Certain Unix and Windows terminals, and also certain R GUIs, e.g. RStudio, support styling terminal output using special control sequences (ANSI sequences).

num_ansi_colors() detects if the current R session supports ANSI sequences, and if it does how many colors are supported.

## Usage

num_ansi_colors(stream = "auto")

detect_tty_colors()

## Arguments

stream

The stream that will be used for output, an R connection object. It can also be a string, one of "auto", "message", "stdout", "stderr". "auto" will select stdout() if the session is interactive and there are no sinks, otherwise it will select stderr().

## Value

Integer, the number of ANSI colors the current R session supports for stream.

## Details

The detection mechanism is quite involved and it is designed to work out of the box on most systems. If it does not work on your system, please report a bug. Setting options and environment variables to turn on ANSI support is error prone, because they are inherited in other environments, e.g. knitr, that might not have ANSI support.

If you want to turn off ANSI colors, set the NO_COLOR environment variable to a non-empty value.

The exact detection mechanism is as follows:

1. If the cli.num_colors options is set, that is returned.

2. If the R_CLI_NUM_COLORS environment variable is set to a non-empty value, then it is used.

3. If the crayon.enabled option is set to FALSE, 1L is returned. (This is for compatibility with code that uses the crayon package.)

4. If the crayon.enabled option is set to TRUE and the crayon.colors option is not set, then the value of the cli.default_num_colors option, or if it is unset, then 8L is returned.

5. If the crayon.enabled option is set to TRUE and the crayon.colors option is also set, then the latter is returned. (This is for compatibility with code that uses the crayon package.)

6. If the NO_COLOR environment variable is set, then 1L is returned.

7. If we are in knitr, then 1L is returned, to turn off colors in .Rmd chunks.

8. If stream is "auto" (the default) and there is an active sink (either for "output" or "message"), then we return 1L. (In theory we would only need to check the stream that will be be actually used, but there is no easy way to tell that.)

9. If stream is not "auto", but it is stderr() and there is an active sink for it, then 1L is returned. (If a sink is active for "output", then R changes the stdout() stream, so this check is not needed.)

10. If R is running inside RGui on Windows, or R.app on macOS, then we return 1L.

11. If R is running inside RStudio, with color support, then the appropriate number of colors is returned, usually 256L.

12. If R is running on Windows, inside an Emacs version that is recent enough to support ANSI colors, then the value of the cli.default_num_colors option, or if unset 8L is returned. (On Windows, Emacs has isatty(stdout()) == FALSE, so we need to check for this here before dealing with terminals.)

13. If stream is not the standard output or standard error in a terminal, then 1L is returned.

14. Otherwise we use and cache the result of the terminal color detection (see below).

The terminal color detection algorithm:

1. If the COLORTERM environment variable is set to truecolor or 24bit, then we return 16 million colors.

2. If the COLORTERM environment variable is set to anything else, then we return the value of the cli.num_default_colors option, 8L if unset.

3. If R is running on Unix, inside an Emacs version that is recent enough to support ANSI colors, then the value of the cli.default_num_colors option is returned, or 8L if unset.

4. If we are on Windows in an RStudio terminal, then apparently we only have eight colors, but the cli.default_num_colors option can be used to override this.

5. If we are in a recent enough Windows 10 terminal, then there is either true color (from build 14931) or 256 color (from build 10586) support. You can also use the cli.default_num_colors option to override these.

6. If we are on Windows, under ConEmu or cmder, or ANSICON is loaded, then the value of cli.default_num_colors, or 8L if unset, is returned.

7. Otherwise if we are on Windows, return 1L.

8. Otherwise we are on Unix and try to run tput colors to determine the number of colors. If this succeeds, we return its return value. If the TERM environment variable is xterm and tput returned 8L, we return 256L, because xterm compatible terminals tend to support 256 colors (https://github.com/r-lib/crayon/issues/17) You can override this with the cli.default_num_colors option.

9. If TERM is set to dumb, we return 1L.

10. If TERM starts with screen, xterm, or vt100, we return 8L.

11. If TERM contains color, ansi, cygwin or linux, we return 8L.

12. Otherwise we return 1L.

Other ANSI styling: ansi-styles, combine_ansi_styles(), make_ansi_style()
num_ansi_colors()