Add Get The SHA256 For A File as a Unix TIL

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).
 
-_1488 TILs and counting..._
+_1489 TILs and counting..._
 
 ---
 
@@ -1443,6 +1443,7 @@ _1488 TILs and counting..._
 - [Generate Random 20-Character Hex String](unix/generate-random-20-character-hex-string.md)
 - [Get A List Of Locales On Your System](unix/get-a-list-of-locales-on-your-system.md)
 - [Get Matching Filenames As Output From Grep](unix/get-matching-filenames-as-output-from-grep.md)
+- [Get The SHA256 Hash For A File](unix/get-the-sha256-hash-for-a-file.md)
 - [Get The Unix Timestamp](unix/get-the-unix-timestamp.md)
 - [Global Substitution On The Previous Command](unix/global-substitution-on-the-previous-command.md)
 - [Globbing For All Directories In Zsh](unix/globbing-for-all-directories-in-zsh.md)

unix/get-the-sha256-hash-for-a-file.md

@@ -0,0 +1,34 @@
+# Get The SHA256 Hash For A File
+
+Unix systems come with a `sha256sum` utility that we can use to compute the
+SHA256 hash of a file. This means the contents of file are compressed into a
+256-bit digest.
+
+Here I use it on a SQL migration file that I've generated.
+
+```bash
+$ sha256sum migrations/0001_large_doctor_spectrum.sql
+b75e61451e2ce37d831608b1bc9231bf3af09e0ab54bf169be117de9d4ff6805  migrations/0001_large_doctor_spectrum.sql
+```
+
+Each file passed to this utility gets output to a separate line which is why we
+see the filename next to the hash. Since I am only running it on a single file
+and I may want to pipe the output to some other program, I can clip off just
+the part I need.
+
+```bash
+sha256sum migrations/0001_large_doctor_spectrum.sql | cut -d ' ' -f 1
+b75e61451e2ce37d831608b1bc9231bf3af09e0ab54bf169be117de9d4ff6805
+```
+
+We can also produce these digests with `openssl`:
+
+```bash
+$ openssl dgst -sha256 migrations/0001_large_doctor_spectrum.sql
+SHA2-256(migrations/0001_large_doctor_spectrum.sql)= b75e61451e2ce37d831608b1bc9231bf3af09e0ab54bf169be117de9d4ff6805
+
+$ openssl dgst -sha256 migrations/0001_large_doctor_spectrum.sql | cut -d ' ' -f 2
+b75e61451e2ce37d831608b1bc9231bf3af09e0ab54bf169be117de9d4ff6805
+```
+
+See `sha256sum --help` or `openssl dgst --help` for more details.