appearance as their default configuration. Two popular emulators continue to supply black-on-white by default to this day: Xorg’s xterm and Apple’s


Windows no longer supplies a terminal emulator: the “command prompt” as we know it now uses the MSVCRT API routines to interact and does not make use of terminal sequences, even ignoring those sequences that MS-DOS family of systems previously interpreted through the ANSI.SYS driver, though it continues to default to white-on-black.

Bold is bright

Where Bold is used, it should be assumed to be *Bright*.

Due to the influence of early graphics adapters providing a set of 8 “low-intensity” and 8 “high intensity” versions of the first, the term “bold” for terminals sequences is synonymous with “high intensity” in almost all circumstances.

History of bold as “wide stroke”

In typography, the true translation of “bold” is that a font should be displayed with emphasis. In classical terms, this would be achieved by pen be re-writing over the same letters. On a teletype or printer, this was similarly achieved by writing a character, backspacing, then repeating the same character in a form called overstriking.

To bold a character, C, one would emit the sequence C^HC where ^H is backspace (0x08). To underline C, one would would emit C^H_.

Video terminals do not support overstriking. Though the mdoc format for manual pages continue to emit overstriking sequences for bold and underline, translators such as mandoc will instead emit an appropriate terminal sequence.

Many characters previously displayable by combining using overstriking of ASCII characters on teletypes, such as: ±, ≠, or ⩝ were delegated to a code page or lost entirely until the introduction of multibyte encodings.

Much like the “ink paper” introduction in windowing systems for terminal emulators, “wide stroke” bold was introduced only much later when combined with operating systems that provided font routines such as TrueType.

Enforcing white-on-black

In conclusion, white-on-black should be considered the default. If there is a need to enforce white-on-black for terminal clients suspected to be defaulted as black-on-white, one would want to trust that a combination of term.home + term.white_on_black + term.clear should repaint the entire emulator’s window with the desired effect.

However, this cannot be trusted to all terminal emulators to perform correctly! Depending on your audience, you may instead ensure that the entire screen (including whitespace) is painted using the on_black mnemonic.

Beware of customized color schemes

A recent phenomenon is for users to customize these first 16 colors of their preferred emulator to colors of their own liking. Though this has always been possible with ~/.XResources, the introduction of PuTTy and iTerm2 to interactively adjustment these colors have made this much more common.

This may cause your audience to see your intended interface in a wildly different form. Your intended presentation may appear mildly unreadable.

Users are certainly free to customize their colors however they like, but it should be known that displaying term.black_on_red("DANGER!") may appear as “grey on pastel red” to your audience, reducing the intended effect of intensity.

256 colors can avoid customization

The first instinct of a user who aliases ls(1) to ls -G or colorls, when faced with the particularly low intensity of the default blue attribute is to adjust their terminal emulator’s color scheme of the base 16 colors.

This is not necessary: the environment variable LSCOLORS may be redefined to map an alternative color for blue, or to use bright_blue in its place.

Furthermore, all common terminal text editors such as emacs or vim may be configured with “colorschemes” to make use of the 256-color support found in most modern emulators. Many readable shades of blue are available, and many programs that emit such colors can be configured to emit a higher or lower intensity variant from the full 256 color space through program configuration.

Monochrome and reverse

Note that reverse takes the current foreground and background colors and reverses them. In contrast, the compound formatter black_on_red would fail to set the background or foreground color on a monochrome display, resulting in the same stylization as normal – it would not appear any different!

If your userbase consists of monochrome terminals, you may wish to provide “lightbars” and other such effects using the compound formatter red_reverse. In the literal sense of “set foreground color to red, then swap foreground and background”, this produces a similar effect on both color and monochrome displays.

For text, very few {color}_on_{color} formatters are visible with the base 16 colors, so you should generally wish for black_on_{color} anyway. By using {color}_reverse you may be portable with monochrome displays as well.

Multibyte Encodings and Code pages

A terminal that supports both multibyte encodings (UTF-8) and legacy 8-bit code pages (ISO 2022) may instruct the terminal to switch between both modes using the following sequences:

  • \x1b%G activates UTF-8 with an unspecified implementation level from ISO 2022 in a way that allows to go back to ISO 2022 again.
  • \x1b%@ goes back from UTF-8 to ISO 2022 in case UTF-8 had been entered via \x1b%G.
  • \x1b%/G switches to UTF-8 Level 1 with no return.
  • \x1b%/H switches to UTF-8 Level 2 with no return.
  • \x1b%/I switches to UTF-8 Level 3 with no return.

When a terminal is in ISO 2022 mode, you may use a sequence to request a terminal to change its code page. It begins by \x1b(, followed by an ASCII character representing a code page selection. For example \x1b(U on the legacy VGA Linux console switches to the IBM CP437 code page, allowing North American MS-DOS artwork to be displayed in its natural 8-bit byte encoding. A list of standard codes and the expected code page may be found on Thomas E. Dickey’s xterm control sequences section on sequences following the Control-Sequence-Inducer.

For more information, see What are the issues related to UTF-8 terminal emulators? by Markus Kuhn of the University of Cambridge.

One can be assured that the connecting client is capable of representing UTF-8 and other multibyte character encodings by the Environment variable LANG. If this is not possible or reliable, there is an intrusive detection method demonstrated in the example program

Alt or meta sends Escape

Programs using GNU readline such as bash continue to provide default mappings such as ALT+u to uppercase the word after cursor. This is achieved by the configuration option altSendsEscape or metaSendsEscape

The default for most terminals, however, is that the meta key is bound by the operating system (such as META + F for find), and that ALT is used for inserting international keys (where the combination ALT+u, a is used to insert the character ä).

It is therefore a recommendation to avoid alt or meta keys entirely in applications, and instead prefer the ctrl-key combinations, so as to avoid instructing your users to configure their terminal emulators to communicate such sequences.

If you wish to allow them optionally (such as through readline), the ability to detect alt or meta key combinations is achieved by prefacing the combining character with escape, so that ALT+z becomes Escape + z (or, in raw form \x1bz). Blessings currently provides no further assistance in detecting these key combinations.

Backspace sends delete

Typically, backspace is ^H (8, or 0x08) and delete is ^? (127, or 0x7f).

On some systems however, the key for backspace is actually labeled and transmitted as “delete”, though its function in the operating system behaves just as backspace.

It is highly recommend to accept both KEY_DELETE and KEY_BACKSPACE as having the same meaning except when implementing full screen editors, and provide a choice to enable the delete mode by configuration.

The misnomer of ANSI

When people say ‘ANSI Sequence’, they are discussing:

  • Standard ECMA-48: Control Functions for Coded Character Sets
  • ANSI X3.64 from 1981, when the American National Standards Institute adopted the ECMA-48 as standard, which was later withdrawn in 1997 (so in this sense it is not an ANSI standard).
  • The ANSI.SYS driver provided in MS-DOS and clones. The popularity of the IBM Personal Computer and MS-DOS of the era, and its ability to display colored text further populated the idea that such text “is ANSI”.
  • The various code pages used in MS-DOS Personal Computers, providing “block art” characters in the 8th bit (int 127-255), paired with ECMA-48 sequences supported by the MS-DOS ANSI.SYS driver to create artwork, known as ANSI art.
  • The ANSI terminal database entry and its many descendants in the terminfo database. This is mostly due to terminals compatible with SCO UNIX, which was the successor of Microsoft’s Xenix, which brought some semblance of the Microsoft DOS ANSI.SYS driver capabilities.
  • Select Graphics Rendition (SGR) on vt100 clones, which include many of the common sequences in ECMA-48.
  • Any sequence started by the Control-Sequence-Inducer is often mistakenly termed as an “ANSI Escape Sequence” though not appearing in ECMA-48 or interpreted by the ANSI.SYS driver. The adjoining phrase “Escape Sequence” is so termed because it follows the ASCII character for the escape key (ESC, \x1b).