Cat program

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The cat program is a program that copies its standard input to its standard output. It is named after the Unix command cat, although this command is actually more powerful. The cat program is a popular problem.

cat has also been described humorously as a programming language in which every program is a quine. The programming language is obviously of very limited computational usefulness. See HQ9+ and Text.

Hello world and quine program

#!/usr/bin/cat
Hello, world!

You must chmod +x the program file to make it executable:

$ chmod +x file

To execute it:

$ ./file

(assuming file is the filename of the program)

Reverse cat

Other shell commands could be used in similar way, so there is reverse variant of cat language too. It's called rev:

#!/usr/bin/rev
!dlrow ,olleH