Move a sed TIL from unix to the sed section

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

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+# 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)