为什么我不能重定向 gcc -H 的输出

Question

当我执行时

gcc -H myfile.c 2>&1 > gcc.log

我在屏幕上得到以下输出：

. /usr/include/argp.h
.. /usr/include/stdio.h
... /usr/include/features.h
.... /usr/include/x86_64-linux-gnu/sys/cdefs.h
..... /usr/include/x86_64-linux-gnu/bits/wordsize.h
.... /usr/include/x86_64-linux-gnu/gnu/stubs.h
..... /usr/include/x86_64-linux-gnu/gnu/stubs-64.h
... /usr/lib/gcc/x86_64-linux-gnu/4.8/include/stddef.h
... /usr/include/x86_64-linux-gnu/bits/types.h
.... /usr/include/x86_64-linux-gnu/bits/wordsize.h
.... /usr/include/x86_64-linux-gnu/bits/typesizes.h
... /usr/include/libio.h
.... /usr/include/_G_config.h
..... /usr/lib/gcc/x86_64-linux-gnu/4.8/include/stddef.h
..... /usr/include/wchar.h
.... /usr/lib/gcc/x86_64-linux-gnu/4.8/include/stdarg.h
... /usr/include/x86_64-linux-gnu/bits/stdio_lim.h
... /usr/include/x86_64-linux-gnu/bits/sys_errlist.h
.. /usr/include/ctype.h
... /usr/include/endian.h
.... /usr/include/x86_64-linux-gnu/bits/endian.h
.... /usr/include/x86_64-linux-gnu/bits/byteswap.h
..... /usr/include/x86_64-linux-gnu/bits/wordsize.h
..... /usr/include/x86_64-linux-gnu/bits/byteswap-16.h
... /usr/include/xlocale.h
.. /usr/include/getopt.h
.. /usr/lib/gcc/x86_64-linux-gnu/4.8/include-fixed/limits.h
... /usr/lib/gcc/x86_64-linux-gnu/4.8/include-fixed/syslimits.h
.... /usr/lib/gcc/x86_64-linux-gnu/4.8/include-fixed/limits.h
..... /usr/include/limits.h
...... /usr/include/x86_64-linux-gnu/bits/posix1_lim.h
....... /usr/include/x86_64-linux-gnu/bits/local_lim.h
........ /usr/include/linux/limits.h
...... /usr/include/x86_64-linux-gnu/bits/posix2_lim.h
.. /usr/include/errno.h
... /usr/include/x86_64-linux-gnu/bits/errno.h
.... /usr/include/linux/errno.h
..... /usr/include/x86_64-linux-gnu/asm/errno.h
...... /usr/include/asm-generic/errno.h
....... /usr/include/asm-generic/errno-base.h
. /usr/include/argz.h
.. /usr/include/errno.h
.. /usr/include/string.h
... /usr/lib/gcc/x86_64-linux-gnu/4.8/include/stddef.h
. /usr/include/stdlib.h
.. /usr/lib/gcc/x86_64-linux-gnu/4.8/include/stddef.h
.. /usr/include/x86_64-linux-gnu/bits/waitflags.h
.. /usr/include/x86_64-linux-gnu/bits/waitstatus.h
.. /usr/include/x86_64-linux-gnu/sys/types.h
... /usr/include/time.h
... /usr/lib/gcc/x86_64-linux-gnu/4.8/include/stddef.h
... /usr/include/x86_64-linux-gnu/sys/select.h
.... /usr/include/x86_64-linux-gnu/bits/select.h
..... /usr/include/x86_64-linux-gnu/bits/wordsize.h
.... /usr/include/x86_64-linux-gnu/bits/sigset.h
.... /usr/include/time.h
.... /usr/include/x86_64-linux-gnu/bits/time.h
... /usr/include/x86_64-linux-gnu/sys/sysmacros.h
... /usr/include/x86_64-linux-gnu/bits/pthreadtypes.h
.... /usr/include/x86_64-linux-gnu/bits/wordsize.h
.. /usr/include/alloca.h
... /usr/lib/gcc/x86_64-linux-gnu/4.8/include/stddef.h
.. /usr/include/x86_64-linux-gnu/bits/stdlib-float.h
.. /usr/lib/gcc/x86_64-linux-gnu/4.8/include/stdbool.h
.. /usr/lib/gcc/x86_64-linux-gnu/4.8/include/stdint.h
... /usr/include/stdint.h
.... /usr/include/x86_64-linux-gnu/bits/wchar.h
.... /usr/include/x86_64-linux-gnu/bits/wordsize.h
Multiple include guards may be useful for:
/usr/include/limits.h
/usr/include/linux/errno.h
/usr/include/wchar.h
/usr/include/x86_64-linux-gnu/asm/errno.h
/usr/include/x86_64-linux-gnu/bits/byteswap-16.h
/usr/include/x86_64-linux-gnu/bits/byteswap.h
/usr/include/x86_64-linux-gnu/bits/endian.h
/usr/include/x86_64-linux-gnu/bits/errno.h
/usr/include/x86_64-linux-gnu/bits/local_lim.h
/usr/include/x86_64-linux-gnu/bits/select.h
/usr/include/x86_64-linux-gnu/bits/sigset.h
/usr/include/x86_64-linux-gnu/bits/stdio_lim.h
/usr/include/x86_64-linux-gnu/bits/stdlib-float.h
/usr/include/x86_64-linux-gnu/bits/sys_errlist.h
/usr/include/x86_64-linux-gnu/bits/time.h
/usr/include/x86_64-linux-gnu/bits/typesizes.h
/usr/include/x86_64-linux-gnu/bits/waitflags.h
/usr/include/x86_64-linux-gnu/bits/waitstatus.h
/usr/include/x86_64-linux-gnu/gnu/stubs-64.h
/usr/include/x86_64-linux-gnu/gnu/stubs.h
/usr/lib/gcc/x86_64-linux-gnu/4.8/include-fixed/syslimits.h

当我查看 gcc.log 时，它是空的。到底是怎么回事？为什么我无法捕获gcc -H 的输出？

消息Multiple include guards may be useful for: 是什么意思？

感谢任何帮助。

Answer 1

在 OSX（使用 clang）上，gcc -H 确实写入标准错误。但是，您的 shell 在重定向 stderr 之后重定向 stdin，因为它在处理 2>&1 之前处理 > gcc.log。

相反，您可以颠倒重定向的顺序：

$ gcc -H myfile.c > gcc.log 2>&1

这将首先执行stdin到gcc.log的重定向，然后将stderr重定向到stdin，现在是gcc.log。

或者，您可以同时明确表示：

$ gcc -H myfile.c > gcc.log 2> gcc.log

Answer 2

您以错误的顺序进行重定向，并且 AFAIK，您只需要重定向标准错误，因为标准输出上没有任何内容。

$ gcc -H myFile.c 2>gcc.log

在>gcc.log 之前执行2>&1 将首先将标准错误重定向到控制台（标准输出所在的位置），然后 将标准输出重定向到文件。