Add Get The SHA256 For A File as a Unix TIL

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

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+# 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.