Spend the time to organize your file system when you start a new project. Your future self will thank you!
Always save a write-protected copy of your raw data.
A path that refers to a particular location in a file system.
Absolute paths are usually written with respect to the file system’s
and begin with either “/” (on Unix) or “\” (on Microsoft Windows).
See also: relative path.
A value given to a function or program when it runs.
The term is often used interchangeably (and inconsistently) with parameter.
A remark in a program that is intended to help human readers understand what is going on,
but is ignored by the computer.
Comments in Python, R, and the Unix shell start with a # character and run to the end of the line;
comments in SQL start with --,
and other languages have other conventions.
current working directory
The directory that relative paths are calculated from;
the place where files referenced by name only are searched for.
Every process has a current working directory.
The current working directory is usually referred to using the shorthand notation . (pronounced “dot”).
A set of files, directories, and I/O devices (such as keyboards and screens).
A file system may be spread across many physical devices,
or many file systems may be stored on a single physical device;
the operating system manages access.
The portion of a file’s name that comes after the final “.” character.
By convention this identifies the file’s type:
.txt means “text file”, .png means “Portable Network Graphics file”,
and so on. These conventions are not enforced by most operating systems:
it is perfectly possible (but confusing!) to name an MP3 sound file homepage.html.
Since many applications use filename extensions to identify the MIME type of the file,
misnaming files may cause those applications to fail.
A program that transforms a stream of data.
Many Unix command-line tools are written as filters:
they read data from standard input,
process it, and write the result to standard output.
A terse way to specify an option or setting to a command-line program.
By convention Unix applications use a dash followed by a single letter,
such as -v, or two dashes followed by a word, such as --verbose,
while DOS applications use a slash, such as /V.
Depending on the application, a flag may be followed by a single argument, as in -o /tmp/output.txt.
A loop that is executed once for each value in some kind of set, list, or range.
See also: while loop.
graphical user interface
A user interface based on selecting items and actions from a graphical display,
usually controlled by using a mouse.
See also: command-line interface.
The default directory associated with an account on a computer system.
By convention, all of a user’s files are stored in or below her home directory.
A set of instructions to be executed multiple times. Consists of a loop body and (usually) a
condition for exiting the loop. See also for loop and while loop.
MIME (Multi-Purpose Internet Mail Extensions) types describe different file types for exchange on the Internet,
for example images, audio, and documents.
Software that manages interactions between users, hardware, and software processes. Common
examples are Linux, OS X, and Windows.
A variable named in a function’s declaration that is used to hold a value passed into the call.
The term is often used interchangeably (and inconsistently) with argument.
The directory that “contains” the one in question.
Every directory in a file system except the root directory has a parent.
A directory’s parent is usually referred to using the shorthand notation .. (pronounced “dot dot”).
A connection from the output of one program to the input of another.
When two or more programs are connected in this way, they are called a “pipeline”.
A running instance of a program, containing code, variable values,
open files and network connections, and so on.
Processes are the “actors” that the operating system manages;
it typically runs each process for a few milliseconds at a time
to give the impression that they are executing simultaneously.
A character or characters display by a REPL to show that
it is waiting for its next command.
(in the shell):
Using quotation marks of various kinds to prevent the shell from interpreting special characters.
For example, to pass the string *.txt to a program,
it is usually necessary to write it as '*.txt' (with single quotes)
so that the shell will not try to expand the * wildcard.
(REPL): A command-line interface that reads a command from the user,
executes it, prints the result, and waits for another command.
To send a command’s output to a file rather than to the screen or another command,
or equivalently to read a command’s input from a file.
A pattern that specifies a set of character strings.
REs are most often used to find sequences of characters in strings.
A path that specifies the location of a file or directory
with respect to the current working directory.
Any path that does not begin with a separator character (“/” or “\”) is a relative path.
See also: absolute path.
The top-most directory in a file system.
Its name is “/” on Unix (including Linux and Mac OS X) and “\” on Microsoft Windows.