These definitions are not exact as I've tried to simplify the
concepts. They may be incorrect in some circumstances. They are
definitely not complete. Please see e.g.
http://www.webopedia.com/ for more comprehensive definitions.
Please send suggestions (and definitions!) to me.
- ADS - Astronomical Data Service - NASA's database to find
all published papers and recent preprints. Go to
- Algorithm - A set of instructions on how to solve a
problem - usually programmed into the computer in the form
- Application - Another word for a program, typically a
program used by a normal user.
- awk - One of the original scripting languages - useful
for short operations.
- astro-ph - The astrophysics section of
arXiv.org. Probably the one you should read.
- Backup - Storing another copy of data to make sure it
doesn't accidently get lost. See tar.
- Bash - The GNU Bourne-Again SHell - a good shell for
- Bit - A binary digit - Either a 1 or a 0. bits can
hold different numbers.
- Bitmap - An image consisting of a set of pixels or dots,
each capable of different values or colours. Observation
astronomical data often consists of bitmaps stored in a FITS file.
Unlike Vector Graphics, bitmaps are often ``jaggy'' when scaled.
- Byte - 8 bits of information - typically holds value from
0 to 255 (). One byte can store one character in English (but
some programs use Unicode to store text, which may require more than
one byte per character).
- C - The C programming language (successor to BCPL) is
widely used outside science, but is useful for scientific
programming. Most of UNIX is written in C.
- C++ - C++ is a development of the C programming language,
but with Object-Oriented and Generic Programming features (I won't
define those for you). The language is widely used outside science,
but not so widely in science. It is famed for its steep learning
curve (which doesn't have to be so steep) and its power.
- Client - A computer or a program which is being given
information by another computer or program (the Server).
- Command - Some text to tell the computer to do something
- may refer to a command in a programming language, an executable,
something interpreted by a shell, or a instruction to another
program (e.g. IRAF).
- Compiler - A program which takes text in a particular
programming language, and produces either an Executable or an Object
- Configuration file - A file containing commands to modify
the behaviour of a program. See Dot Files.
- CPU - Central Processing Unit - the part of the computer
which executes a program. There are different sorts of CPUs (e.g.
Sparc, Alpha, i386). Typically programs written to run on one CPU do
not run on another sort (plenty of exceptions here, though).
- csh - The C Shell - A C-like shell. Don't use this.
- Data - Data consists of a series of bytes on most
machines. Data in a file can either be stored as it is in the
computer's memory (raw or unencoded), or stored in text format
- Data reduction - Taking unprocessed astronomical data and
getting it in a more understandable form (e.g. calibrating it).
- Device - Something attached to your computer - can be
internal to the machine like a disk, or external like a mouse, or a
logical device-like entity. Devices include disks, tape drives,
mice, keyboards, terminals. In UNIX devices often have an associated
device file in the /dev directory which represents the device and
can be used to operate it. Special devices include /dev/null which
throws everything away, and /dev/zero which provides infinite
numbers of zeros for you enjoyment.
- Directory - A name for a set of files and directories.
Directories can contain other directories (This can make a ``tree''
of directories). Directories can also refer to different devices to
their parent - hard disks, floppy disks, network drives. All
directories in UNIX have the root directory (``/'') back in their
- Disk - See Hard Disk or Floppy Disk (also others). A disk
is usally a permanent (hopefully!) storage device.
- Dot File - A file with a name starting with a dot. These
files are hidden from a normal ls (try ls -a instead). Dot files in
your home directory are typically configuration files. The other
common dot files are ``.'' and ``..'' which exist in every
directory, meaning the current directory and the parent directory.
- Editor - A program for editing (usually) a text file.
Examples are emacs, vi, ed and nedit. People are very opinionated
about which is the best editor! (Search on the internet for
- emacs - A very popular and powerful editor. You can do
anything in emacs but it may not be easy to find out how to do
it. The editor can be extended using Lisp code to do any sort of
text manipulation you like.
- Executable - File stored on a disk which can be run on
the computer. Executables are marked with the executable attribute
(See the chmod command). Executables are stored in a particular
format (e.g. ELF), and consist of headers plus machine code.
- Extension (of a filename) - Conventional ending for a
file name (usually dot-something) to tell you what sort of file it
is (e.g. .a - library, .c - C source code, .h - C source header
code, .html - Web page, .pl - Perl script, .sh - sh shell script,
.tex - LATEXor TEXinput file...)
- File - Where data are permanently stored on ``normal''
computers. On UNIX consists of a sequence of bytes up to a certain
length. Files may contain normal ASCII text or a random sequence of
bytes which can be interpreted by a program. Files are stored in
Directories on Disks.
- Finger - See who's on a computer, e.g. finger @cass00.
- FITS file - Standard astronomical file format. Usually
used for observational data but also suitable for theoretical
work. FITS files consist of a series of ``extensions'' and a set of
- Floating point - How real numbers are stored on a
computer. Numbers are stored as a sign (, +1 or -1), a mantissa
(, ), and an exponent () in a similar form to
, e.g. 512.43 in base 10 would be
. The number of bytes used for each part
is larger for doubles than single floats, meaning operations are
more accurate. Floating point works well but you can get big
problems if you aren't careful (e.g. don't subtract large numbers
from each other, don't compare floating point numbers to be an exact
number). There are long documents on floating point numbers (see
the IEEE standard). See for very useful information if you're
doing numerical work http://docs.sun.com/db?p=/doc/800-7895
- Floppy - A small transferable disk holding about 1.4
- Fork - Splitting a process into two - a Parent and a
Child. This is the way other processes are run on UNIX.
- FORTRAN - FORmula TRANslation - The original scientific
programming language. FORTRAN 77 (an earlier dialect) lacks modern
features (such as dynamic memory allocation) but is in wide-spread
use. Fortran 90/95 is a much better version, but is less widly used.
- Home directory - A personal directory belonging to a
particular user. Home directories are normally given the name
- FTP - File Transfer Protocol - a method of transferring
files between different computers on the internet. It's to be
avoided if you need to type a password to get the file as the file
goes over the network in Plain Text, which can be ``sniffed''. Use
scp or sftp instead if possible.
- Gigabyte - 1024 Megabytes, bytes. Still a fair
chunk of information.
- GIF - A bitmap image format. Use a PNG instead if you can.
- GPL - GNU Public Licence - A licence applied to a program
to specify it can be distributed and modified to/by anyone, but if a
modified version is distributed, the source must be distributed too
(this is the general idea). See http://www.gnu.org/
- GNU - GNU is Not Unix - A set of programs written by the
Free Software Foundation to provide a Free (as in libre) UNIX
framework. GNU is a large component of a Linux distribution. GNU
software include emacs and gcc. See http://www.gnu.org/ . GNU
software is licenced under the GPL.
- grep - The grep command lets you find lines in file(s)
with particular text or regular expressions.
- Grid - A form of Parallel computing, allowing jobs to be
submitted to a set of machines.
- GSL - GNU Scientific Library - Free library for
scientific and numerical work, including integration and equation
- Hard Disk - A large non-portable disk attached to (or in)
a computer capable of holding a large quantity of data.
- HTML - HyperText Markup Language - The language by which
Web Pages are described. HTML is simple text with added codes to
- HTTP - HyperText Transfer Protocol - The protocol by
which Web Pages are transmitted. Other things can use this protocol
- HTTPS - A secure version of HTTP which encrypts data.
- IDL - A proprietary programming environment with strong
numerical and graphing features. The down sides are that you need a
licence to run this on all machines, and the syntax is particularly
poor. It is also quite slow for some programming cases.
- info - A system like man but providing more information
about GNU programs, like emacs. Run info to try it.
- Internet - The global network of computers running
TCP/IP, which the IoA is connected to.
- IP address - Internet Protocol address - each computer on
the internet (with exceptions) has a unique 4-byte number
identifying it, which is called an IP address. A name like
www.bbc.co.uk is converted to an IP address by a DNS (Directory Name
Service) server. This is useful as you don't need to remember lots
of numbers like 188.8.131.52
- IRAF - The Image Reduction and Analysis Facility - A
standard astronomical package for data reduction.
- JANET - The Joint Academic NETwork - The network linking
universities in the UK together. It is linked to the Internet.
- Job - Using a program with particular inputs. e.g.
submitting a job.
- JPEG, JPG - The JPEG format is a lossy (in that it loses
data from the original, but can look very good) bitmap image format.
It is widely used on the internet. The advantage of this format is
that it doesn't take much space up.
- K, Kilobyte - A kilobyte, or 1024 bytes ().
Anyone claiming this is 1000 bytes is wrong.
- Kernel - See UNIX
- Killing - Stopping a process permanently. Processes can
be killed ``nicely'' with kill PID, or hastily with kill -9 PID.
- Library - A set of Object Files combined together into
one file. Libraries are either static or dynamic. Static libraries
are linked into an executable when it is made (.a extension).
Dynamic libraries are linked when the executable is run. (.so
extension). Using static libraries is simpler than using dynamic
ones, but they need more space in the executable. Libraries are
sometimes standard (come with UNIX) like the C-runtime library, or
optional (like gtk). Lots more could be said here.
- Lisp - A computer language often used for text
processing. Used as the internal language in emacs.
- Machine - A commonly used word meaning a computer.
- Maple - An proprietary mathematics program. Maple can do
numerical work and symbolic work (like doing integrals). Might be
useful if you're a paper and pencil theorist, but cannot be relied
upon to do all your work. See Mathematica.
- man - The UNIX manual system. Very useful. Type man man.
- Mathematica - The original proprietary mathematics system
(see Maple). Maple and Mathematica claim to do similar things, but
with varying success.
- Megabyte - 1024 kilobytes, or 1048576 bytes ().
- Memory - See RAM
- Mount - The act of making a disk or network drive
available for use to the users of a system. Undoing this is called
- Mount point - A disk or network drive is ``mounted'' into
the file system at a mount point, which looks like a directory,
- N-Body - A way of simulating something by considering it
as separate bodies, linked by gravity.
- NAG - Numerical Algorithms Group - A proprietary computer
library for numerical work.
- Network - A system for allowing computers to
communicate using the transmission of packets of
information. They do this using a Protocol like TCP/IP. Networks can
be linked together (e.g. JANET, Internet)
- Newsgroup - A newsgroup is a discussion forum. There are
many newsgroups, for many subjects. Newsreading programs include
mozilla, trn and pine.
- NFS - Network Filing System - A system for accessing
files over the network as if they were on your local machine. Often
used for home directories.
- Object file - Compiled source code which can be Linked
together with other Object Files or Libraries to produce an
executable or library. Object Files typically have an .o extension.
- Parallel - Running code on more than one machine or CPU
in a machine at once.
- Perl - A very powerful scripting language but has arcane
syntax. Code can be very hard to read once written.
- PID - Process ID - Each process has an ID number (PID)
(normally 0 to ) which can be used to kill it or stop it.
- Ping - See whether a computer is on the network (e.g.
- Pipe - A connection between the output of one program and
the input of another. e.g. ls more, takes the output of ``ls'' (a
list of files), and displays it one screen at a time with the more
- PNG - Portable Network Graphics - An lossless bitmap
image format. Should be used instead of GIF which has patent
- Process - When a program is started it becomes a process.
You can run more than one copy of a program and you get more than
one process. Processes are started from a shell (or another process)
and can be killed using the kill or top shell commands.
- Program - Refers to a set of instructions which can be
``run'' on a computer. Programs are stored in a file (an Executable)
and are started from a shell (or indirectly from other processes).
The word program can also refer to the source code of the program.
- Programming language - A standard which specifies how
(sort of) human readable text is run on a computer. Programming
languages vary quite a lot in style (e.g. Procedural - made up of
blocks run from each other, Object-Oriented - program made of a set
of communicating isolated objects, Functional - everything is a
function!, ...), and make some things easier than others. There are
low-level languages which give you lots of control over the computer
and things run quickly, or higher level where you can get the
computer to do lots with little work but have less total control,
however these catagories aren't fixed. Examples include C, FORTRAN,
Perl, Python, BASIC, Pascal, FORTH...).
- Protocol - An accepted standard for transmission of data,
like TCP, HTTP, FTP.
- Python - A scripting language. Designed for simplicity
and clarity. Less widly used that Perl, however, but is a good
choice for large programs.
- Quota - The maximum amount of space you are allowed to
use on a disk. The quota can have a soft-limit (which may be
exceeded for some time), and a hard-limit (which may not be
- RAM - Random Access Memory (typically just called memory)
- fast memory in a computer which holds currently running programs
and data being processed. When the computer is switched off anything
in memory is lost.
- Recursive - See Recursive.
- Redirect - Send the output of one thing to another
(e.g. redirect to a file).
- Regular expression - A useful way of getting a program to
match particular bits of text. Often used in scripting languages to
process data files.
- Scanner - A device for transferring text or images from
paper to a computer.
- Script - A set of shell commands in an executable file.
- Scripting languages - A computer language designed for
ease of writing. Scripting languages are often used to process text
files or run external programs. Examples include Perl, Python, tcl
- SCP - Secure Copy - Sends files over SSH
- SCSI - A way of talking to devices on a computer like
disks, tape drives or scanners.
- sed - The Stream EDitor - A program for simple
manipulation of the contents of files.
- Server - A machine or program which is giving data to
another machine or program (the Client).
- SFTP - Secure FTP - Like SCP, but more like FTP.
- sh - The Bourne Shell - very basic but standard shell.
- Shell - A program which takes instructions (either by
someone typing them in or from a shell script file), and follows
them, including running programs. Shells include, sh (The Bourne
shell), csh (The C shell), bash (The Bourne-Again shell - bad pun!),
tcsh (An extended C shell). Shells vary in their features and ease
of use. Some shells are horrible (e.g. csh, tcsh) :-).
- Socket - A communication mechanism between programs. Data
can be sent over the network, making a Network Socket. This is the
primary mechanism of the Internet.
- Source code - Human readable (hopefully!) program text.
Source code can either be compiled and then stored as an executable
on the computer, or interpreted by an ``interpreter'' to run
directly (e.g. Python, shell, BASIC), though some things are between
compiler and interpreter.
- SPH - Smooth Particle Hydrodynamics - A way of simulating
- SSH - Secure SHell - A way to access another machine.
Data is sent encrypted between the machines making it hard to grab
information like passwords.
- Submit - To send something to another system - e.g. the
grid engine or a printer.
- Tape - A tape is used to back up data. DO THIS! A tape
looks like an audio cassette. Tapes come in different sizes and
- tar - A UNIX command used to combine (and optionally
compress) files together.
- tcl - Tool Command Language - An older scripting
language. Less powerful and harder to use than modern languages like
Perl or Python.
- TCP/IP - Transmission Control Protocol / Internet
Protocol - The Protocols the Internet uses to transmit data.
- tcsh - The exTended C-Shell - the default astronomy
shell. Not very good for programming.
- Telnet - An old protocol for connecting to a different
machine. Don't use it as it sends passwords in plain text. Use SSH
- Terminal - A terminal is a device (or application) which
allows you to type commands into a Shell. See XTerm.
- UNIX - The environment (Operating System) your computer
is running, providing the framework for your programs. There are
lots of different sorts of UNIX which have lots of subtle
differences (e.g. Linux, Solaris, Tru64, HPUX...). UNIX is made up
of a Kernel which provides common interfaces to the hardware, the
Runtime Library which bridges applications (or programs you write)
and the Kernel, and the Shell which lets you run programs. UNIX is
a pretty amorphous concept and also consists of the huge suit of
programs you can run, e.g. sed, ls, rm, ping...
- Vector graphics - An image consisting of geometrical
shapes, like lines, circles and text. Vector graphics make suitable
figures for papers and scale well. See Bitmap also.
- Web - The World Wide Web - often known as the
``internet'', though it is only part.
- Web comic - How to spend your PhD doing very little.
- Web page - A page intended to be viewed with a web browser.
- Window Manager - The program which controls the windows
(the boxes around the programs) on your screen. Common examples are
fvwm, twm, sawfish.
- Windows - Do not confuse this Microsoft system with the
superior X-Windows System.
- windows - The rectangles on your screen programs run in.
- XTerm - The X-Terminal application allows you to type
commands into your shell and run programs. This is the most useful X
application. Many versions of this exist (e.g. kterm,
- X-Windows (or X) - The X-Windows System - The system UNIX
machines typically use to display graphical programs. X-Windows uses
a Window Manager to control the windows on the desktop. X-Windows
can also run a program over the network from a remote machine (very
useful!). This can be done with SSH.
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The translation was initiated by Jeremy Sanders on 2011-10-02