June 8, 2012

One of the on-going themes here at Programming Praxis involves the re-implementation of the text utilities from Unix V7. Today we look at the cat command:


cat – catenate and print


cat [ -u ] file …


Cat reads each file in sequence and writes it on the standard output. Thus

cat file

prints the file and

cat file1 file2 >file3

concatenates the first two files and places the result on the third.

If no file is given, or if the argument ‘-‘ is encountered, cat reads from the standard input. Output is buffered in 512-byte blocks unless the standard output is a terminal or the -u option is present.


pr(1), cp(1)


Beware of ‘cat a b >a’ and ‘cat a b >b’, which destroy input files before reading them.

Your task is to write the Unix V7 cat program. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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