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.
Hello world and quine program
#!/usr/bin/cat Hello, world!
chmod +x the program file to make it executable:
$ chmod +x file
To execute it:
(assuming file is the filename of the program)
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