Add Output Only Lines Involved In A Substitution as a sed 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://tinyletter.com/jbranchaud).
 
-_1076 TILs and counting..._
+_1077 TILs and counting..._
 
 ---
 
@@ -933,6 +933,7 @@ _1076 TILs and counting..._
 
 - [Apply Multiple Substitutions To The Input](sed/apply-multiple-substitutions-to-the-input.md)
 - [Extract Value From Command Output With Sed](sed/extract-value-from-command-output-with-sed.md)
+- [Output Only Lines Involved In A Substitution](sed/output-only-lines-involved-in-a-substitution.md)
 - [Use An Alternative Delimiter In A Substitution](sed/use-an-alternative-delimiter-in-a-substitution.md)
 
 ### Shell

sed/output-only-lines-involved-in-a-substitution.md

@@ -0,0 +1,38 @@
+# Output Only Lines Involved In A Substitution
+
+When you run a basic `sed` command, it will _autoprint_ the pattern space (a
+line of input) once it is done running the script against it. That means every
+line will get sent to stdout.
+
+You can supress the autoprint functionality with the `-n` flag like so:
+
+```bash
+$ seq 100 | sed -n 's/1$/one/'
+```
+
+You can then add the `p` flag to the end of the substitute command to tell it
+to _print_ any line that was affected by that substitution after the
+substitution has been applied.
+
+```bash
+$ seq 100 | sed -n 's/1$/one/p'
+one
+1one
+2one
+3one
+4one
+5one
+6one
+7one
+8one
+9one
+```
+
+For all numbers between 1 and 100, this matches those that end in `1` and
+substitutes that `1` for `one`. And then it is only those lines that go to
+stdout.
+
+If you used the `p` flag without `-n`, every line would autoprint and then
+you'd get duplicate output for each line that had a substitution.
+
+See `man sed` for more details.