Saturday, October 29, 2022

[SOLVED] grep and tail -f for a UTF-16 binary file - trying to use simple awk

Issue

How can I achieve the equivalent of:

tail -f file.txt | grep 'regexp'

to only output the buffered lines that match a regular expression such as 'Result' from the file type:

$ file file.txt
file.txt:Little-endian UTF-16 Unicode text, with CRLF line terminators

Example of the tail -f stream content below converted to utf-8:

Package end.

Total warnings: 40
Total errors: 0
Elapsed time: 24.4267192 secs.
...Package Executed.

Result: Success

Awk?

The problems in piping to grep led me to awk as a on-stop-shop solution for stripping the offending characters and also producing matched lines from regex.

awk seems to be giving the most promising results, however, I am finding that it returns the whole stream rather than individual matching lines:

tail -f file.txt | awk '{sub("/[^\x20-\x7F]/", "");/Result/;print}'
Package end.

Total warnings: 40
Total errors: 0
Elapsed time: 24.4267192 secs.
...Package Executed.

Result: Success

What I have tried

  • converting the stream and piping to grep

    tail -f file.txt | iconv -t UTF-8 | grep 'regexp'

  • using luit to change terminal encoding as per this post 

    luit -encoding UTF-8 -- tail -f file.txt | grep 'regexp'

  • delete non ASCII characters, described here, then piping to grep

    tail -f file.txt | tr -d '[^\x20-\x7F]' | grep 'regexp'
tail -f file.txt | sed 's/[^\x00-\x7F]//' | grep 'regexp'

  • various combinations of the above using grep flags --line-buffered, -a as well as sed -u

  • using luit -encoding UTF-8 -- pre-pended to the above
  • using a file with the same encoding containing the regular expression for grep -f

Why they failed

  • Most attempts, simply nothing is printed to the screen because grep searches 'regexp' when in fact the text is something like '\x00r\x00e\x00g\x00e\x00x\x00p' - for example 'R' will return the line 'Result: Success' but 'Result' won't
  • If a full regular expression gets a match, such as in the case of using grep -f, it will return the whole stream and doesn't seem to just return the matched lines
  • piping through sed or tr or iconv seems to break the pipe to grep and grep seems to still only be able to match individual characters

Edit

I looked at the raw file in it's utf-16 format using xxd with an aim of using regex to match the encoding, which gave the following output:

$ tail file.txt | xxd
00000000: 0050 0061 0063 006b 0061 0067 0065 0020  .P.a.c.k.a.g.e.
00000010: 0065 006e 0064 002e 000d 000a 000d 000a  .e.n.d..........
00000020: 0054 006f 0074 0061 006c 0020 0077 0061  .T.o.t.a.l. .w.a
00000030: 0072 006e 0069 006e 0067 0073 003a 0020  .r.n.i.n.g.s.:.
00000040: 0034 0030 000d 000a 0054 006f 0074 0061  .4.0.....T.o.t.a
00000050: 006c 0020 0065 0072 0072 006f 0072 0073  .l. .e.r.r.o.r.s
00000060: 003a 0020 0030 000d 000a 0045 006c 0061  .:. .0.....E.l.a
00000070: 0070 0073 0065 0064 0020 0074 0069 006d  .p.s.e.d. .t.i.m
00000080: 0065 003a 0020 0032 0034 002e 0034 0032  .e.:. .2.4...4.2
00000090: 0036 0037 0031 0039 0032 0020 0073 0065  .6.7.1.9.2. .s.e
000000a0: 0063 0073 002e 000d 000a 002e 002e 002e  .c.s............
000000b0: 0050 0061 0063 006b 0061 0067 0065 0020  .P.a.c.k.a.g.e.
000000c0: 0045 0078 0065 0063 0075 0074 0065 0064  .E.x.e.c.u.t.e.d
000000d0: 002e 000d 000a 000d 000a 0052 0065 0073  ...........R.e.s
000000e0: 0075 006c 0074 003a 0020 0053 0075 0063  .u.l.t.:. .S.u.c
000000f0: 0063 0065 0073 0073 000d 000a 000d 000a  .c.e.s.s........
00000100: 00

Solution

I realised a simple regex to ignore any characters between letters in the search string might work...

This matches 'Result' whilst allowing any one character between each letter...

$ tail -f file.txt | grep -a 'R.e.s.u.l.t'
Result: Success

$ tail -f file.txt | awk '/R.e.s.u.l.t./'
Result: Success

or as per this answer: to avoid typing all the tedious dots...

search="Result"
tail -f file.txt | grep -a -e "$(echo "$search" | sed 's/./&./g')"


Answered By - Alexander McFarlane
Answer Checked By - Katrina (WPSolving Volunteer)
at