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Cat program
(Redirected from Reverse cat program)
- Not to be confused with Cat Program (language), nor Cat (Language).
- For the concatenative language, see catlangwiki:Cat.
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. Unix command line utilities in this space include both rev (reversing characters within each line) and tac (reversing the order of lines in the input; cat backwards).
$ echo '!dlrow ,olleH' | rev Hello, world!
$ (echo 'world!'; echo 'Hello,') | tac Hello, world!
$ (echo '!dlrow'; echo ',olleH') | tac | rev # or ... | rev | tac Hello, world!
A simple "reverse cat" program could simply output the bytes of the input backwards, which for a conventional newline-delimited text file has roughly the effect of rev and tac combined (except for the treatment of the file's trailing newline).
While a regular cat program requires only O(1) storage space, a reverse cat program requires O(n) storage space, so as a popular problem it is a slightly more ambitious achievement.
Implementations
綠酒千杯腸已爛
與君六千七百九十八杯酒。可以窮歡宴。綠酒一杯歌一遍。 與君六千七百九十七杯酒。可以窮歡宴。綠酒一杯歌一遍。 …… 與君三杯酒。可以窮歡宴。綠酒一杯歌一遍。 與君二杯酒。可以窮歡宴。綠酒一杯歌一遍。 與君一杯酒。可以窮歡宴。綠酒一杯歌一遍。 綠酒千杯腸已爛。
‰
i.
^English
Print the user’s input.
!!brainfeed
^.
... Bottles of beer on the wall
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bottles of beer on the wall, 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...................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................... bottles of beer. Take one down and pass it around, 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bottles of beer on the wall. …… (a lot of characters ommited) ... bottles of beer on the wall, ... bottles of beer. Take one down and pass it around, .. bottles of beer on the wall. .. bottles of beer on the wall, .. bottles of beer. Take one down and pass it around, . bottle of beer on the wall. . bottle of beer on the wall, . bottle of beer. Take one down and pass it around, no bottles of beer on the wall. No bottles of beer on the wall, no bottles of beer. Go to the store and buy some more, 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bottles of beer on the wall.
Assemble
$st "" ipt st dvr st pas brk
brainfuck
,[.,]
Befunge
~,
C
This program takes in a user input and prints it, then takes in another user input and prints it, and so on...
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[]) {
FILE *fp;
void filecopy(FILE *, FILE *);
if (argc == 1) {
filecopy(stdin, stdout);
}
else {
while (--argc > 0) {
if ((fp = fopen(*++argv, "r")) == NULL) {
printf("cat: Can't open file '%s'", *argv);
return 1;
} else {
filecopy(fp, stdout);
fclose(fp);
}
}
}
return 0;
}
void filecopy(FILE *ifp, FILE *ofp) {
int c;
while ((c = getc(ifp)) != EOF) {
putc(c, ofp);
}
}
Dotfuck
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brainfuck
,[.,]
Deadfish+
ofjpog
Español
Leer la pagina en esta wiki sobre "Cat program" Este programa simula ese programa.
ExampleFuck
Hello World FizzBuzz Polyglot Self Interpreter 99 Bottles of Beer
Fish Code
<,.{0}><
Pepe
REEe Reee
Python
print(input())
RAGE!!!
<rant> WHY DOES K MEAN THE SAME THING AS WHAT YOU'RE ABOUT TO TELL ME SCREAM K!!! </rant>
Regards,
Reads until the end of input, one character at a time.
Subject: Fwd: Fwd: Fwd: FW: read this Hi all, FYI, forwarding this exactly as I received it. Just to level-set, letter is 0. Reading between the lines. Per my last email, while we're over -1 on letter: > Let me spell it out. > Reading between the lines. Best, Ihor
Simpl
print(usrinp())
SpoilerFuck
,.,
Truths-machine
truths(i, o=i out(o))
Tol
toltoLToLTOlTOL
Unary (compressed)
56623
Zidryx myno
dfwbh(wbdih())