Grep by Example

We are in the middle of a multipart series. Each post focuses on one member of the command-line text-processing trifecta: Grep, Sed and Awk. In this first part, we will explore Grep.

Grep stands for global regular expression print. As its name implies, it searches for text that matches a regular expression. In this post, we'll work through several examples.

Whenever "rhyme.txt" is referenced, assume it contains the following content:

Hickory dickory dock

The mouse ran up the clock

The clock struck one

The mouse ran down

Hickory dickory dock

-- "Hickory, Dickory, Dock" (public domain)

Example 1: Basic Search

The most basic use of Grep is to search for a word in a file:

grep "mouse" rhyme.txt

Which would produce the following output:

The mouse ran up the clock

The mouse ran down

Note that, by default, only lines that match the pattern are returned.

Example 2: Case Insensitive Search

To perform a case insensitive search, use the "-i" flag:

grep -i "MOUSE" rhyme.txt

Which would produce the same output as Example 1:

The mouse ran up the clock

The mouse ran down

Example 3: Line Numbers

To display line numbers in the output, use the "-n" flag:

grep -n "mouse" rhyme.txt

Which would produce:

2:The mouse ran up the clock

4:The mouse ran down

Example 4: Count Matches

To count the number of matching lines, use the "-c" flag:

grep -c "mouse" rhyme.txt

Which would output:

2

Example 5: Regular Expressions

Grep supports regular expressions. For example, to find lines that start with "The":

grep "^The" rhyme.txt

Which would output:

The mouse ran up the clock

The clock struck one

The mouse ran down

Example 6: Invert Match

To find lines that don't match a pattern, use the "-v" flag:

grep -v "mouse" rhyme.txt

Which would output:

Hickory dickory dock

The clock struck one

Hickory dickory dock

Example 7: Multiple Files

Grep can search multiple files at once:

grep "mouse" rhyme.txt poem.txt story.txt

When searching multiple files, Grep will prefix each match with the filename:

rhyme.txt:The mouse ran up the clock

rhyme.txt:The mouse ran down

poem.txt:The mouse was very quiet

story.txt:A mouse lived in the wall

Example 8: Recursive Search

To search all files in a directory (and its subdirectories), use the "-r" flag:

grep -r "mouse" .

This will search for "mouse" in all files in the current directory and its subdirectories.

Example 9: Context

To show lines before and after the match, use the "-A" (after) and "-B" (before) flags:

grep -A 1 -B 1 "struck" rhyme.txt

Which would output:

The mouse ran up the clock

The clock struck one

The mouse ran down

Example 10: Word Boundaries

To match whole words only, use the "-w" flag:

grep -w "the" rhyme.txt

This would match "the" but not "there" or "these".

Conclusion

These examples demonstrate the most commonly used features of Grep. In the next post, we'll explore Sed, which will allow us to not only search text but also modify it.