Enhance/fix `isatty` using `command test`.

  Presently, `isatty` only works on a handful of keywords. Here it
  is rewritten to be able to take any path, device or fd number as
  an argument, and eliminates errors printed to stdout.
  Per discussion in #1228, using `builtin test -c` within a pipe to
  test special file descriptors is not viable, so this implementation
  specifcially uses `command test`. Additionally, a note has been
  added to the documentation of `test` regarding this potential
  aberration from the expected output of the test utility under the
  'Standards' section.

1 files changed, 18 insertions, 19 deletions

diff --git a/share/functions/isatty.fish b/share/functions/isatty.fish
index d4f2c43e..f49b3aa4 100644
--- a/share/functions/isatty.fish
+++ b/share/functions/isatty.fish
@@ -1,28 +1,27 @@
+function isatty -d "Test if a file or file descriptor is a tty."
 
-function isatty -d "Tests if a file descriptor is a tty"
-	set -l fd 0
-	if count $argv >/dev/null
-		switch $argv[1]
+# Use `command test` because `builtin test` doesn't open the regular fd's.
 
-			case -h --h --he --hel --help
-				__fish_print_help isatty
-				return 0
+  switch "$argv"
 
-			case stdin
-				set fd 0
+    case '-h*' '--h*'
+      __fish_print_help isatty
 
-			case stdout
-				set fd 1
+    case ''
+      command test -c /dev/stdin
 
-			case stderr
-				set fd 2
+    case '*'
+      if test -e "$argv" # The eval here is needed for symlinks. Unsure why.
+        command test -c "$argv"; and eval tty 0>"$argv" >/dev/null
 
-			case '*'
-				set fd $argv[1]
+      else if test -e /dev/"$argv"
+         command test -c /dev/"$argv"; and tty 0>/dev/"$argv" >/dev/null
 
-		end
-	end
-
-	eval "tty 0>&$fd >/dev/null"
+      else if test -e /dev/fd/"$argv"
+         command test -c /dev/fd/"$argv"; and tty 0>/dev/fd/"$argv" >/dev/null
 
+      else
+         return 1
+    end
+  end
 end