grep¶

`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

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
grep -r 'pattern' --include='*.txt' --include='*.md' /path/to/dir
- searches pattern recursively, only including specific files
- An equivalent expression is grep -r 'pattern' --include='*.{txt,md}' /path/to/dir

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

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