I am trying to grep a Linux system (Deb) for flags in a capture the flag competition. The following does not return any results and we know there are hundreds for flag{ } instances of text in a directory.

grep -rnw 'Downloads/' -e  'flag{' | more

Why? How can we search for flag{? While below works fine.

grep -rnw 'Downloads/' -e  'flag' | more
  • escape the {. – DopeGhoti May 23 '18 at 21:48
  • 5
    Please define does not work. – Stéphane Chazelas May 23 '18 at 21:57
  • How do you escape the { ? – icebowl May 23 '18 at 22:18
  • 1
    It would be useful to see an example of something that you are expecting would match. – Kusalananda May 24 '18 at 7:00

With grep -w, you are asking grep to only return matches that are immediately preceded by or followed by a newline (at the very start or end of a line) or a non-word character (a word character is a letter, digit or underscore).

This means that the text this is flag{a} will not be matched by grep -w 'flag{', since the character following the { in the text is a word character.

Similarly, this is theflag{ would not be matched by grep -w 'flag{'.

You may dispose of the -w option and instead use an explicit zero-width word boundary pattern, for example at the beginning, so that you may match flag{a} but not theflag{. With GNU grep, this may be done using

grep -rn '\<flag{' Downloads


grep -rn '\bflag{' Downloads

The \< pattern matches a word boundary at the start of a word (and \> matches at the end), while \b matches at both start and end. \< and \> were originally inherited from the ex and vi editors and are more portable than \b.

GNU grep does not support [[:<:]] and [[:>:]] (which work the same way as \< and \>) to match at the beginning or end of words, but BSD grep does. BSD grep does not support \b.

  • This worked! grep -rn '\bflag{' Downloads\ Thank you! – icebowl May 24 '18 at 17:09

Grep with -e (denoting an expression will follow) uses Basic Regex by default. From man grep, under "Basic vs Extended Regular Expressions" (online manual):

Traditional egrep did not support the { meta-character, and some egrep implementations support \{ instead, so portable scripts should avoid { in grep -E patterns and should use [{] to match a literal {.

GNU grep -E attempts to support traditional usage by assuming that { is not special if it would be the start of an invalid interval specification. For example, the command grep -E '{1' searches for the two-character string {1 instead of reporting a syntax error in the regular expression. POSIX allows this behavior as an extension, but portable scripts should avoid it.

So the most portable option would be:

grep -rnw -e 'flag[{]' 'Downloads/'  | more
  • 1
    There's no grep implementation where grep 'foo{'without -E will be a problem. Having options after non-option arguments would be a problem in most though. So grep -rnw 'flag{' Downloads would be enough. – Stéphane Chazelas May 24 '18 at 8:21
echo 'flag{' | grep -F 'flag{'

echo 'flag{' | grep 'flag{'

echo 'flag{' | grep -E 'flag{'

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.