Change the zsh heading to unix.

unix/global-substitution-on-the-previous-command.md

@@ -0,0 +1,28 @@
+# Global Substitution On The Previous Command
+
+Let's say we just executed the following command:
+
+```bash
+$ grep 'foo' foo.md
+```
+
+It gave us the information we were looking for and now we want to execute
+a similar command to find the occurrences of `bar` in `bar.md`. The `^`
+trick won't quite work here.
+
+```bash
+$ ^foo^bar<tab>
+$ grep 'bar' foo.md
+```
+
+What we need is a global replace of `foo` in our previous command. The `!!`
+command can help when we sprinkle in some `sed`-like syntax.
+
+```bash
+$ !!gs/foo/bar<tab>
+$ grep 'bar' bar.md
+```
+
+For a short command like this, we haven't gained much. However, for large
+commands that span the length of the terminal, this can definitely save us
+a little trouble.