1 files changed, 34 insertions, 0 deletions
diff --git a/unit-tests/cmdline-redirect-stdin.mk b/unit-tests/cmdline-redirect-stdin.mk
new file mode 100644
index 000000000000..994016b5ec87
--- /dev/null
+++ b/unit-tests/cmdline-redirect-stdin.mk
@@ -0,0 +1,34 @@
+# $NetBSD: cmdline-redirect-stdin.mk,v 1.1 2021/02/01 20:31:41 rillig Exp $
+#
+# Demonstrate that the '!=' assignment operator can read individual lines
+# from make's stdin.
+#
+# This edge case is an implementation detail that has no practical
+# application.
+
+all: .PHONY
+	@printf '%s\n' "first line" "second line" \
+	| ${MAKE} -f ${MAKEFILE} read-lines
+
+.if make(read-lines)
+line1!=		read line; echo "$$line"
+line2!=		read line; echo "$$line"
+
+.if ${line1} != "first line"
+.  error line1="${line1}"
+
+.elif ${line2} == ""
+# If this branch is ever reached, the shell from the assignment to line1
+# probably buffers its input.  Most shells use unbuffered stdin, and this
+# is actually specified by POSIX, which says that "The read utility shall
+# read a single line from standard input".  This is the reason why the shell
+# reads its input byte by byte, which makes it terribly slow for practical
+# applications.
+.  error The shell's read command does not read a single line.
+
+.elif ${line2} != "second line"
+.  error line2="${line2}"
+.endif
+
+read-lines: .PHONY
+.endif