Cat (Unix)

cat is a standard Unix utility that reads files sequentially, writing them to standard output. The name is derived from its function to (con)catenate files (from Latin catenare, "to chain"). It has been ported to a number of operating systems. The other primary purpose of cat, aside from concatenation, is file printing allowing the computer user to view the contents of a file. Printing is the most common use of cat. cat was part of the early versions of Unix, e.g., Version 1, and replaced pr, a PDP-7 and Multics utility for copying a single file to the screen. It was written by Ken Thompson and Dennis Ritchie. The version of cat bundled in GNU coreutils was written by Torbjorn Granlund and Richard Stallman. The ReactOS version was written by David Welch, Semyon Novikov, and Hermès Bélusca. Over time, alternative utilities such as tac and bat also became available, bringing different new features. The cat utility serves a dual purpose: concatenating and printing. With a single argument, it is often used to print a file to the user's terminal emulator (or historically to a computer terminal or teletype). With more than one argument, it concatenates several files. The combined result is by default also printed to the terminal, but often users redirect the result into yet another file. Hence printing a single file to the terminal is a special use-case of this concatenation program. Yet, this is its most common use. The Single Unix Specification defines the operation of cat to read files in the sequence given in its arguments, writing their contents to the standard output in the same sequence. The specification mandates the support of one option flag, u for unbuffered output, meaning that each byte is written after it has been read. Some operating systems, like the ones using GNU Core Utilities, do this by default and ignore the flag. If one of the input filenames is specified as a single hyphen (-), then cat reads from standard input at that point in the sequence. If no files are specified, cat reads from standard input only.