为什么 grepping 相同的联机帮助页有时会导致错误？

Question

完全相同的命令：

man curl | grep -Pzo 'EXIT CODES(.|\n)*AUTHORS' | grep '  6  '

有时会给出预期的输出：

       6      Couldn't resolve host. The given remote host was not resolved.

有时会报错：

Binary file (standard input) matches

例如：

$ man curl | grep -Pzo 'EXIT CODES(.|\n)*AUTHORS' | grep '  6  '
       6      Couldn't resolve host. The given remote host was not resolved.

$ man curl | grep -Pzo 'EXIT CODES(.|\n)*AUTHORS' | grep '  6  '
Binary file (standard input) matches

$ man curl | grep -Pzo 'EXIT CODES(.|\n)*AUTHORS' | grep '  6  '
Binary file (standard input) matches

$ man curl | grep -Pzo 'EXIT CODES(.|\n)*AUTHORS' | grep '  6  '
       6      Couldn't resolve host. The given remote host was not resolved.

相关包的版本：

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 16.04.1 LTS
Release:        16.04
Codename:       xenial

$ grep --version
grep (GNU grep) 2.25
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Mike Haertel and others, see <http://git.sv.gnu.org/cgit/grep.git/tree/AUTHORS>.

$ man --version
man 2.7.5

$ curl --version
curl 7.47.0 (x86_64-pc-linux-gnu) libcurl/7.47.0 GnuTLS/3.4.10 zlib/1.2.8 libidn/1.32 librtmp/2.3
Protocols: dict file ftp ftps gopher http https imap imaps ldap ldaps pop3 pop3s rtmp rtsp smb smbs smtp smtps telnet tftp
Features: AsynchDNS IDN IPv6 Largefile GSS-API Kerberos SPNEGO NTLM NTLM_WB SSL libz TLS-SRP UnixSockets

我真的为这个而摸不着头脑。

我通过将-a 标志放入我的 grep 中解决了我的问题：man curl | grep -Pzoa 'EXIT CODES(.|\n)*AUTHORS' | grep -a ' 6 '

但我真的很困惑为什么有时它只会出错？...

Answer 1

因为使用了-z 选项，所以第一个 grep 将一个 NUL 字符附加到输出的末尾。接下来会发生什么取决于缓冲的变幻莫测。如果第二个 grep 在分析文件之前看到该 NUL，则它确定该文件是二进制文件。如果没有，它会找到您想要的匹配项。

所以，这恰好对我有用：

$ man curl | grep -Pzo 'EXIT CODES(.|\n)*AUTHORS' | grep '  6  '
       6      Couldn't resolve host. The given remote host was not resolved.

但是，如果我将第一个 grep 的输出放在一个临时文件中并要求第二个 grep 读取它，那么第二个 grep 总是会抱怨输入是二进制的：

$ man curl | grep -Pzo 'EXIT CODES(.|\n)*AUTHORS' >tmpfile;  grep '  6  ' tmpfile
Binary file tmpfile matches

替代方案：使用 awk

避免 NUL 字符问题以及减少所需进程数量的一种方法是使用 awk：

$ man curl | awk '/EXIT CODES/,/AUTHORS/{if (/   6   /) print}'
       6      Couldn't resolve host. The given remote host was not resolved.

替代方案：使用 sed

$ man curl | sed -n '/EXIT CODES/,/AUTHORS/{/   6   /p}'
       6      Couldn't resolve host. The given remote host was not resolved.

替代方案：使用 greps 和 tr

正如tripleee 建议的那样，另一种选择是使用tr 将NUL 替换为换行符：

$ man curl | grep -Pzo 'EXIT CODES(.|\n)*AUTHORS' | tr '\000' '\n' | grep '  6  '
       6      Couldn't resolve host. The given remote host was not resolved.