Add grep.md (#1223)

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Co-authored-by: Rico Sta. Cruz <rstacruz@users.noreply.github.com>

grep.md

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+---
+title: GNU grep
+category: CLI
+layout: 2017/sheet
+updated: 2021-08-23
+---
+
+### Usage
+{: .-prime}
+
+```bash
+grep <options> pattern <file...>
+```
+
+### Matching options
+
+```bash
+-e, --regexp=PATTERN
+-f, --file=FILE
+-i, --ignore-case
+-v, --invert-match
+-w, --word-regexp
+-x, --line-regexp
+```
+
+### Pattern options
+
+```bash
+-F, --fixed-strings   # list of fixed strings
+-G, --basic-regexp    # basic regular expression (default)
+-E, --extended-regexp # extended regular expression
+-P, --perl-regexp     # perl compatible regular expression
+```
+
+### Expressions
+
+#### Basic Regular Expressions (BRE)
+
+In BRE, these characters have a special meaning unless they are escaped with a backslash:
+
+`^ $ . * [ ] \`
+
+However, these characters do not have any special meaning unless they are escaped with a backslash:
+
+`? + { } | ( )`
+	
+#### Extended Regular Expressions (ERE)
+
+ERE gives all of these characters a special meaning unless they are escaped with a backslash:
+
+`^ $ . * + ? [ ] ( ) | { }`
+
+#### Perl Compatible Regular Expressions (PCRE)
+
+PCRE has even more options such as additional anchors and character classes, lookahead/lookbehind, conditional expressions, comments, and more. See the [regexp cheatsheet](/regexp).
+
+### Output Options
+
+```bash
+-c, --count           # print the count of matching lines. suppresses normal output
+    --color[=WHEN]    # applies color to the matches. WHEN is never, always, or auto
+-m, --max-count=NUM   # stop reading after max count is reached
+-o, --only-matching   # only print the matched part of a line
+-q, --quiet, --silent
+-s, --no-messages     # suppress error messages about nonexistent or unreadable files
+```
+
+### Context Options
+
+```bash
+-B NUM, --before-context=NUM  # print NUM lines before a match
+-A NUM, --after-context=NUM   # print NUM lines after a match
+-C NUM, -NUM, --context=NUM   # print NUM lines before and after a match
+```
+
+### Examples
+
+```bash
+# Case insensitive: match any line in foo.txt
+# that contains "bar"
+grep -i bar foo.txt
+
+#  match any line in bar.txt that contains
+# either "foo" or "oof"
+grep -E "foo|oof" bar.txt
+
+# match anything that resembles a URL in
+# foo.txt and only print out the match
+grep -oE "https?:\/\/((\w+[_-]?)+\.?)+" foo.txt
+
+# can also be used with pipes:
+# match any line that contains "export" in
+# .bash_profile, pipe to another grep that
+# matches any of the first set of matches
+# containing "PATH"
+grep "export" .bash_profile | grep "PATH"
+
+# follow the tail of server.log, pipe to grep
+# and print out any line that contains "error"
+# and include 5 lines of context
+tail -f server.log | grep -iC 5 error
+```