Add Extract Value From Command Output With Sed 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://tinyletter.com/jbranchaud).
 
-_1011 TILs and counting..._
+_1012 TILs and counting..._
 
 ---
 
@@ -940,6 +940,7 @@ _1011 TILs and counting..._
 - [Display The Contents Of A Directory As A Tree](unix/display-the-contents-of-a-directory-as-a-tree.md)
 - [Do Not Overwrite Existing Files](unix/do-not-overwrite-existing-files.md)
 - [Exclude A Directory With Find](unix/exclude-a-directory-with-find.md)
+- [Extract Value From Command Output With Sed](unix/extract-value-from-command-output-with-sed.md)
 - [Figure Out The Week Of The Year From The Terminal](unix/figure-out-the-week-of-the-year-from-the-terminal.md)
 - [File Type Info With File](unix/file-type-info-with-file.md)
 - [Find Files With fd](unix/find-files-with-fd.md)

unix/extract-value-from-command-output-with-sed.md

@@ -0,0 +1,36 @@
+# Extract Value From Command Output With Sed
+
+As part of a shell script, you may need to extract a value from one command to
+be used as part of a subsequent command.
+
+For instance, [I recently wrote a
+script](https://gist.github.com/jbranchaud/3cda6be6e1dc69c6f55435a387018dac)
+that needed to determine the version of the currently running Postges server.
+The `postgres` command can tell me that.
+
+```bash
+$ postgres -V
+postgres (PostgreSQL) 12.3
+```
+
+However, the output includes extra fluff that I don't need, namely the leading
+`postgres (PostgreSQL) ` part.
+
+The output of `postgres` can be piped into a `sed` command that can extract
+just what I need.
+
+```bash
+$ postgres -V | sed -n 's/postgres (PostgreSQL) \(.*\)/\1/p'
+12.3
+```
+
+The `sed` command receives this single line of output and attempts a
+substituation. It matches on `postgres (PostgresSQL) ` followed by a capture
+group (`\(.*\)`) for the remaining characters. This capture group matches the
+version part of the output. `sed` replaces everything in the first part of the
+substitution with `\1`, which is `12.3`, and outputs that.
+
+The output of this could then be piped to another command or captured in a
+variable to be used in the remainder of a script.
+
+[source](https://stackoverflow.com/a/24572880/535590)