Add $OUTPUT and $LAST eshell variables

These contain the output from the last eshell command. Cool how easily
I implemented this.

ha-eshell.org

@@ -210,6 +210,78 @@ This allows us to replace some of our aliases with functions:
     (eshell-fn-on-files 'find-file-other-window 'find-file-other-window args))
 #+end_src
 We’ll leave the =e= alias to replace the =eshell= buffer window.
+** Last Results
+The [[https://github.com/mathiasdahl/shell-underscore][shell-underscore]] project looks pretty cool, where the =_= character represents a /filename/ with the contents of the previous command (you know, like if you were planning on it, you’d =tee= at the end of every command). An interesting idea that I could duplicate.
+
+While diving into the =eshell= source code, I noticed that the special variable, =$$= can be used /sometimes/ as the output of the last command. For instance:
+#+begin_example
+$ echo "hello world"
+hello world
+$ echo $$
+hello world
+#+end_example
+However, what I would like is something like this to work:
+#+begin_example
+$ ls *.org(U)
+a.org b.org f.org
+$ rg "foobar" $$
+#+end_example
+
+The problem /may/ be between calling Emacs functions versus external commands, as the =echo= works, but the call to =ls= doesn’t:
+#+begin_example
+$ ls *.org(U) b.txt
+a.org b.org f.org b.txt
+
+$ echo Nam $$
+("Nam" nil)
+#+end_example
+
+However, we could add a hook that runs /after/ every command to copy the output to a variables of our choosing:
+#+begin_src emacs-lisp
+  (defvar OUTPUT ""
+    "Contains the output from the last eshell command executed.")
+
+  (defvar LAST nil
+    "Contains a list of elements from the last eshell command executed.")
+#+end_src
+
+Why two variables? Well unlike the behavior of the original shell (and most of its descendents, like =bash=), =eshell= doesn’t automatically split on whitespace. For instance, =echo= called this way:
+#+begin_example
+$ echo a b *.txt
+("a" "b"
+ ("b.txt" "date today.txt"))
+#+end_example
+Is given a list of /three elements/: =a=, =b=, and a list of all files in the current directory with an =.org= extension. An interesting side-effect is that spaces in filenames are /often okay/. So we want =$OUTPUT= to contain the command’s output /as a string/, and we have, =$LAST= contains the same stuff, but separated by spaces, into a list. So, if we are passing the output from =ls= to =grep=, we would use =$LAST= to represent files.
+
+The following function does the work of saving the output of the last command. We can get this because after every command, eshell updates two variables, [[elisp:(describe-variable 'eshell-last-input-end)][eshell-last-input-end]] (the start of the output), and [[elisp:(describe-variable 'eshell-last-output-start)][eshell-last-output-start]] (the end of the output):
+#+begin_src emacs-lisp
+  (defun ha-eshell-store-last-output ()
+    "Store the output from the last eshell command.
+  Called after every command by connecting to the `eshell-post-command-hook'."
+    (setq OUTPUT
+          (s-trim
+           (buffer-substring-no-properties eshell-last-input-end eshell-last-output-start)))
+    (setq LAST
+          (split-string (rx (one-or-more space)) OUTPUT)))
+#+end_src
+
+Now we save this output after every command by adding it to the [[elisp:(describe-variable 'eshell-post-command-hook)][eshell-post-command-hook]]:
+#+begin_src emacs-lisp
+  (add-hook 'eshell-post-command-hook 'ha-eshell-store-last-output)
+#+end_src
+
+Success:
+#+begin_example
+$ ls *.org(U) b.txt
+a.org  b.txt
+
+$ rg Nam $LAST
+a.org
+8:Nam vestibulum accumsan nisl.
+
+b.txt
+1:Nam euismod tellus id erat.
+#+end_example
 * Special Prompt
 Following [[http://blog.liangzan.net/blog/2012/12/12/customizing-your-emacs-eshell-prompt/][these instructions]], we build a better prompt with the Git branch in it (Of course, it matches my Bash prompt). First, we need a function that returns a string with the Git branch in it, e.g. ":master"
 #+begin_src emacs-lisp :tangle no