I have lots of files I'm searching through to try and find the ones that contain a string of 8 characters - nothing else.

So far I've tried: -

grep -x '.\{8,8\}'

Which is showing me files that contain a string of exactly 8 characters, but also show files that contain lots of other stuff as well as the 8 characters. I want the files that contain a string of exactly 8 characters and nothing more.

Any help would be much appreciated. Thanks.

  • 6
    All strings that are 9 (or more) bytes long contain strings of exactly 8 characters. Did you mean lines that are exactly 8 characters long (plus the newline), or files that contain lines like that, or files/lines that contain white-space separated "words" of exactly 8 characters, or something completely different? – ilkkachu May 22 '18 at 10:33

You are looking for files that are 9 bytes long, eight characters and a newline. This assumes that you are looking for single-byte characters only.

find . -type f -size 9c -exec grep -l -E '^.{8}$' {} +

This finds all regular files in or below the current directory that are exactly 9 bytes long. To verify that they only contain a single line, we run grep over them and try to match a line with exactly eight characters. We let grep output the names of the matching files with its -l option.

  • This is fantastic - you're a wizard! – crabsticks May 22 '18 at 10:31
  • That assumes all characters in all the files are single-byte. – Stéphane Chazelas May 22 '18 at 10:40
  • @StéphaneChazelas Yes, it does. I will add a note about this. – Kusalananda May 22 '18 at 10:42
  • 1
    @twalberg No. We have to make sure there are no other newlines in the file than at the end, otherwise we'd find a file whose contents is red\ncat\n\n for example. – Kusalananda May 22 '18 at 16:06

With GNU awk to find regular files that contain only one line, containing exactly 8 characters (not counting the newline character if any):

find . -type f -size +7c -size -50c -exec gawk '
  BEGINFILE               {p = 0}
  FNR == 1 && length == 8 {p = 1}
  FNR == 2                {p = 0; nextfile}
  ENDFILE                 {if (p) print FILENAME}' {} +

With find, we restrict to files whose size is in between 8 and 49 bytes. 8 for a file with 8 one-byte characters and no newline, 49 for 8 6-byte characters (the maximum in UTF-8, you may want to adapt for other charsets) and a newline.

Or with zsh:

has_one_line_of_8_characters() {
  local c
  ! read -ru0 -k10 c && [[ $c =~ $'^[^\n]{8}\n?$' ]]
} < ${1-$REPLY}

printf '%s\n' **/*(.DL+7L-50+has_one_line_of_8_characters)
gawk '/^.{9}$/{print FILENAME}' RS='\0' *

I chose 9 chars, because the 8 characters and the newline character in the end of line = 9 characters in total. If you need strictly 8 characters with the newline included, you should use the 8 number in the pattern.


I have four files in the test folder:

$ tail -n +1 -- *
==> 11_chars <==

==> 5_chars <==

==> 7_chars <==

==> 9_chars <==


$ gawk '/^.{9}$/{print FILENAME}' RS='\0' *

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