path: root/2022/captions/emacsconf-2022-eshell--top-10-reasons-why-you-should-be-using-eshell--howard-abrams--main.vtt

WEBVTT captioned by howard

NOTE Introduction

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I have 10 minutes to talk you into

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giving Eshell a second chance.

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Have the right perspective and expectation,

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and I think you’ll really enjoy it.

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Just remember eshell is a shell,

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not a terminal emulator.

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I use both Eshell and vterm.

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I’m going to talk and type fast,

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as I have 10 reasons for you to try Eshell again.

NOTE 1. It’s an Emacs REPL

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1. It’s an Emacs REPL.

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I mean, check this out.

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Let’s start up Eshell here.

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Let’s just type a Lisp expression.

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It works.

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As a shell, the parens are kinda optional.

NOTE 2. It’s also a shell

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2. It’s also a shell.

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While eshell may look like a shell, like Bash

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you should view it as a REPL

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with parenthesis-less s-expressions.

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This makes sense, because a shell command with options,

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like this ls command,

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looks like an s-expression.

NOTE 3. You can mix these two modes

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3. You can mix these two modes.

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Shells can call subshells

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which return their output like a function call,

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like this Bash command.

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In this Eshell example,

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I use the output of a text file

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as command line arguments to ripgrep.

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Notice how I use braces

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to state that it is a call to an eshell expression.

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We can mix Lisp-expressions and Shell-expressions.

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Allow me a contrived example.

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Notice I use good ol' setq to create a variable.

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Yes, those are global Emacs variables available everywhere.

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In Eshell, the wildcard actually creates a list.

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This variable assignment doesn’t work as you might expect,

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as setq in Eshell is still setq,

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and it assigns variables in pairs.

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To make a list in Eshell, we use listify:

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Without parens, Eshell is in “shell mode”,

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which means that words are strings,

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and variables need to be prefixed with dollar signs.

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A command can have both Eshell and Lisp expressions.

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As you can see here,

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I have a call to ripgrep,

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but part of it is an s-expression.

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Remember the differences:

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With parens, eshell treats it as Lisp,

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like the last line in my example.

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With braces, eshell follows these shell-like rules:

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First, if it looks like a number, it's a number.

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Otherwise, eshell converts it to a string

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(quotes, like a shell, groups words).

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What about this mix between functions and executables

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for the first word?

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Functions that begin with eshell are called first.

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Next in priority are executables on your $PATH,

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then matching Lisp functions.

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You can actually switch this order

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with the `eshell-prefer-lisp-functions` variable.

NOTE 4. Emacs is better than shell

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4. Emacs is actually better than shell.

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If the following works, why would you call

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expr or bc or dc, or any of those other calculators?

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You can just call a Lisp expression.

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Why call less or more when you could call view-file?

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Here, I’ve aliased less to view-file.

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Load it up, and it shows up in an Emacs mode.

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Just like with less, if you hit q,

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you go back to your Eshell terminal.

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I do have an improvement, though.

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The problem with view-file is

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it takes a single file as an argument.

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In a shell, we might want to view more than one.

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So let’s make a solution to that.

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This function will call the first function

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with the first argument,

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and the second function with each of the rest.

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This allows me to make a version of less

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that calls view-file on the first [argument] given,

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but open in another window for each additional file.

NOTE 5. Better regular expressions

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5. Better regular expressions.

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Can’t remember regular expressions when calling

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grep or some other search function? Use the rx macro.

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Here I call ripgrep again, but this time,

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I’m using a Lisp expression calling the rx macro

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to look for UUIDs in the files in my current directory.

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But I have another improvement for this.

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While the rx macro is freaking cool for Emacs Lisp,

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it doesn’t always translate to regular expressions

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accepted by most commands.

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The (I have no idea how to pronounce this) pcre2el project

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can convert from a Lisp regular expression

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to Perl-compatible regular expressions (PCRE)

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acceptable by most search commands.

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I’ve created a new macro here, prx,

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that translates the output of the rx macro.

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This allows me to type something much more readable,

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and probably easier to remember.

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Certainly easier than this freaking regular expression.

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I’ve got an even better improvement.

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The rx macro with regular expression snippets

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can be assigned to key words

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that I can then take advantage of.

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Now our command would be much simpler to type.

NOTE 6. Loops are better with predicates

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6. Loops are better with predicates.

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Let’s say you want to remove the execute bit

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from files that have it.

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In a shell like bash, you need both a for loop and an if,

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as you can see in this example.

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With eshell, use a predicate to combine into a simple loop.

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The paren x after a file glob

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filters for only files marked as executable.

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Now here is another improvement.

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Since we often type loops to execute on one command,

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what about creating a function

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that can do this all in one go?

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This do function splits the arguments on that double colon,

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where the left side is a single statement to run,

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and the right side is a list of files.

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I have to append and flatten it

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in order for it to work.

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It loops through each file,

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creating an eshell command with the file appended.

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With this, I can remove the execute bit

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on all CSV files that have it.

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I see that my example wasn’t too good, as most commands

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like chmod accept multiple files, but you get the idea.

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In my final, larger form on my website,

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I don’t assume the command expression accepts

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a file as a final argument,

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as I can also replace underscores with the filename.

NOTE 7. Output of last command

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7. Output of last command.

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Most shells have a special variable

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like $? for the exit code of the last command.

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While reading through the source code,

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I noticed that the $$ refers to

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the output of the last command.

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This seems pretty cool.

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However, Eshell returns true or nil

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when running external commands,

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so accessing the output from a call to ls

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doesn’t work as expected.

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But this is Emacs.

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We can fix that.

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After running any command, eshell sets these four variables.

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I can hook a function call after every Eshell command.

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Using buffer-substring,

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I store the output into a global variable,

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and extend Eshell’s special variables list.

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In my Emacs configuration,

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I turned this variable into a ring,

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so while $$ works,

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so does array sub-scripting on that variable.

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This allows me to run a command

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and use the output from that command more than once.

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The code for this is a bit longer,

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so you’ll need to see my Emacs configuration for details.

NOTE 8. Redirection back to Emacs

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8. Redirection back to Emacs.

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Output of any command

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can go to kill-ring (or the clipboard).

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Think of the implications.

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You don’t have to go into text selection mode.

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Just grab the output.

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In fact, with our $$ improvement,

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we can always copy the output from the last command

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to the clipboard.

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Better yet, let’s write the output

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to our engineering notebook.

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Here’s my idea.

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First, create a capture template that takes a string,

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or if called interactively, the region,

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and that does an immediate-finish after inserting

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that string to the default notes file.

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Next, create a wrapper function

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to call org-capture-string to run that template.

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Finally, we add our new function to eshell-virtual-targets.

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Let’s see this in action.

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I have a CSV file of user information.

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I can use grep and cut to extract some of that

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and write it out to this month’s engineering notebook.

NOTE 9. Using Emacs buffers

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9. Using Emacs buffers.

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Why leave the results of eshell commands

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in the *eshell* buffer?

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Send the output into a buffer where you can use it.

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Here’s a call to ripgrep

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that searches for lines with email addresses

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using a complicated regular expression

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that I added to my prx macro.

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When I switch to this almost-grep buffer,

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I can turn on grep-mode.

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Now I can jump around as if I just called grep directly.

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Perhaps I’m proficient with my prx macro

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to filter out entries,

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but not good with shell commands

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that I can use in pipes to extract just one…

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the address column, for instance?

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Let’s just extract it,

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send it to a buffer called email-list,

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and now I can use Emacs commands that I know and love

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to edit the data directly.

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We currently have an over-sight

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that the Eshell’s built-in cat command

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doesn’t pipe buffer contents as standard in.

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So I created a bcat, a buffer cat, function to do this.

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So this command works

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to grab my email addresses I just extracted

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and send them to another program.

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If you’re interested, I have a more elaborate

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and yet simpler workflow surrounding sending data

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back and forth from Eshell to Emacs buffers.

NOTE 10. cd to remote systems

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10. Did I mention that you can cd to remote systems?

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This command uses SSH to jump to my host, goblin,

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start a root session, and jump to the etc directory.

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Remember that Tramp can be finicky

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if you start blinging your remote hosts with oh-my-zshell,

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and funky prompts and things like that,

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so your mileage may vary.

NOTE Summary

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In summary: Use eshell if you want

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a quick way to run commands and Emacs functions as a REPL,

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or to run an OS program but process the output with Emacs.

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Keep in mind that Eshell has two types of subshells,

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and you can mix and match during a command call.

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The rx macro is really cool.

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Eshell loops are better with filters and predicates …

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if you can remember them.

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Take advantage of Emacs buffers

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to really enhance your shell experience.

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You’ve now seen that just like Emacs,

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I’ve crafted Eshell to be my own shell creation,

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tailored to my workflow.

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So, steal my spells, cast your own magic,

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but feel free to share your incantations back to me.

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I’ve gone over my time allotment, so we’ll have to

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continue this discussion on the intertubes.

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Why yes, I have joined the birdless diaspora,

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so toot me over there.

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Thanks.