We are currently working on new rules for what content should and shouldn't be allowed on this website, and are looking for feedback! See Esolang:2026 topicality proposal to view and give feedback on the current draft.
Cat program
(Redirected from Reverse cat)
- 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
.............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................. bottles of beer on the wall, .............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................. bottles of beer. Take one down and pass it around, ............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................. 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, .............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................. bottles of beer on the wall.
Assemble
$st "" ipt st dvr st pas brk
B+*
#include <stdio>
int main() {
make str a;
getinp("%s", a);
printf("%s", a);
}
brainfuck
,[.,]
Befunge
~,
C
#include <stdio.h>
int main(){while (1){printf("%c", getchar());}}
Doki!
MC[Choose Option]"Type Something!" Act 3:Monika[Var]"Cat", Input(Line 1) MC[Give Poem]Cat Act 4:Sayori[Thank You]
Dotfuck
.................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. .................................................. ................................................
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}><
Mesmerizer
Teto:
Ready for Performance! Quiz Time! “Type something!” Resist Teto Wait Resist Teto Send Message Output Resist Teto Shut Down…
Miku:
Ready for Performance! Wait Wait Set the Stage “Output” to Input! Wait Wait Wait Shut Down…
Pepe
REEe Reee
Portals
| _ | _ | _ | _ | _ | _ | _ | _ |
| | | | | ||||||
| | | | | ||||||
| E | < | ? | T | *C | | | ||
| | | | | ||||||
| | | | | ||||||
| _ | _ | _ | _ | _ | _ | _ | _ |
Chell
Walk Forward 2 Pick Up Get User Input for String Add “Cat” to Storage Cube with Value “” Edit Storage Cube’s “Cat” to Input of Line 2 Walk Forward 1 Place Down Face S Walk Forward 1 Face W Walk Forward 1 Face N Walk Forward 1 Face W Walk Forward 2
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
Unary
000000000 ... (a lot of chars omitted) 000000000
Vegetable Juice
You like it, don’t you? “How long is your string?:” “Length” flavored Vegetable Juice is on sale now for 200 yen! Come on! Come on! I have decided now that Length must love this drink! Aggressively Advertise Vegetable Juice at 5% off for 17 Yen Length times! You like it, don’t you? “Give Miku a Value between 0 and 255:” “Cat” flavored Vegetable Juice is on sale now for 200 yen! Come on! Come on! I have decided now that Cat must love this drink! You like it, don’t you? “Do 0 if you want number, 1 if you want ASCII.” “ASCII” flavored Vegetable Juice is on sale now for 200 yen! Come on! Come on! I have decided now that ASCII must love this drink! I’ve decided, ASCII=0 Aaaaaaaaaa! Cat! I’ve decided just now. Now drink it! Cat! So drink it! All sold out!
Zidryx myno
dfwbh(wbdih())