The grep command is the magic Unix utility for searching text. It accepts regular expressions and can produce reports in various formats.

1. Search stdin for lines that match a pattern:

        $ echo -e "this is a word\nnext line" | grep word 
        this is a word

2. Search a single file for lines that contain a given pattern:

        $ grep pattern filename
        this is the line containing pattern

Alternatively, this performs the same search:

        $ grep "pattern" filename
        this is the line containing pattern

3. Search multiple files for lines that match a pattern:

        $ grep "match_text" file1 file2 file3 ... 

4. To highlight the matching pattern, use the -color option. While the option position does not matter, the convention is to place options first.

        $ grep -color=auto word filename
        this is the line containing word

5. The grep command uses basic regular expressions by default. These are a subset of the rules described earlier. The -E option will cause grep to use the Extended Regular Expression syntax. The egrep command is a variant of grep that uses extended regular expression by default:

        $ grep -E "[a-z]+" filename

Or:

        $ egrep "[a-z]+" filename

6. The -o option will report only the matching characters, not the entire line:

        $ echo this is a line. | egrep -o "[a-z]+\."
        line

7. The -v option will print all lines, except those containing match_pattern:

        $ grep -v match_pattern file

The -v option added to grep inverts the match results.

8. The -c option will count the number of lines in which the pattern appears:

        $ grep -c "text" filename
        10

It should be noted that -c counts the number of matching lines, not the number of times a match is made. Consider this example:

        $ echo -e "1 2 3 4\nhello\n5 6" | egrep  -c "[0-9]"
        2

Even though there are six matching items, grep reports 2, since there are only two matching lines. Multiple matches in a single line are counted only once.

9. To count the number of matching items in a file, use this trick:

        $ echo -e "1 2 3 4\nhello\n5 6" | egrep -o "[0-9]" | wc -l
        6

10. The -n option will print the line number of the matching string:

        $ cat sample1.txt
        gnu is not unix
        linux is fun
        bash is art
        $ cat sample2.txt
        planetlinux
        $ grep linux -n sample1.txt
        2:linux is fun

Or

        $ cat sample1.txt | grep linux -n

If multiple files are used, the -c option will print the filename with the result:

        $ grep linux -n sample1.txt sample2.txt
        sample1.txt:2:linux is fun
        sample2.txt:2:planetlinux

11. The -b option will print the offset of the line in which a match occurs. Adding the -o option will print the exact character or byte offset where the pattern matches:

        $ echo gnu is not unix | grep -b -o "not"
        7:not

Character positions are numbered from 0, not from 1.

12. The -l option lists which files contain the pattern:

        $ grep -l linux sample1.txt sample2.txt
        sample1.txt
        sample2.txt

The inverse of the -l argument is -L. The -L argument returns a list of nonmatching files.