implement CLICOLOR_FORCE environment variable

src/libcmd/markdown.cc

@@ -64,7 +64,7 @@ static std::string doRenderMarkdownToTerminal(std::string_view markdown)
     if (!rndr_res)
         throw Error("allocation error while rendering Markdown");
 
-    return filterANSIEscapes(std::string(buf->data, buf->size), !isTTY());
+    return filterANSIEscapes(std::string(buf->data, buf->size), !shouldANSI());
 }
 
 std::string renderMarkdownToTerminal(std::string_view markdown)

src/libmain/progress-bar.cc

@@ -557,7 +557,7 @@ class ProgressBar : public Logger
 
 Logger * makeProgressBar()
 {
-    return new ProgressBar(isTTY());
+    return new ProgressBar(shouldANSI());
 }
 
 void startProgressBar()

src/libmain/shared.cc

@@ -6,6 +6,8 @@
 #include "loggers.hh"
 #include "progress-bar.hh"
 #include "signals.hh"
+#include "terminal.hh"
+
 
 #include <algorithm>
 #include <exception>
@@ -370,7 +372,7 @@ int handleExceptions(const std::string & programName, std::function<void()> fun)
 
 RunPager::RunPager()
 {
-    if (!isatty(STDOUT_FILENO)) return;
+    if (!isOutputARealTerminal(StandardOutputStream::Stdout)) return;
     char * pager = getenv("NIX_PAGER");
     if (!pager) pager = getenv("PAGER");
     if (pager && ((std::string) pager == "" || (std::string) pager == "cat")) return;

src/libutil/logging.cc

@@ -54,7 +54,7 @@ class SimpleLogger : public Logger
         : printBuildLogs(printBuildLogs)
     {
         systemd = getEnv("IN_SYSTEMD") == "1";
-        tty = isTTY();
+        tty = shouldANSI();
     }
 
     bool isVerbose() override {

src/libutil/terminal.cc

@@ -61,14 +61,28 @@ inline std::pair<int, size_t> charWidthUTF8Helper(std::string_view s)
 
 namespace nix {
 
-bool isTTY()
+bool isOutputARealTerminal(StandardOutputStream fileno)
 {
-    static const bool tty =
-        isatty(STDERR_FILENO)
-        && getEnv("TERM").value_or("dumb") != "dumb"
-        && !(getEnv("NO_COLOR").has_value() || getEnv("NOCOLOR").has_value());
+    return isatty(int(fileno)) && getEnv("TERM").value_or("dumb") != "dumb";
+}
 
-    return tty;
+bool shouldANSI(StandardOutputStream fileno)
+{
+    // Implements the behaviour described by https://bixense.com/clicolors/
+    // As well as https://force-color.org/ for compatibility, since it fits in the same shape.
+    // NO_COLOR CLICOLOR CLICOLOR_FORCE Colours?
+    // set      x        x              No
+    // unset    x        set            Yes
+    // unset    x        unset          If attached to a terminal
+    //                                  [we choose the "modern" approach of colour-by-default]
+    auto compute = [](StandardOutputStream fileno) -> bool {
+        bool mustNotColour = getEnv("NO_COLOR").has_value() || getEnv("NOCOLOR").has_value();
+        bool shouldForce = getEnv("CLICOLOR_FORCE").has_value() || getEnv("FORCE_COLOR").has_value();
+        bool isTerminal = isOutputARealTerminal(fileno);
+        return !mustNotColour && (shouldForce || isTerminal);
+    };
+    static bool cached[2] = {compute(StandardOutputStream::Stdout), compute(StandardOutputStream::Stderr)};
+    return cached[int(fileno) - 1];
 }
 
 std::string filterANSIEscapes(std::string_view s, bool filterAll, unsigned int width)

src/libutil/terminal.hh

@@ -5,11 +5,43 @@
 #include <string>
 
 namespace nix {
+
+enum class StandardOutputStream {
+    Stdout = 1,
+    Stderr = 2,
+};
+
+/**
+ * Determine whether the output is a real terminal (i.e. not dumb, not a pipe).
+ *
+ * This is probably not what you want, you may want shouldANSI() or something
+ * more specific. Think about how the output should work with a pager or
+ * entirely non-interactive scripting use.
+ *
+ * The user may be redirecting the Lix output to a pager, but have stderr
+ * connected to a terminal. Think about where you are outputting the text when
+ * deciding whether to use STDERR_FILENO or STDOUT_FILENO.
+ *
+ * \param fileno file descriptor number to check if it is a tty
+ */
+bool isOutputARealTerminal(StandardOutputStream fileno);
+
 /**
  * Determine whether ANSI escape sequences are appropriate for the
  * present output.
+ *
+ * This follows the rules described on https://bixense.com/clicolors/
+ * with CLICOLOR defaulted to enabled (and thus ignored).
+ *
+ * That is to say, the following procedure is followed in order:
+ * - NO_COLOR or NOCOLOR set             -> always disable colour
+ * - CLICOLOR_FORCE or FORCE_COLOR set   -> enable colour
+ * - The output is a tty; TERM != "dumb" -> enable colour
+ * - Otherwise                           -> disable colour
+ *
+ * \param fileno which file descriptor number to consider. Use the one you are outputting to
  */
-bool isTTY();
+bool shouldANSI(StandardOutputStream fileno = StandardOutputStream::Stderr);
 
 /**
  * Truncate a string to 'width' printable characters. If 'filterAll'

src/nix-env/nix-env.cc

@@ -1091,7 +1091,7 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
         return;
     }
 
-    bool tty = isTTY();
+    bool tty = shouldANSI();
     RunPager pager;
 
     Table table;

src/nix/main.cc

@@ -412,7 +412,7 @@ void mainWrapped(int argc, char * * argv)
     settings.verboseBuild = false;
 
     // If on a terminal, progress will be displayed via progress bars etc. (thus verbosity=notice)
-    if (nix::isTTY()) {
+    if (nix::isOutputARealTerminal(StandardOutputStream::Stderr)) {
         verbosity = lvlNotice;
     } else {
         verbosity = lvlInfo;

src/nix/prefetch.cc

@@ -191,7 +191,7 @@ static int main_nix_prefetch_url(int argc, char * * argv)
 
         Finally f([]() { stopProgressBar(); });
 
-        if (isTTY())
+        if (isOutputARealTerminal(StandardOutputStream::Stderr))
           startProgressBar();
 
         auto store = openStore();