tests: install: check the behavior when standard input is the source

* tests/install/stdin.sh: New file.
* tests/local.mk: Add the new test.

tests/install/stdin.sh

@@ -0,0 +1,58 @@
+#!/bin/sh
+# Test 'install' when the source is standard input.
+
+# Copyright (C) 2026 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ ginstall
+
+# work around a dash bug when redirecting
+# from symlinked ttys in the background
+tty=$(readlink -f /dev/stdin)
+
+test -r "$tty" 2>&1 \
+  || skip_ '/dev/stdin is not readable'
+
+echo a >a || framework_failure_
+echo b >b || framework_failure_
+
+# Test the behavior when the source is a pipe and the destination does
+# not exist.
+cat a | ginstall /dev/stdin file1 >out 2>err || fail=1
+compare a file1 || fail=1
+compare /dev/null out || fail=1
+compare /dev/null err || fail=1
+
+# Test the behavior when the source is a file and the destination does
+# not exist.
+ginstall /dev/stdin file2 <b >out 2>err || fail=1
+compare b file2 || fail=1
+compare /dev/null out || fail=1
+compare /dev/null err || fail=1
+
+# Test the behavior when the source is a file and the destination exists.
+ginstall /dev/stdin file1 <file2 >out 2>err || fail=1
+compare b file1 || fail=1
+compare /dev/null out || fail=1
+compare /dev/null err || fail=1
+
+# Test the behavior when the source is a pipe and the destination exists.
+cat b | ginstall /dev/stdin file1 >out 2>err || fail=1
+compare b file1 || fail=1
+compare /dev/null out || fail=1
+compare /dev/null err || fail=1
+
+Exit $fail

tests/local.mk

@@ -679,6 +679,7 @@ all_tests =					\
   tests/install/install-C.sh			\
   tests/install/install-C-selinux.sh		\
   tests/install/install-Z-selinux.sh		\
+  tests/install/stdin.sh			\
   tests/install/strip-program.sh		\
   tests/install/trap.sh				\
   tests/ln/backup-1.sh				\