grep should find a hex value in a file but doesn't - Unix & Linux Stack Exchange

相关文章推荐

道上混的领结 · 从多个文件上的grep搜索中获取最后一行开发者社区· 3 周前 ·

逆袭的甘蔗 · visjs入门--模块groups__尊哥的 ...· 2 年前 ·

强健的大白菜 · python ...· 2 年前 ·

狂野的卤蛋 · Android报错：Fatal ...· 2 年前 ·

爱旅游的牙膏 · Python为Jupyter ...· 3 年前 ·

Stack Exchange network consists of 183 Q&A communities including Stack Overflow , the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. Visit Stack Exchange

Unix & Linux Stack Exchange is a question and answer site for users of Linux, FreeBSD and other Un*x-like operating systems. It only takes a minute to sign up.

Teams

Q&A for work

Connect and share knowledge within a single location that is structured and easy to search.

Learn more about Teams

Here is the beginning of a file:

# hexdump -n 550 myFile
0000000 f0f2 f5f0 f7f9 f1f1 f1f0 f0f0 e3f1 f3c8
0000010 f3f5 0000 0000 000c 0000 0000 0000 000c
0000020 0000 0c00 0000 0000 0000 0c00 0000 0000
0000030 000c 0000 0000 0000 000c 0000 0c00 0000
0000040 0000 0000 0c00 0000 0000 000c 0000 0000
0000050 0000 000c 0000 0c00 0000 0000 0000 0c00
0000060 0000 0000 000c 0000 0000 0000 000c 0000
00000b0 0000 0000 000c 0000 0000 0000 0000 0000
00000c0 0000 0000 0000 0c00 0000 0000 0000 0c00
00000d0 0000 0000 000c 0000 0000 0000 000c 0000
00000e0 0c00 0000 0000 0000 0c00 0000 0000 000c
00000f0 0000 0000 0000 000c 0000 0c00 0000 0000
0000100 0000 0c00 0000 0000 000c 0000 0000 0000
0000110 000c 0000 0c00 0000 0000 0000 0c00 0000
0000120 0000 0000 0c00 0000 0000 0000 0c00 0000
0000160 0000 0000 0c00 0000 0000 0000 0000 0000
0000170 0000 0000 0000 0000 000c 0000 0000 0000
0000180 000c 0000 0c00 0000 0000 0000 0c00 0000
0000190 0000 000c 0000 0000 0000 000c 0000 0c00
00001a0 0000 0000 0000 0c00 0000 0000 000c 0000
00001b0 0000 0000 000c 0000 0c00 0000 0000 0000
00001c0 0c00 0000 0000 000c 0000 0000 0000 000c
00001d0 0000 0000 0000 000c 0000 0000 0000 000c
0000210 0000 0000 0000 000c 0000 0000 0000 0000
0000220 0000 0000 0a00
0000226
in which we can see the hex values 0c and 0a
I don't understand why grep finds 0c but not 0a:
# grep -P '\x0c' myFile
Fichier binaire myFile correspondant
# grep -P '\x0a' myFile
<nothing in the output>
I am using CentOS.
                What grep is this? Are you on Linux? Is this GNU grep? Can you give us an example of the file itself so we can try to reproduce it?
– terdon
                Aug 18, 2021 at 15:48
                I suspect you need -zP otherwise the newlines are stripped out before the pattern matching takes place
– steeldriver
                Aug 18, 2021 at 15:51
                @steeldriver I think you're absolutely right, you may as well post an answer. Test with grep -zP '\x0a' /bin/ln vs grep -P '\x0a' /bin/ln.
– terdon
                Aug 18, 2021 at 15:53
                I'm on CentOS. The file is binary. You can see its 550 first characters in the dump. The same problem remains with a file containing only its 16 last bytes (hexdump: 000c 0000 0000 0000 0000 0000 0000 0a00).
– mcoulont
                Aug 18, 2021 at 15:54
\x0a isn't just any hex value - it's the hex value corresponding to the ASCII linefeed character.
Since grep is (by default) line-based, the linefeed characters are stripped out before pattern matching takes place. At least with GNU grep, you can change this behavior with the -z option:
   -z, --null-data
          Treat  input  and  output  data  as  sequences  of  lines,  each
          terminated by a zero byte (the ASCII NUL character) instead of a
          newline. 
however note that this will strip out ASCII nulls, so that you will no longer be able to grep for those.
                I suppose you can still at least count the NULLs with something like grep -zP '.' file | wc -l.
– terdon
                Aug 18, 2021 at 16:18

Stack Exchange Network