Nesting text decoration using VT100 escape sequences

Question

Is there a way to nest VT100 escape sequences? For example, if I want a bold faced word within red text as below:

is there a straightforward way to do it? If I do the following, the first \e[m intended to close the bold face would also close the red color.

"\e[31mfoo \e[01mbar\e[m baz\e[m"

Answer 1

No - they do not "nest". As a rule, video modes can be set or reset. Some resets affect multiple modes, e.g., \e[m which resets bold, underline, reverse, etc.

The DEC VT100 implemented a subset of an ANSI standard, while also providing extensions. The corresponding standard was made obsolete by ISO-6429, which is readily available as ECMA-48.

A close reading of the standard may demonstrate that some of it is ambiguous, i.e., can be read as alluding to different types of behavior. For instance, there is no enumeration of the possible video attributes in the SGR (set graphic rendition) section, and how they may be interrelated. Because of this ambiguity, there exist terminals using "VT100 escape sequences" which have noticeably different color behavior (see for example the ncurses FAQ My terminal shows some uncolored spaces).

Because the standard does not specify everything, we rely upon common practice and conventions to determine what a terminal emulator can be expected to do, and where one might develop a new feature which extends the standard. For instance, ECMA-48 does not mention the VT100 scrolling feature, which is widely used. Also, it does not actually say that SGR 0 resets color. While existing terminal emulators happen to do this, early on before the convention was common, it was possible to have terminal emulators which relied on different escape sequences to reset color. (The dynamic colors feature in xterm is an example of this).

As noted, it is possible to provide extensions. ECMA-48 outlines encoding for private use (implementation-dependent) control sequences. Someone could (no one has...) design a terminal which provides the type of feature suggested. If that did not fill a perceived need by many people, or if it were cumbersome to use, it would be ignored. Because this type of escape sequence differs from existing practice, there is no support for it in curses and similar libraries. That would make it cumbersome to use.

This does not say that it is impossible to provide such a feature. As an example, xterm since patch #251 has provided a feature (incorporated into GNU screen) which sidesteps the longstanding problem of retrieving window titles (viewed as a security problem) by adding a mode where titles are saved on a stack. GNU screen uses this feature where it exists to automatically restore the original window title on exit.

Answer 2

\e[m resets all graphical rendition options, including color. To clear the bold option only, you can use \e[22m, although not all terminals support this sequence.