I switched from bash to zsh a few years ago and I am never looking back. It
has awesome tab completion: for example cd doc/sh/pl becomes cd
Documents/shared/planning. It also expands git commands and branches,
environment variables and other things. It has extended file globbing which
provides a good replacement for find. If you spend a lot of time in the
shell, you should really give zsh a try.
Whatever shell you use, it’s worth spending a few minutes configuring your
prompt. The prompt is something you will see literally thousands of times a
day. So why not make it useful?
Here is mine. First, let’s open a new shell in the home directory:
Simple and to the point. ~ sweet ~! Let’s go into some directory:
It displays the path, from the home directory, with the current directory
displayed in bold (“planning”). To keep the prompt short, we only display the
last 3 subdirectories:
And now let’s move into a git repo:
The prompt displays the path starting at the root of the repo (“org”), and also
the current branch in green (“master”). Now let’s move into a subdirectory in
the same repo:
The root of the repo is highlighted (“org”), and the current directory is
displayed in bold like before. We could also display something indicating if
the repo is clean or if it has uncommitted changes, but I prefer to keep it
Notice the function current_git_branch: this is the fastest way I have found
to get the name of the branch. The trick is to make the execution of the prompt
as fast as possible, since it gets executed every time you hit Enter.
This is the second article in a series about Emacs Org mode. It assumes that you read
the first one.
Today we’ll talk a bit about
literate programming. The
idea is to write an Org mode document that includes snippets of code, and
possibly the result of their execution as well. It kind of reverses the way you
think about documenting code: instead of adding comments inside your program,
you add your program’s code inside the documentation file. In the end the
result is the same, you still get executable code.
Org mode includes a feature named
Babel which allows for
embedding code in any programming language you fancy. Let’s get started.
First, we need to make sure that Babel is enabled for a few other languages
than ELisp (the only one by default). Add the following in your emacs
configuration and evaluate it with M-C-x (or restart Emacs):
First open a new buffer literate.org, with this content:
The #+TITLE directive is just to add a title to the Org file; it is not
really needed. The other directives we use are #+begin_src and #+end_src
which respectively introduce and close a code block for a particular
language. Note that Emacs automatically applies the proper syntax highlighting.
Type C-c C-c to evaluate the code block under the point. After confirmation,
Babel will insert a new block #+RESULTS with the result of the evaluation,
What happened here? Emacs forked a process to execute the code using Python,
and got back the returned value, and inserted it into the buffer.
By default the result is the returned value i.e. the last expression that was
executed, but you can also use the output of the program as the result with a
directive like: #+begin_src python :result output. For example, evaluate the
shell statement below, again using C-c C-c:
Calling code blocks
Babel also allows you to refer to code blocks from elsewhere in your document,
by labeling each block with a name, using directive #+name. Let’s say we have
some Ruby code to revert a string (yes, I know we could use the native
We can now call this block. Note that we get the result of the block
evaluation. So if you want to use the result of a function in the block, you
also need to add the call to that function (see the last line).
To call the block, add a #+call directive referring to the block’s name and
binding the input variable str, and type C-c C-c with the point onto it:
Here is a more practical example to illustrate the power of Babel, using
different languages to get the job done. This is actually similar to an iPython
Suppose I want to produce a report containing statistics about the words that
are used in a collection of files. Assuming all these files are in the same
directory (let’s say they are Markdown files), this shell script counts the
number of unique words in each file:
The result has multiple lines, one per file. Therefore it is stored into an Org
Now let’s compute the standard deviation. We could follow up with another shell
script, but it is easier to do this in ELisp. Add this block in the buffer:
The #+begin_src directive uses the :var keyword to bind the Lisp variable
samples to the output of the words code block (which is the table
above). In Lisp that samples variable will be set with a list of
lists. Basically each row in the table is a sublist, and since the table has
only one column the sublists only contain a single number, like this:
If you evaluate the block with C-c C-c, you get the final result:
You can of course re-run the report anytime you want. Once we have our report
ready, Org mode is able to export it into plenty of different formats, choosing
exactly what should be included and excluded. But that’s a discussion for
If you are a programmer, there probably isn’t a day that goes by when you don’t
need a calculator, for example for things like base conversion. After all, like
Dijkstra said, programming is math.
Emacs includes a calculator, which is actually quite sophisticated. It is
capable of both algebraic and symbolic calculations; it has a ton of functions
and it can handle matrices or solve equations, if you need that sort of
things. It can even create graphs if you have GnuPlot installed on your
Here we will just review the basics. Check out the
manual for more. There
a nice Info manual embedded in Emacs which you can bring up using C-h i g
Start the calculator using M-x calc. It will show up at the bottom of the
screen. Type q to quit. Once the calculator is started, you can hide and
redisplay it using C-x * *.
Calc uses the RPN
(Reverse Polish Notation) logic. Which is only fair, since Lisp uses the Polish
notation! Anyway, if you are a fan of the HP calculators, you will be just at
Calc displays a stack of numbers. When you type a number, it accumulates in the
stack. When you type an operator such as + or *, it operates on the last 2
numbers of the stack, and replaces those numbers with the result.
Give it a try: Type M-x calc, then the following:
P (no need to press Enter)
The P inserts π. Now you have 3 numbers in the stack, and you can calculate
the circumference of a circle of radius 10 (C = 2 π r). Press * once, the
stack now shows 2 and 31.4159265359. Press * another time and you get a
single number which is the result.
Because the dash key (-) is reserved for subtraction, you should use an
underscore (_) to enter a negative number. Note that the calculator supports
arbitrarily big numbers. If you want to challenge it, try calculating 2^200
(that’s 2 Enter, 200 Enter, then ^). The default precision is 12 but you
can change it using p.
Type n to negate the last number, and TAB to flip the last 2 numbers. The
delete key deletes the last number on the stack.
If you need to use complex numbers, enter them like (a,b). If you need
fractions, enter them like 1:3 (1/3). You can also enter an algebraic
expression directly by typing ' (the quote key) then your expression, such as
2 + (10 / 5). The result will be put on top of the stack.
Most functions use a single key. For example, type s for Sin, c for
Cos, t for Tan, etc. The more complex commands are modal: they start with a
prefix key, which leads to a prompt for the subcommand key. If you get lost,
type ? and the mini-buffer will show you what is available.
If you make a mistake, type U to undo and D to redo. Of course this is
Emacs, you have an infinite number of undos and redos. You can also reset the
calculator using C-x * 0.
You can enter any number in the format base#number. Example: 16#FF is
immediately converted to 255.
For the reverse, you need to set the output display mode. In this example, d r
16 followed by Enter sets the display to base 16. d r 2 sets it to
binary. Set it to base 10 to get the default behaviour again.
In this short post we’ll review a few features of Emacs that are not well
known, but really worth knowing.
The simplest way to repeat a command is to type Control-a number, then the
command. For example if I type C-6 C-0 ~ (that’s 6 then 0 with the Control
key down, then the tilde character), I get a line with 60 tilde:
Another way is to use the universal argument: we saw how to write an
interactive Lisp function that accepts a numeric argument in the
previous article. forward-line is such a function;
it goes down N lines (by default one line). For example C-u 1 0 DOWN
(Control-u then 1 then 0 then the down arrow key) goes down 10 lines. You can
do the same by calling the function explicitly with C-u 1 0 M-x forward-line.
Registers are used for saving temporary data in the current session. They can
store positions or bookmarks, text, numbers etc. Registers have a name which is
a letter or a number: a to z, A to Z (so case-sensitive) and 1
to 9, which gives you a total of 62 registers. That’s more than enough.
C-x r is the prefix for all register operations. Generally you type this
prefix, then a key to specify what operation you want (e.g. save or read), then
the register name which is one extra key.
For example, open any file you have in one of your projects, go to some
position in the file, and hit C-x r SPC a (where SPC is the space
bar). This command stores the buffer position in register a. Now go to a
different buffer such as the scratch buffer, and type C-x r j a: this command
will jump back to the buffer and the position you just stored in register a.
Note that if you close the file and then retype the same command, Emacs will
ask you if you want to reopen the file, and it will bring you back to the same
position again. This can be useful if you want to keep bookmarks in a large
project, for example if you keep going to the same files and bits within these
files, but you don’t want to keep them open all the time.
You can view the content of register a with command M-x view-register then
a at the prompt. It will show a window saying that this register stores a
position in your buffer (note that the position is a number, which represents
the offset from the beginning of the buffer). Type C-x 1, or q in the other
window, to dismiss it.
A bookmark is similar to a buffer position but with a twist: you give it a name
and it is persisted between Emacs sessions. Note that bookmarks are not
actually associated with registers, but they use the same C-x r prefix.
The command is C-x r m (mark) which prompts for a name. By default it
proposes the current buffer name but you can choose whatever you want. You can
view the list of bookmarks with C-x r l (list). Just click on a bookmark to
jump to it. You can also jump to a bookmark with C-x r b (bookmark) which
prompts for the name using auto-complete. The nice thing about bookmarks is
that they are saved on the filesystem when you exit Emacs, and they are
available when you restart it. You can also force save with M-x
Sometimes you want to save a snippet of text somewhere, so you can paste it
later. One way is to use the kill ring: M-w to save the selected text
(which is what Emacs calls the region), then C-y to paste. The kill ring
saves all copy operations you have done so far, so if you want to paste the
second previous thing you copied, type C-y followed by M-y (repeat M-y to
go back in history).
Another way is to save the region in a register, which you do with something
like C-x r s a (save in register a). Now if you want to insert the content
of a register at the current point, type C-x r i a (insert the content of
This is less useful than the above, but you can also save a window
configuration in a register. For example, split the screen horizontally with
C-x 2, then split the current window vertically with C-x 3. You now have 3
windows displayed, which you can resize as you see fit.
Let’s save this layout in register a with C-r w a. Now dismiss all other
windows than the current one using C-x 1. If you want to restore the layout,
type C-r j a (it is the same key for jumping to a buffer position). Note that
this only saves window configurations and not buffers content: if you close one
of the buffers Emacs will not reopen it for you.
The table below summarizes the keys we just learned.
C-x r SPC a
Store the current position to register a.
C-x r j a
Jump to the position stored in register a, or restore the window positions stored in register a.
C-x r s a
Save the selected region in register a.
C-x r i a
Insert the text saved into register a.
C-x r w a
Save window positions in register a.
C-x r m
Save a bookmark.
C-x r b
Go to a bookmark.
C-x r l
Editing macros are a very powerful feature of Emacs. After all, Emacs stands
for “Editing Macros” 1.
There are several keys for macros, but really you only need to remember two of
them: F3 and F4. The first one records a new macro. The second one
terminates the recording, if you were recording a macro; otherwise it executes
the macro. If you make an mistake while recording a macro, hit C-g to abort,
and start over.
Let’s take an example. Suppose I have this text:
Now suppose I want to make each line start with a capital letter and end with a
period. I could edit the text manually because it is only 3 lines, but just
imagine that it is much longer for argument’s sake, in order to make the use of
a macro more compelling.
The way to do this with a macro is simple: fix the first line, while recording
a macro. Then execute the macro N times, one time per remaining line. To record
the macro do the following:
Move the cursor to the beginning (e.g. M-<).
F3 to start recording.
M-c to capitalize the first word (The).
C-e to go to the end of the line.
. to insert a period. Now the first line is good.
C-a to go to the beginning of the line (where we started), and the down
arrow to go to the next line.
Now type F4 to stop recording. Then F4 again to run the macro on the second
line. Then F4 again to run the macro on the 3rd line. You’re done!
You could also run the macro N times using Control-a number then F4, as we
saw earlier. You can also apply the macro to a whole region by selecting the
region and running M-x apply-macro-to-region-lines, which is neat.
If you want to see macros at their best (and incidentally Emacs beat Vim
at its own game), check out this quick video:
The entire series of Emacs Rocks is worth watching.
That’s it for today. Lots more to come. Stay tuned!
Note that Emacs editing macros have nothing to do with Lisp macros: one is a trick to save a sequence of keys and repeat it, the other is a Lisp function that executes twice, at compilation time and at run time. ↩
This is the second article in a series about Lisp; it assumes you read
the first one.
If you use Emacs but don’t know Lisp, you are missing a lot: Emacs is
infinitely customizable with Emacs Lisp. This post is an introduction to ELisp,
hopefully giving you enough basics to write useful functions. Today we will
mostly focus on the language itself, as opposed to the gazillion of
Emacs-specific APIs for editing text.
My goal is not to review every function of the language: it would take a book
to do so. My goal instead is to give a good high-level overview of Elisp. If
you find yourself looking for a function or variable, you can browse the
Emacs elisp site
or you can use M-x apropos which displays anything that matches a given
Let’s start with…
The Basic Types
Strings are double quoted and can contain newlines. Use backslash to escape
A string is a sequence of characters. The syntax for a character is ?x:
question mark followed by character. Some need to be escaped, like for example
?\(, ?\) and ?\\.
There are many functions operating on strings, like for example:
Note that none of these functions have any side effect, as it is the case with
most functions in Lisp - they are pure functions. They create a new object and
Integers have 29 bits of precision (I don’t know why) and doubles have 64
bits. Binary starts with “#b”, octal with “#o” and hexadecimal with “#x”.
The most useful data structure in Lisp is a list, but the language also has
arrays, hash tables, and objects. An array is called a vector, and you can
create one like so: [ "the" "answer" "is" 42]. Like lists, they can contain
objects of various types. You use spaces to separate the values; comas are part
of the Lisp syntax but they are used for something else as we will soon see.
The quote is a special character in the Lisp syntax that prevents an expression
from being evaluated. For instance:
The quote prevents the evaluation of the symbol “a” on the first line, and the
list on the second line, otherwise they would be considered as a variable and a
function call respectively.
The backquote is like a quote, except that any element preceded by a coma is
evaluated. The backquote is very handy for defining macros, e.g. functions that
generate code. For example:
Lisp is a dynamically-typed language, like Ruby or Python and unlike Java or
C++. You don’t need to declare the type of a variable, and a variable can hold
objects of different types over time.
We already saw in the previous post how to declare a global variable with
defvar and set it with setq. Another way to use variables is function
Here we define a function add with 2 arguments, which returns the sum of its
arguments. Then we call it. message is an Emacs function similar to C’s
printf: it prints a message in the mini-buffer and in the messages
Every time you call add, Lisp creates new bindings to hold the values of
x and y within the scope of the function call. A single variable can have
multiple bindings at the same time; for example the parameters of a recursive
function are rebound for each call of the function.
The let form declares local variables. The syntax is (let (variable*) body)
where each variable is either a variable name, or a list (variable-name
value). Variables declared with no value are bound to nil. For example:
The scope of the variable bindings is the body of the let form. After the
let, the variables refer to whatever, if anything, they referred to before
the call to let. You can bind the same variable multiple times:
Note that let binds variables in parallel and not sequentially. That means
that you cannot declare a variable whose value depends on another variable
declared in the same let. For example this is wrong:
There are two ways to fix the code above: you could use a second let within
the first, or you could replace let with let*: it binds variables
sequentially, one after the other. The key to understand that is to remember
that the origin of Lisp is the
Lamda Calculus, where
everything is a function call. The first let form above is equivalent to
calling an anonymous function like this:
Here we define a lambda (anonymous) function with 2 arguments, and we call it
with the values of the arguments. The syntax of a lambda is (lambda
(arguments*) body), and we call it like any other function by putting it in a
second pair of parentheses with the arguments.
The equivalent of a let* requires multiple function calls:
The first lambda binds x to 1 and the second lambda binds y to x * 10.
In ELisp and Common Lisp, nil is used to mean false, and everything that is
not nil is true, including the constant t which means true. Therefore a
symbol is true, a string is true and a number is true (even 0). nil is the
same as () and it is considered good taste to use the former when you mean
false (or void) and the latter when you mean empty list. Note that Clojure and
Scheme treat boolean logic differently: for them the empty list and false are
Let’s start with simple boolean functions. not returns the negation of its
argument, so that (not t) returns nil and vice versa. Like most functions in
Lisp, and and or can take any number of arguments. and returns the value
of the last argument that is true, or nil if it finds an argument that is not
true. or returns the value of the first argument that is true, or nil if none
of them are true. For example:
You can compare for equality using = for numbers, string= for strings or
eq for same address in memory. There is also a generic equal function that
tests if the objects are equal no matter what type they are, so that’s the only
one you need to remember.
(if then else*) is a special form that is equivalent to C’s ternary operator
?:. It must have at least a then form and can only have one. It may have
one or more else forms. It returns the value of the then form or the value
of the last else form. For example:
If you just want a then or an else, it is better to use when and unless
because they can have multiple then or else forms. They return the value of
the last form or nil. Here is an example:
Finally cond is like a super-charged version of C’s switch/case: it chooses
between an arbitrary number of alternatives. Its arguments are a collection of
clauses, each of them being a list. The car of the clause is the condition,
and the cdr is the body to be executed if the condition is true (the body can
have as many forms as you like). cond executes the body of the first clause
for which the condition is true. For example:
The code above uses predicates like numberp which returns t if the argument
is a number. The function current-buffer returns a buffer object which is
neither a number, string, list or symbol (it is an instance of a class). Notice
the last clause: the condition is t which is obviously always true. This is
the “otherwise” clause guaranteed to fire if everything else above has failed.
The simplest loop is a while:
dotimes takes a variable and a count, and sets the variable from 0 to count -
dolist takes a variable and a list, and sets the variable to each item in the
If you need anything more complicated, take a look at the documentation of the
loop macro. This is a very powerful macro with lot of options that takes an
(almost) English sentence as argument and generates what you mean. For example,
a C “for” loop can be expressed like so:
Another example is the following code which iterates over a “plist” (property
list) which is a collection like (key1 value1 key2 value2) using cddr to
move by 2 items at a time and skipping the properties where the key is an even
Elisp also has exceptions, try/catch/finally and anything else you would expect.
Lisp uses several keywords for declaring arguments within a defun.
&optional introduces optional arguments, which if not specified are bound
to nil. For example (defun foo (a b &optional c d) ...) makes c and d
&rest takes all remaining arguments and concatenates them into a list. For
example the signature of the list function is simply (&rest objects).
&key introduces a keyword argument, that is an optional argument specified
by a keyword with a default value of your choice. For example:
Functions are first class objects in Lisp. You can store them in a variable and
call them later. For example:
The syntax #'foo is sugar for (function foo) which returns the definition
of the function stored in symbol foo. It basically returns a pointer to the
code. funcall calls the function with a given list of arguments. Note that
Emacs is very tolerant and (setq f 'list) (e.g. setting f to the symbol
“list”) will also work.
apply works like funcall but it applies the function to a list of
An interesting example of using apply is mapcar which applies a function to
each element of a list and returns a list of the results:
Let’s use our fresh knowledge to do something useful.
Sometimes I want to include a separator in a comment, e.g. a sequence of dashes
or tilde that fills up the rest of the line until the 80 character column (the
fill-column variable defines that limit). For example, if I type “// Begin of
test” I want a magic key to do this:
Elisp functions must be declared “interactive” if you want to call then using
Meta-x or bind them to a key. You do this declaration by calling the
interactive special form (it’s not a function) as the first form in the body of
end-of-line move the cursor to the end of the line, as you probably
guessed. The let form calculates the number of characters to insert before it
reaches the end of the line using the variable fill-column (which should be
set to 79) and the current-column function which returns the cursor’s column
number. The insert function inserts a character or string at the position of
the cursor. Finally global-set-key binds the function to a key chord. Note
that this is a simple implementation; it might be more efficient to create a
string with n characters using (make-string num-chars ?~).
Let’s write another one. Suppose you work in an organization that has created
its own code style, and suppose that said code style proclaims that lines
longer than 80 characters are a cardinal sin. Believe me, such code styles do
exist. So let’s write an interactive function that will find the next “long”
line in the current buffer, from the position of the cursor. It could look like
This interactive function takes a numeric argument which is the max length of
lines. The “P” string in the call to interactive specifies that we use an
argument (in raw form; see the documentation of interactive for
details). Either the user invokes this function with M-x goto-long-line, in
which case the argument len is set to nil, or she invokes the function with
C-u 7 9 M-x goto-long-line, in which case the argument len is set to 79 (for
instance). The first setq line is used to set a default value to len:
either it is the number that the user specified or it is the value of variable
Without going into too much details, the rest of the code is a while loop
until we have found a line or we reached the end of the buffer (predicate
eobp). At each step we go down one line (forward-line) and we check the
length of the line. Note that the Emacs function point returns the position
of the cursor as an offset into the file (the current character number if you
will). Our function is designed to be called both interactively and within a
program, so it tests how we are called using predicate called-interactively-p
before deciding to print a message or not. point-min returns the position of
the first character in the buffer (should be 1) and goto-char goes to a given
Note that sometimes the compiler complains when you call a function that is
designed to be used interactively in your code (these functions are marked as
such using a property). Usually the warning says you should use another
function, supposedly more efficient because doing less tests.
That’s it for today. Lots more to come. Stay tuned!
A right click in the mini-buffer pops up the message buffer. That’s a nice trick for debugging if you have a lot of traces. ↩