Home / ubuntu / Questions / 1026045
Accepted
Socrates
Asked: 2018-04-19 00:19:10 +0800 CST

Limit grep output to short lines

  • 772

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?

command-line grep text-processing

2 Answers

  1. Best Answer

    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}' {} +
    • 16

  2. 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
    • 3