cat is a standard Unix utility that reads files sequentially, writing them to standard output. The name is derived from its function to concatenate files.
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.[2]

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.
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