The grep utility searches any given input files, selecting lines that match one or more patterns. By default, a pattern matches an input line if the regular expression (RE) in the pattern matches the input line without its trailing new line. An empty expression matches every line. Each input line that matches at least one of the patterns are written to the standard output.
grep is used for simple patterns and basic regular expressions (BREs). Below are the frequent use cases which will be handy for developers in their day to day activity.
- Case insensitive Search
- Get Context
- Invert Match
- Show Filenames
- Recursive Search
- Print Matching Part
- Regular Expressions
- Search Binaries
Case insensitive Search
grep -i : Output of both commands below will be same as -i option does the case insensitive search.
grep -i hello file.txt
grep -i HELLO file.txt
To get the context of your search use the below commands.
grep -A :
grep -A 3 hello file.txt
This will show 3 lines of context after your match i.e "hello" word in this case.
grep -B :
grep -B 3 hello file.txt
This will show 3 lines of context before your match i.e "hello" word in this case.
grep -C :
grep -C 3 hello file.txt
This will show 3 lines of context before and after your match i.e "hello" word in this case.
grep -v : Find all lines that don't match the given pattern.
grep -v hello file.txt
grep -l : Show only the filenames of the files that matched
grep -l hello *.txt
Above command lists all .txt files which has "hello" word in the current directory.
grep -r : Search all the files in a directory
grep -r hello */*.txt
Above command lists all .txt files inside nested directory which has "hello" word in it.
Print Matching Part
grep -o hello file.txt
Above command prints only matching part of the line (not the complete line).
grep -E : This option is used for matching Regular Expressions.
grep -E 'hel*' file.txt
Above command searches for all the lines which have words starting with hel.
grep -E 'hel*|wo*' file.txt
Above command searches for all the lines which have words starting with hel OR wo
grep -a : Treat binary data like its text instead of ignoring it.
Treat all files as ASCII text. Normally grep will simply print ``Binary file ... matches'' if files contain binary characters. Use of this option forces grep to output lines matching the specified pattern.
grep -a <pattern> <binary file>