grep

grep stands for global rgular expression and print and allows us to search for text within files.

grep does not work with directories. If something like grep "search" ./ is tried, an error is returned.

  • grep "text to search" file_name
    • text to search goes inside the double quotes
    • nothing is printed in case of no results
    • each line containing the searched text is printed
  • grep -w "text" file
    • -w will search for whole words
    • for instance, if two lines contain the text you and yours, and we use grep -w "you" file, only the line containing you will be returned
    • grep "you" file returns both the lines
    • printf "you\nyours\n" | grep "you" 
        you
  yours
    • printf "you\nyours\n" | grep -w "you" 
        you
  • grep -i
    • will do a case insensitive search
  • grep -n
    • will add a line number as well to the output
    • printf "you are happy\nhow do you do\nyours truly\n" | grep -n -w "you" 
        1:you are happy
  2:how do you do
  • Multiple flags can be combined together as -win
    • grep -win will match the full word in a case insensitive fashion and print the line numbers int eh result
  • grep -B num
    • prints num lines before the match as well so that we can get an idea of the context of the searched word
    • printf "1\n2\n3\n4\n5\n6\n7\n8\n" | grep -wn -B 2 "5" 
        3-3
  4-4
  5:5
  • grep -A num
    • will return num lines after the matches
    • both the flags can be used in the same query
    • printf "1\n2\n3\n4\n5\n6\n7\n8\n" | grep -wn -B 2 -A 3 "5" 
        3-3
  4-4
  5:5
  6-6
  7-7
  8-8
  • grep -C num
    • will return num lines before and after the matches
    • -C num is equivalent to -A num -B num

Searching multiple files

  • grep "search" ./*
    • will serch in every file in the current directory
    • an error will be returned
    • can combine with find using the -exec flag
  • grep -r "search" ./*
    • performs a recursive search within all the directories as well
    • in case of many subdirectories/files, recursive search might get too slow
  • grep -l "search" ./
    • only returns the list of files where at least one match was found
    • a file will appear only once irrespective of the number of matches
  • grep -c "search" ./
    • similar to the -l flag, but will also return the number of matches against each file

Piping outputs to grep

  • some command | grep "keyword"
    • will search the keyword in the output of some command
    • multiple grep commands can be chained together to form something similar to and filters

Regular Expressions

  • By default, grep uses posix compatible regular expressions, while programs like python use perl compatible regular expressions
  • -P flag will force grep to use perl compatible regular expressions on linux systems
  • Mac uses BSD version of grep and does not support the -P flag. GNU version of grep, which is used in linux, needs to be installed on Mac to use this flag

References