[clang] [llvm] [lit] Make builtin cat work with stdin (PR #158447)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/158447

cat with no files passed to it is supposed to read from STDIN according
to POSIX. The builtin cat lacking this behavior led to the clang test in
dev-fd-fs.c to fail because it expected this behavior. This is a simple
modification and I do not think it is possible to rewrite the test
without this easily while preserving the semantics around named pipes.


>From 5bd8d4f925f3b5f82d85ef693861b6b1067d9f38 Mon Sep 17 00:00:00 2001
From: Aiden Grossman <aidengrossman at google.com>
Date: Sat, 13 Sep 2025 22:54:58 +0000
Subject: [PATCH] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20initia?=
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Created using spr 1.3.6
---
 clang/test/Misc/dev-fd-fs.c                    | 1 -
 llvm/utils/lit/lit/builtin_commands/cat.py     | 3 +++
 llvm/utils/lit/tests/Inputs/shtest-cat/cat.txt | 4 ++++
 3 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/clang/test/Misc/dev-fd-fs.c b/clang/test/Misc/dev-fd-fs.c
index ea94d950b0716..b989ab8a439cf 100644
--- a/clang/test/Misc/dev-fd-fs.c
+++ b/clang/test/Misc/dev-fd-fs.c
@@ -1,6 +1,5 @@
 // Check that we can operate on files from /dev/fd.
 // REQUIRES: dev-fd-fs
-// REQUIRES: shell
 
 // Check reading from named pipes. We cat the input here instead of redirecting
 // it to ensure that /dev/fd/0 is a named pipe, not just a redirected file.
diff --git a/llvm/utils/lit/lit/builtin_commands/cat.py b/llvm/utils/lit/lit/builtin_commands/cat.py
index ddab555662045..2797e0cbb4154 100644
--- a/llvm/utils/lit/lit/builtin_commands/cat.py
+++ b/llvm/utils/lit/lit/builtin_commands/cat.py
@@ -49,6 +49,9 @@ def main(argv):
             import os, msvcrt
 
             msvcrt.setmode(sys.stdout.fileno(), os.O_BINARY)
+    if len(filenames) == 0:
+        sys.stdout.write(sys.stdin.read())
+        sys.exit(0)
     for filename in filenames:
         try:
             contents = None
diff --git a/llvm/utils/lit/tests/Inputs/shtest-cat/cat.txt b/llvm/utils/lit/tests/Inputs/shtest-cat/cat.txt
index 4014b0fca1f24..c5b5d247c2f95 100644
--- a/llvm/utils/lit/tests/Inputs/shtest-cat/cat.txt
+++ b/llvm/utils/lit/tests/Inputs/shtest-cat/cat.txt
@@ -70,3 +70,7 @@
 # NP-CAT-OUTPUT-NEXT:M-HM-IM-JM-KM-LM-MM-NM-OM-PM-QM-RM-SM-TM-UM-VM-WM-XM-YM-ZM-[
 # NP-CAT-OUTPUT-NEXT:M-\M-]M-^M-_M-`M-aM-bM-cM-dM-eM-fM-gM-hM-iM-jM-kM-lM-mM-nM-o
 # NP-CAT-OUTPUT-NEXT:M-pM-qM-rM-sM-tM-uM-vM-wM-xM-yM-zM-{M-|M-}M-~M-^?
+
+## Test that cat will pipe stdin to stdout if no other files are specified.
+# RUN: echo test | cat | FileCheck --check-prefix=CAT-STDIN %s
+# CAT-STDIN: test