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#+TITLE: How to get all Emacs daemon buffers from a shell script
#+DATE: 2021-05-01T22:38:32-04:00
#+DRAFT: false
#+DESCRIPTION: A small script that will print all Emacs daemon buffers to stdout
#+TAGS[]: emacs
#+KEYWORDS[]: emacs
#+SLUG:
#+SUMMARY:
#+ATTR_HTML: :title Emacs daemon buffers
#+ATTR_HTML: :alt Emacs darmon buffers
[[file:cover.png]]
A while back I posted a response to someone's question on [[https://www.reddit.com/r/emacs/comments/ljtify/is_it_there_a_way_to_export_the_list_of_opened/gnhatdu/?context=3][reddit]] about
how to get a list of all Emacs daemon buffers from a shell script. It
was a pretty interesting problem so I thought I'd explain my answer
here.
The question was "Is it there a way to export the list of opened
buffers to STDOUT?".
In the comments I left a rather byzantine looking snippet of code that
I'd managed to produce.
#+begin_src
emacs --batch --eval "(require 'server)" --eval "(mapc #'print (read (server-eval-at \"server\" '(format \"%s\" (mapcar (lambda (buffer) (format \"\\\"%s\\\"\n\" buffer)) (buffer-list))))))" | sed '/^$/d; s/^"//g; s/"$//g'
#+end_src
I've simplified the lisp slightly since I answered that question.
Here's the updated version.
#+begin_src
emacs --batch --eval "(require 'server)" --eval "(mapc #'princ (read (server-eval-at \"server\" '(prin1-to-string (mapcar (lambda (buffer) (format \"%s\\n\" buffer)) (buffer-list))))))" 2>/dev/null
#+end_src
Here it is written in a way that's easier to read.
#+begin_src
emacs --batch \
--eval "(require 'server)" \
--eval "(mapc #'princ
(read (server-eval-at \"server\"
'(prin1-to-string (mapcar (lambda (buffer)
(format \"%s\\n\" buffer))
(buffer-list))))))" \
2>/dev/null
#+end_src
Let's break it down.
- =emacs= Emacs itself!
- =--batch= Runs Emacs in [[https://www.gnu.org/software/emacs/manual/html_node/elisp/Batch-Mode.html][=batch=]] mode, which executes commands
non-interactively and stops it from opening a window. This is
usually used for running Emacs lisp as a script.
- =--eval= Evaluates the following piece of elisp in the batch Emacs
- =(require 'server)= Loads the built-in =server= package. This is
used to connect to the running Emacs daemon
- =--eval= Since the previous elisp snippet was a complete
s-expression I evaluate the next expression as a new argument. I
could have also wrapped them both in a =progn=, but this felt cleaner.
- =mapc FUNCTION SEQUENCE= Apply =FUNCTION= to every object in the
list =SEQUENCE=.
- =princ= Outputs the printed form of an object to
=standard-out=. It's used here because it doesn't surround the
string in quotes like =print= does.
- =read STREAM= Read =STREAM=, in this case a string, and turn it
into a lisp object
- =server-eval-at \"server\" FORM= Evaluates =FORM= on the Emacs
daemon and returns the result. The quotes are escaped
because it's already inside a quote because it's a command
line argument.
- =prin1-to-string OBJECT= Return a string containing the
printed representation of =OBJECT=. I use this instead of
=princ= because that outputs the result to the minibuffer of
the Emacs daemon instead of returning it.
- =mapcar FUNCTION SEQUENCE= Applies FUNCTION to each
element of SEQUENCE and returns a list of the result.
- =(lambda (buffer) (format \"%s\\n\" buffer)= An
anonymous function that takes a buffer and returns the
string version of its name followed by a newline. The
quotes and newline are escaped because it's already
inside a quote because it's a command line argument.
- =buffer-list= Returns a list of all buffers in Emacs
- =2>/dev/null= Send the Emacs startup text, which is outputted to
=stderr=, to =/dev/null=. We don't want to see it.
The result is a single line command that outputs the name of every
buffer in the Emacs daemon, one per line, to =stdout=.
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