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Masahiro Yamada 913ab9780f kbuild: use more portable 'command -v' for cc-cross-prefix 
To print the pathname that will be used by shell in the current
environment, 'command -v' is a standardized way. [1]

'which' is also often used in scripts, but it is less portable.

When I worked on commit bd55f96fa9 ("kbuild: refactor cc-cross-prefix
implementation"), I was eager to use 'command -v' but it did not work.
(The reason is explained below.)

I kept 'which' as before but got rid of '> /dev/null 2>&1' as I
thought it was no longer needed. Sorry, I was wrong.

It works well on my Ubuntu machine, but Alexey Brodkin reports noisy
warnings on CentOS7 when 'which' fails to find the given command in
the PATH environment.

  $ which foo
  which: no foo in (/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin)

Given that behavior of 'which' depends on system (and it may not be
installed by default), I want to try 'command -v' once again.

The specification [1] clearly describes the behavior of 'command -v'
when the given command is not found:

  Otherwise, no output shall be written and the exit status shall reflect
  that the name was not found.

However, we need a little magic to use 'command -v' from Make.

$(shell ...) passes the argument to a subshell for execution, and
returns the standard output of the command.

Here is a trick. GNU Make may optimize this by executing the command
directly instead of forking a subshell, if no shell special characters
are found in the command and omitting the subshell will not change the
behavior.

In this case, no shell special character is used. So, Make will try
to run it directly. However, 'command' is a shell-builtin command,
then Make would fail to find it in the PATH environment:

  $ make ARCH=m68k defconfig
  make: command: Command not found
  make: command: Command not found
  make: command: Command not found

In fact, Make has a table of shell-builtin commands because it must
ask the shell to execute them.

Until recently, 'command' was missing in the table.

This issue was fixed by the following commit:

| commit 1af314465e5dfe3e8baa839a32a72e83c04f26ef
| Author: Paul Smith <psmith@gnu.org>
| Date:   Sun Nov 12 18:10:28 2017 -0500
|
|     * job.c: Add "command" as a known shell built-in.
|
|     This is not a POSIX shell built-in but it's common in UNIX shells.
|     Reported by Nick Bowler <nbowler@draconx.ca>.

Because the latest release is GNU Make 4.2.1 in 2016, this commit is
not included in any released versions. (But some distributions may
have back-ported it.)

We need to trick Make to spawn a subshell. There are various ways to
do so:

 1) Use a shell special character '~' as dummy

    $(shell : ~; command -v $(c)gcc)

 2) Use a variable reference that always expands to the empty string
    (suggested by David Laight)

    $(shell command$${x:+} -v $(c)gcc)

 3) Use redirect

    $(shell command -v $(c)gcc 2>/dev/null)

I chose 3) to not confuse people. The stderr would not be polluted
anyway, but it will provide extra safety, and is easy to understand.

Tested on Make 3.81, 3.82, 4.0, 4.1, 4.2, 4.2.1

[1] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/command.html

Fixes: bd55f96fa9 ("kbuild: refactor cc-cross-prefix implementation")
Cc: linux-stable <stable@vger.kernel.org> # 5.1
Reported-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Alexey Brodkin <abrodkin@synopsys.com>
2019-06-08 00:38:47 +09:00
arch Merge branch 'x86-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-06-02 11:10:01 -07:00
block block: print offending values when cloned rq limits are exceeded 2019-05-31 15:12:34 -06:00
certs treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 36 2019-05-24 17:27:11 +02:00
crypto treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 174 2019-05-30 11:26:41 -07:00
Documentation mm, memcg: consider subtrees in memory.events 2019-06-01 15:51:31 -07:00
drivers Merge branch 'efi-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-06-02 11:06:13 -07:00
fs for-linus-20190601 2019-06-02 09:27:44 -07:00
include Merge branch 'akpm' (patches from Andrew) 2019-06-02 08:51:30 -07:00
init treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 167 2019-05-30 11:26:39 -07:00
ipc treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 52 2019-05-24 17:36:42 +02:00
kernel Merge branch 'perf-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-06-02 11:08:12 -07:00
lib SPDX fixes for 5.2-rc3, round 2 2019-06-02 10:22:38 -07:00
LICENSES LICENSES: Rename other to deprecated 2019-05-03 06:34:32 -06:00
mm mm, compaction: make sure we isolate a valid PFN 2019-06-01 15:51:32 -07:00
net SPDX update for 5.2-rc3, round 1 2019-05-31 08:34:32 -07:00
samples treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 206 2019-05-30 11:29:53 -07:00
scripts kbuild: use more portable 'command -v' for cc-cross-prefix 2019-06-08 00:38:47 +09:00
security Merge branch 'next-fixes-for-5.2-rc' of git://git.kernel.org/pub/scm/linux/kernel/git/zohar/linux-integrity 2019-05-31 11:08:44 -07:00
sound treewide: fix typos of SPDX-License-Identifier 2019-06-01 18:29:58 +02:00
tools Merge branch 'perf-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2019-06-02 11:08:12 -07:00
usr user/Makefile: Fix typo and capitalization in comment section 2018-12-11 00:18:03 +09:00
virt Fixes for PPC and s390. 2019-06-02 10:19:39 -07:00
.clang-format Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net 2019-04-17 11:26:25 -07:00
.cocciconfig
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: set git diff driver for C source code files 2016-10-07 18:46:30 -07:00
.gitignore .gitignore: exclude .get_maintainer.ignore and .gitattributes 2019-05-18 11:49:54 +09:00
.mailmap A reasonably busy cycle for docs, including: 2019-05-08 12:42:50 -07:00
COPYING COPYING: use the new text with points to the license files 2018-03-23 12:41:45 -06:00
CREDITS MAINTAINERS: Farewell Martin Schwidefsky 2019-05-31 10:14:11 +02:00
Kbuild Kbuild updates for v5.1 2019-03-10 17:48:21 -07:00
Kconfig kconfig: move the "Executable file formats" menu to fs/Kconfig.binfmt 2018-08-02 08:06:55 +09:00
MAINTAINERS Merge branch 'i2c/for-current' of git://git.kernel.org/pub/scm/linux/kernel/git/wsa/linux 2019-06-02 10:18:11 -07:00
Makefile kbuild: teach kselftest-merge to find nested config files 2019-06-05 02:33:10 +09:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.