I often use grep to find files having a certain entry like this:

grep -R 'MyClassName'

The good thing is that it returns the files, their contents and marks the found string in red. The bad thing is that I also have huge files where the entire text is written in one big single line. Now grep outputs too much when finding text within those big files. Is there a way to limit the output to for instance 5 words to the left and to the right? Or maybe limit the output to 30 letters to the left and to the right?

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    Pipe your results thru cut – Rinzwind Apr 18 '18 at 8:26
  • So, let's say the pattern you're looking for is at position 50, but you said you only want 30 letters.What do you want to do then ? Ignore that line or also include it into output but trim it ? What exactly do you want to limit - the search or the lines themselves ? – Sergiy Kolodyazhnyy Apr 18 '18 at 8:48
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    @Rinzwind I don't quite understand what you want to achieve with cut, as it only splits by delimiter or by count of characters. Though when I find a line with MyClassName it may be anywhere in the line and not always at the same position. Furthermore, there may be a variation of characters in the front and the back of it, which breaks the possibility to split by delimiter. – Socrates Apr 18 '18 at 9:16
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    @SergiyKolodyazhnyy When a positive line with MyClassName has been found, I want to get as a result the file name and the x characters to the left and to the right. x is any number I provide, for instance 30. The rest of the file contents shall be ignored. This is to get a context to the matching files and limit the overload. – Socrates Apr 18 '18 at 9:19
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    @Rinzwind What type of custom delimiter would you suggest with cut if there are three files with the following input: oiadfaosuoianavMyClassNameionaernaldfajd and /(/&%%§%/(§(/MyClassName&((/$/$/(§/$& and public class MyClassName { public static void main(String[] args) { } }? – Socrates Apr 18 '18 at 10:12

grep itself only has options for context based on lines. An alternative is suggested by this SU post:

A workaround is to enable the option 'only-matching' and then to use RegExp's power to grep a bit more than your text:

grep -o ".\{0,50\}WHAT_I_M_SEARCHING.\{0,50\}" ./filepath

Of course, if you use color highlighting, you can always grep again to only color the real match:

grep -o ".\{0,50\}WHAT_I_M_SEARCHING.\{0,50\}"  ./filepath | grep "WHAT_I_M_SEARCHING"

As another alternative, I'd suggest folding the text and then grepping it, for example:

fold -sw 80 input.txt | grep ...

The -s option will make fold push words to the next line instead of breaking in between.

Or use some other way to split the input in lines based on the structure of your input. (The SU post, for example, dealt with JSON, so using jq etc. to pretty-print and grep ... or just using jq to do the filtering by itself ... would be better than either of the two alternatives given above.)

This GNU awk method might be faster:

gawk -v n=50 -v RS='MyClassName' '
  FNR > 1 { printf "%s: %s\n",FILENAME, p prt substr($0, 0, n)}
  {p = substr($0, length - n); prt = RT}
' input.txt
  • Tell awk to split records on the pattern we're interested in (-v RS=...), and the number of characters in context (-v n=...)
  • Each record after the first record (FNR > 1) is one where awk found a match for the pattern.
  • So we print n trailing characters from the previous line (p) and n leading characters from the current line (substr($0, 0, n)), along with the matched text for the previous line (which is prt)
    • we set p and prt after printing, so the value we set is used by the next line
    • RT is a GNUism, that's why this is GNU awk-specific.

For recursive search, maybe:

find . -type f -exec gawk -v n=50 -v RS='MyClassName' 'FNR>1{printf "%s: %s\n",FILENAME, p prt substr($0, 0, n)} {p = substr($0, length-n); prt = RT}' {} +
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    Ok, it works. Seems Regex is a valid approach, so thanks for that. The processing time is quite big though. Without Regex as in my above post it takes 4.912s and with Regex as in your post it takes 3m39.312s. – Socrates Apr 18 '18 at 8:55
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    @Socrates see if the awk method I added above performs better – muru Apr 18 '18 at 10:43
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    The fold method can be used only if you are sure that the searched string does not appear at the border, otherwise it would get hidden by grep. – Melebius Apr 18 '18 at 10:45
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    @muru Thanks for your suggestion with gawk. Unfortunately, the suggested command with find outputs random stuff and no file names, when executed on my system. Furthermore, I'm not fluent enough in awk to properly analyse the command. Currently, Regex in combination with grep solves the matter maybe not fast, but reliable. Again, thanks a lot. – Socrates Apr 18 '18 at 12:30
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    @Socrates I think I managed to fix the awk command. My mental model was wrong about which line's RT and prefix, etc. were to be used. – muru Apr 18 '18 at 13:33

Using only-matching in combination with some other options(see below), might be very close to what you are seeking, without the processing overhead of regex mentioned in the other answer

grep -RnHo 'MyClassName'
  • n numeric output, show the line number of the match
  • H filename, show the filename at the start of the line of the match
  • o only matches, only show the matched string, not the whole line
  • While it is true that the result is found much faster, there is missing info. The file path is shown, the line number is shown, but the text output is only my initial search MyClassName. Hence, the context is missing. – Socrates Apr 18 '18 at 9:36
  • grep -RnHo "MyClassName" and grep -Rno "MyClassName" have the same output. – Socrates Apr 18 '18 at 9:37
  • @Socrates output is not the same without H in the same directory – Robert Riedl Apr 18 '18 at 9:44
  • The -o flag might be interesting if the regex had some variable part. For a fixed string, it’s useless to print it each time. OP is most likely interested in the near context. – Melebius Apr 18 '18 at 9:54
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    @Socrates, true - context is missing, but I thought that was the point ? Limit the output ? You can add context again by adding the lines before (-B 1) or after (-A 1). Sorry that I could not be of more help. – Robert Riedl Apr 18 '18 at 10:52

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