Add Find Duplicate Lines In A File as a Unix TIL

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1302 TILs and counting..._
+_1303 TILs and counting..._
 
 ---
 
@@ -1253,6 +1253,7 @@ _1302 TILs and counting..._
 - [File Type Info With File](unix/file-type-info-with-file.md)
 - [Find All Files Matching A Name With fd](unix/find-all-files-matching-a-name-with-fd.md)
 - [Find A File Installed By Brew](unix/find-a-file-installed-by-brew.md)
+- [Find Duplicate Lines In A File](unix/find-duplicate-lines-in-a-file.md)
 - [Find Files With fd](unix/find-files-with-fd.md)
 - [Find Newer Files](unix/find-newer-files.md)
 - [Fix Unlinked Node Binaries With asdf](unix/fix-unlinked-node-binaries-with-asdf.md)

unix/find-duplicate-lines-in-a-file.md

@@ -0,0 +1,20 @@
+# Find Duplicate Lines In A File
+
+Let's say I have a large file in a Ruby project. I want to find instances of a
+`field` declaration being duplicated throughout the file. Just searching for
+duplicate lines within the file is going to result in all kinds of false
+positives (think, lots of duplicate `end` lines).
+
+What I can do is `grep` for a pattern that will just match on the lines that
+are `field` declarations. The results of the `grep` can then be piped to `sort`
+which will order them. This ordering will mean that any duplicates are placed
+next to each other. Lastly, I'll pipe the sorted lines to `uniq` with the `-d`
+flag which will filter the results down to just those lines that are repeated.
+
+Here is what the whole thing looks like:
+
+```
+$ grep -o "field :[a-zA-Z_][a-zA-Z_0-9]*" file.rb | sort | uniq -d
+```
+
+See `man uniq` for more details on the available flags.