In this article, we explain one of the basic Linux commands for beginners, i.e. the grep command in Linux, with its most frequently used command options.
What is Linux?
Linux is a family of open-source operating systems based on the Linux kernel. The first Linux system kernel was released on September 17, 1991, by Linus Torvalds.
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Popular Linux distributions include Debian, Fedora, and Ubuntu, and the commercial distributions include Red Hat Enterprise Linux and SUSE Linux Enterprise Server.
There are also quite a number of customized Linux distributions, such as Kali Linux, REMnux etc. Kali Linux is a Debian-based distribution developed, funded and maintained by Offensive Security for ethical hackers for the purposes of Penetration Testing, Security Research & Assessment, and Computer Computer Forensics & Reverse Engineering. REMnux, on the other hand, is a Linux distro curated for reverse-engineering and malware analysis purposes.
What I find most interesting is how people really have taken Linux and used it in ways and attributes and motivations that I never felt.
Linus Torvalds
Read more educational and inspirational cyber quotes at our page 100+ Best Cyber Security & Hacker Quotes.
grep Command in Linux – Print Lines That Match Patterns
In Linux, grep is a command-line utility to search for a string of characters, i.e., patterns, in a specified file or files. The search strings could vary from simple words to complex regular expressions. When a match is found, the line containing the search string is printed as the grep command’s output. The grep command is specifically useful for searching a string of characters in large files, such as log files.
Search for PATTERNS in each FILE. Basic usage:grep [OPTION] ... PATTERNS [FILE] ...Use -cto suppress normal output and print a count of matching lines instead, use-ito ignore case distinctions in patterns,-nto prefix each line of output with a line number, and-v(invert match) to select non-matching lines.
Using the grep Command
To demonstrate the most frequently used grep command options, we will be using the files illustrated in Figure 1 and 2 in the following examples.
Simple String Search
To print any line from a file that contains the specified pattern (case sensitive), write grep followed by the search pattern and the file name, e.g., grep pattern file-name.
$ grep string file1.txt
grep CommandSearching a Pattern in Multiple Files
To print any line from multiple files that contain the specified pattern (search string), write the file names with spaces in between.
$ grep string file1.txt file2.txt
Searching a Pattern Recursively in a Directory
To search a pattern recursively in multiple files in a directory, use the -r option followed by the search pattern and the directory name.
$ grep -r string grep/
Searching a Pattern with Ignore Case Option
To print any line from a file that contains the specified pattern, disregarding the letter case (case insensitive), use the -i option.
$ grep -i string file1.txt
Inverse Search
To print lines that does not match the specified pattern (invert match), use the -v option.
$ grep -v string file1.txt
Note that, the preceding example can be further filtered with a case insensitive invert match search, by using the -i and -v options together, as demonstrated in Figure 8.
$ grep -vi string file1.txt
Displaying Line Numbers
To prefix each of the matching lines with a line number, use the -n option.
$ grep -n string file1.txt
Printing Count of Matching Lines
To suppress the normal output and print a count of matching lines instead, use the -c option.
$ grep -c string file1.txt
Printing Lines with Whole Word Matches
To select only those lines containing matches that form whole words, use the -w option.
$ grep -w hack file1.txt file2.txt
Listing Names of Files
To suppress the normal output and instead print the names of each input file that would contain the search pattern, use the -l option. To print the names of files that would not contain the pattern, use the -L option.
$ grep -l hack file1.txt file2.txt
Displaying a Number of Files Before and After a Matching Line
To display a specified number of lines after a matching line, use the -A option followed by a number. Similary, use the -B option to display a specified number of lines before a matching line, and use the -C option to display a specified number of lines both before and after a matching line.
$ grep -A 3 hack file1.txt file2.txt
Piping the Output of a Command into the grep
Note that, in Linux everything is represented as a file. In other words, files, directories, device or outputs generated by commands are all files, albeit with different functionalities. In this context, output generated by a command can also be fed into to the grep command by using the pipe (|) operator. In fact, this is one of the most common uses of the grep command.
For instance, to search for the root user in Kali Linux in the /etc/passwd file, you can use the following command:
$ cat /etc/passwd | grep -w root
is equivalent to
$ grep -w root /etc/passwd
grep CommandTo explore more grep command options, please visit the man pages by typing $ man grep on the terminal.
To learn more on Linux, you could also visit our Linux Resources Page.
