Create 012-grep-howto.md

Added short article about grep command, as requested on flashreads#157

linux/012-grep-howto.md

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+---
+id: 012-grep-howto.md
+title: Grep command usage cheatsheet
+tags: 
+  - linux
+  - grep
+author: waltherman
+meta-description: Grep is an ultimate command to search text patterns in files
+date: 2021-10-13 03:30:00 +0300 
+keywords: linux, grep, searching in files
+template: post
+categories:
+  - linux
+cover: ../../images/categories/linux.png
+---
+
+# What is Grep?
+
+Grep stands for "global regular expression print" and it is an ultimate command to search text or any file for lines that contain a match to the specified text pattern, word or regex.
+By default, grep outputs only the matched lines.
+
+## Basic grep usage
+
+```
+$ grep pattern filename
+```
+
+As a result grep would give all patterns found in file named "filename". For example:
+
+```
+$ grep pattern filename
+pattern
+pattern1
+zzzpattern44
+```
+
+Also you could use pipe to filter output of any command. For example, let's find installed PostgreSQL client version on Ubuntu server via next command:
+```
+$ dpkg --list | grep postgresql-client
+
+```
+
+-i parameter used for case-insensetive grep 
+```
+$ grep -i pattern filename
+
+```
+-v - inverted match of grep
+
+```
+$ grep -v pattern filename
+
+```
+
+-w used for search only for words match. For example file named "filename" from very first example had several matches, but with this argument there would be only one:
+```
+$ grep -w pattern filename
+pattern
+```
+
+Recursive search
+Let's find all files matching pattern in this directory and all subdirectories by -r/R argument:
+
+```
+$ grep -R pattern .
+
+```
+Also we could use grep to find file by its content:
+
+```
+$ grep -rnw '/path/to/somewhere/' -e 'pattern'
+
+```