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grep Command

What is grep?

  • grep stands for:

    • Globally search a Regular Expression and Print (common backronym)
    • Global Regular Expression Print
    • Global Regular Expression Parser

  • We can use the grep command to search for a specified pattern within a single or multiple files.

Basic Syntax

grep <pattern> <filename>
  • This command prints all lines in the file(s) that match the specified pattern.

Example

File Contents

Assume we have a file subjects.txt with the following contents:

Sno Subjectname Faculty Fee
1. Python Nagoor 1000
2. Java Sriman 2000
3. Unix Durga 250
4. DevOps Sriman 3500
5. UNIX Durga 400
6. Java Durga 1000

Searching in a Single File

To search for the word "durga" in subjects.txt, you can use any of these commands:

$ grep 'durga' subjects.txt
$ grep "durga" subjects.txt
$ grep durga subjects.txt
  • This will print all lines that contain "durga" (case-sensitive):
3. Unix Durga 250
5. UNIX Durga 400
6. Java Durga 1000

Consider the demo.txt

The most important subject is linux
learning linux is very easy
the knowledge of linux is required everywhere
Your father learn linux, now you are learning.
In the future your kids also required to learn linux
java is a programming language
python is all rounder
Devops is the best technologies to fulfill current market requirements

akshay@myubuntu:~/Desktop$ grep "linux" demo.txt
The most important subject is linux
learning linux is very easy
the knowledge of linux is required everywhere
Your father learn linux, now you are learning.
In the future your kids also required to learn linux

subject.txt

Linux is very important everywhere
Java is like ocean and not that much easy
Python is nursery level programming language
Devops and datascience have future
Linux videos are too good

akshay@myubuntu:~/Desktop$ grep linux demo.txt subjects.txt
demo.txt:The most important subject is linux
demo.txt:learning linux is very easy
demo.txt:the knowledge of linux is required everywhere
demo.txt:Your father learn linux, now you are learning.
demo.txt:In the future your kids also required to learn linux

Search in all file in current working directory

akshay@myubuntu:~/Desktop$ grep java *
demo.txt:java is a programming language

It cannot search in nested directory:

akshay@myubuntu:~/Desktop$ grep linux *
demo.txt:The most important subject is linux
demo.txt:learning linux is very easy
demo.txt:the knowledge of linux is required everywhere
demo.txt:Your father learn linux, now you are learning.
demo.txt:In the future your kids also required to learn linux
grep: dir1: Is a directory
akshay@myubuntu:~/Desktop$

2 To Search in Multiple Files

You can use grep to search for a pattern across multiple files. Here are a few examples:

Example Commands

Searching in Specific Files

$ grep durga subjects.txt career.txt
  • Output:
subjects.txt:3. unix durga 250
subjects.txt:5. UNIX durga 400
subjects.txt:6. Java durga 1000
career.txt:durga
career.txt:durga ksdjfdlakjklfjad

Searching in All Text Files

$ grep durga *.txt
  • Output:
career.txt:durga
career.txt:durga ksdjfdlakjklfjad
subjects.txt:3. unix durga 250
subjects.txt:5. UNIX durga 400
subjects.txt:6. Java durga 1000

Searching in All Files in the Current Directory

$ grep durga *
  • Output:
career.txt:durga
career.txt:durga ksdjfdlakjklfjad
subjects.txt:3. unix durga 250
subjects.txt:5. UNIX durga 400
subjects.txt:6. Java durga 1000

3 To Search Data by Ignoring Case

By default, the grep command is case-sensitive. If you want to ignore case, you should use the -i option.

Example Commands

Case-Sensitive Search

$ grep unix *.txt
  • Output:
career.txt:unix jksadjfklasjdkflajs
subjects.txt:3. unix durga 250

Case-Insensitive Search

$ grep -i unix *.txt
  • Output:
career.txt:unix jksadjfklasjdkflajs
subjects.txt:3. unix durga 250
subjects.txt:5. UNIX durga 400

4 To Display the Number of Occurrences

To count the number of occurrences of a pattern, use the -c option. The c stands for count.

Example Command

$ grep -c unix *.txt
  • Output:
career.txt:1
subjects.txt:1

Comparison of locate, find, and grep

Command Purpose Example Usage
locate Quickly finds files and directories by name or pattern using an indexed database. locate filename.txt
find Searches for files and directories by name, type, size, and other attributes in real-time. find /home -name filename.txt
grep Searches for specific text patterns within the content of files. grep "pattern" filename.txt

Key Differences

  • locate and find: These commands are primarily used to find the location of files and directories based on their names or other attributes.
  • grep: This command is used to search for specific patterns or text within the contents of files.

In essence:

  • Use locate and find to locate files.
  • Use grep to search within the content of those files.

Additional grep Options

5 To Display Line Numbers Before Results

Use the -n option to display line numbers for each matched line in the search results.

Example Command

$ grep -n is *.txt
  • Output:
career.txt:1:Java  It is ocean but ever green!!!!

$ grep -n durga *.txt
  • Output:
career.txt:4:durga
career.txt:7:durga ksdjfdlakjklfjad
subjects.txt:4:3. unix durga 250
subjects.txt:6:5. UNIX durga 400
subjects.txt:7:6. Java durga 1000

6 To Display Only File Names Where Pattern Exists

Use the -l option to display only the names of files that contain the specified pattern.

Example Command

$ grep -l durga *.txt
  • Output:
career.txt
subjects.txt

7 To Print All Lines Except Those That Match the Pattern

Use the -v option to invert the search, displaying lines that do not contain the specified pattern.

Example Command

$ grep durga subjects.txt
  • Output (shows lines with "durga"):
3. unix durga 250
5. UNIX durga 400
6. Java durga 1000

$ grep -v durga subjects.txt
  • Output (shows lines without "durga"):
sno subjectname faculty fee
1. Python Nagoor 1000
2. java Sriman 2000
4. devops sriman 3500

8 To Search for Exact Words in the File

Use the -w option to search for whole words that match the pattern exactly.

Example Commands

$ grep -i unix demo.txt
  • Output (case-insensitive search):
UnixDemo session
unix material

$ grep -iw unix demo.txt
  • Output (case-insensitive, exact word match):
unix material

8 Example 2: Searching for Exact Words with Line Numbers

The -in and -win options allow you to search for exact words, with -i making the search case-insensitive and -n displaying line numbers.

Example Commands

$ grep -in unix demo.txt
  • Output (case-insensitive search with line numbers):
5:UnixDemo session
6:unix material
9:UNIX classes and videos

$ grep -win unix demo.txt
  • Output (whole-word match, case-insensitive, with line numbers):
6:unix material
9:UNIX classes and videos

9 Display Lines Before, After, or Surrounding the Search Results

To display additional lines around each match, use the -A, -B, or -C options.

  • -A <number>: Shows the specified number of lines after each match.
  • -B <number>: Shows the specified number of lines before each match.
  • -C <number>: Shows the specified number of lines both before and after each match.
  • -<number>: The C is optional for showing lines before and after.

Example Commands

$ grep friends demo.txt
  • Output (shows only the matched line):
Hello friends how are you

$ grep -A 2 friends demo.txt
  • Output (shows 2 lines after the matched line):
Hello friends how are you
DataScienceDemo
UnixDemo session

$ grep -B 2 friends demo.txt
  • Output (shows 2 lines before the matched line):
This is java demo
this is python demo
Hello friends how are you

$ grep -C 2 friends demo.txt
  • Output (shows 2 lines before and after the matched line):
This is java demo
this is python demo
Hello friends how are you
DataScienceDemo
UnixDemo session

$ grep -2 friends demo.txt
  • Output (shows 2 lines before and after the matched line, C is optional):
This is java demo
this is python demo
Hello friends how are you
DataScienceDemo
UnixDemo session