Implement "tac": Print lines from a file in reverse

Question

Between the kitten question and seeing this question at U&L about some sed magic, how about implementing tac?

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Objective

Implement a program that will reverse and print the lines in a file.

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Input

A file, provided as a name or via standard input

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Output

The lines, reversed, to standard out.

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Scoring

Bytes of source code.

Answer 1

GS2, 3 bytes

* +

The three bytes are, in order, split lines, reverse, and join lines.

Answer 2

Perl, 11 bytes

$\=$_.$\}{

Behaves exactly like tac. This code requires the -p switch, which I have counted as 1 byte.

Test runs

$ echo -en 'a\nb' | perl -pe'$\=$_.$\}{' | xxd -g 1
0000000: 62 61 0a                                         ba.
$ echo -en 'a\nb\n' | perl -pe'$\=$_.$\}{' | xxd -g 1
0000000: 62 0a 61 0a                                      b.a.

How it works

As explained here, the -p switch basically wraps while (<>) { ... ; print } around the program, so the source code is equivalent to

 while(<>)
 {
   $\ = $_ . $\
 }
 print

For each line of input, we prepend the current line ($_) to $\ (initially undefined), updating the latter with the result.

After all lines have been processed, print prints the value of the local variable $_ (undefined in this scope), followed by the output record separator ($\).

Answer 3

Pyth, 4 bytes

j_.z

.z is the input separated by lines as a list, _ reverses it and j joins it by a character, which by default is \n.

Answer 4

FlogScript, 2 bytes

)"

(Try it on anarchy golf.)

The ) enables --in-out-line-array mode, and the rest of the program is ", reversing the array of lines.

Answer 5

Retina, 7 bytes

!rm`.*$

With a single regex, Retina runs in Match mode. This normally just prints the number of matches, but with ! we configure it to print the actual matches instead (separated by linefeeds).

The actual regex is merely .*$. .* matches any line (potentially empty), because . can match any character except linefeeds. I'll get to the $ in a minute.

How do we make it print the matches in reverse? By making use of .NET's right-to-left matching mode, activated with the r. This means the regex engine starts at the end of the string when looking for matches and works backwards.

Finally, the m makes the $ match the end of a line instead of the end of the string. Why do we even need that? The trouble is that .* generates extraneous matches. Consider the regex substitution

s/a*/$0x/

applied to the input baaababaa. You'd think this would yield baaaxbaxbaax, but it actually gives you baaaxxbaxxbaaxx. Why? Because after matching aaa the engine's cursor is between the a and the b. Now it can't match any more as, but a* is also satisfied with an empty string. This means, after every single match you get another empty match.

We don't want that here, because it would introduce additional empty lines, so we discard those extraneous matches (which are at the beginnings of the lines, due to the right-to-left mode) by requiring that matches include the end of the line.

Answer 6

Haskell, 34 bytes

main=interact$concat.reverse.lines

[edit]

Saved one byte by replacing unlines with concat.

Answer 7

CJam, 7 bytes

qN/W%N*

Reads stdin, prints to stdout.

Explanation:

q       Get input.
N/      Split at newlines.
W%      Reverse list.
N*      Join with newlines.

Answer 8

Pyth, 5 bytes

jb_.z

This reverses the order of the lines with a simple split-reverse-join approach, but not quite like tac.

Try it online.

Answer 9

Befunge-93, 17 bytes

~:1+!#v_
>:#,_@>$

Nothing fancy here; just put everything on the stack, then pop it off.

Answer 10

Pure Bash (no external utilities), 56

mapfile a
for((i=${#a[@]};i--;));{
printf %s "${a[i]}"
}

This is one of the few answers to do exact tac emulation, as asked about in Dennis' comment:

$ echo -en 'a\nb' | ./tacemu.sh | xxd -g 1
0000000: 62 61 0a                                         ba.
$ echo -en 'a\nb\n' | ./tacemu.sh | xxd -g 1
0000000: 62 0a 61 0a                                      b.a.
$ 

Answer 11

Ruby, 16 bytes

puts [*$<].reverse

Answer 12

Python 2, 52 bytes

import sys;print''.join(sys.stdin.readlines()[::-1])

Answer 13

sed, 9 bytes

1!G;h;$!d

No upvote wanted, this is a famous sed one-liner.

Answer 14

Perl, 16 bytes

print reverse<>

Answer 15

Powershell, 41 bytes

$a=$args|%{gc $_};[array]::Reverse($a);$a

Stores the content of a file line by line in a, reverses a and finally prints it.

Answer 16

GolfScript, 7 bytes

n/-1%n*

Online test here.

Answer 17

Burlesque, 6 bytes

ln<-uN

ln splits lines, <- reverses, uN joins lines and formats for raw output.

Answer 18

Bash, 48 43 characters

(Inspired by Digital Trauma's Bash answer. Upvotes for the idea should go to him.)

mapfile -c1 -C's=$2$s;set'
printf %s "$2$s"

Sample run:

bash-4.3$ echo -en 'a\nb' | bash tac.sh | xxd -g 1
0000000: 62 61 0a                                         ba.

bash-4.3$ echo -en 'a\nb\n' | bash tac.sh | xxd -g 1
0000000: 62 0a 61 0a                                      b.a.

Answer 19

JavaScript (SpiderMonkey Shell), 38 bytes

[...read(readline())].reverse().join``

Pretty simple

---

read() reads a file

readline() reads a string from STDIN

[...str] will split str into an array of chars

reverse() will reverse the array

join`` will collpase the array into a string

Answer 20

GNU Awk, 27 characters

(Inspired by Ed Morton's GNU Awk answer. CW as I not intended to hijack his solution.)

{s=$0RT s}END{printf"%s",s}

Note that by changing RT → RS this becomes portable standard Awk but looses the ability to preserve the absence of the final newline.

Sample run:

bash-4.3$ echo -en 'a\nb' | awk '{s=$0RT s}END{printf"%s",s}' | xxd -g 1
0000000: 62 61 0a                                         ba.

bash-4.3$ echo -en 'a\nb\n' | awk '{s=$0RT s}END{printf"%s",s}' | xxd -g 1
0000000: 62 0a 61 0a                                      b.a.

Answer 21

C#, 179 171 bytes

using B=System.Console;class A{static void Main(){var a=new System.Collections.Stack();string b;while((b=B.ReadLine())!=null)a.Push(b);foreach(var c in a)B.WriteLine(c);}}

Reads lines, placing them in a stack, and then writes them backwards. I would use Mathematica for this, but it has no sense of EOF.

Answer 22

SNOBOL, 42 bytes

S S =INPUT CHAR(10) S :S(S)
 OUTPUT =S
END

Answer 23

Gema, 25 characters

*\n=@set{s;$0${s;}}
\Z=$s

Sample run:

bash-4.3$ echo -en 'a\nb' | gema '*\n=@set{s;$0${s;}};\Z=$s'
ba

bash-4.3$ echo -en 'a\nb\n' | gema '*\n=@set{s;$0${s;}};\Z=$s'
b
a

Answer 24

Hassium, 90 Bytes 86 Bytes

use IO;func main(){c=File.readLines(args[0]);for(x=c.length-1;x>=0; println(c[x--]))0;

See expanded here

Answer 25

sed, 7 bytes

G;h;$!d

This works for me (and it's the shortest solution elsewhere), but I don't really want to find out why. I just messed around with the famous 9-byte trick until I found this. I guess Ging the first line does nothing?

Answer 26

JavaScript(Node.js), 91 Bytes

console.log(require('fs').readFileSync(process.argv[2])+"".split(d="\n").reverse().join(d))

Answer 27

Bash + common utilities, 25

tr \\n ^G|rev|tr ^G \\n|rev

Here the ^G is a literal BEL character. I'm assuming the input is only printable ascii.

This transforms the entire input to one line by replacing newlines with BELs, then reverses that line, then transforms back to multiline, then reverses each line again, to get the desired output.

Answer 28

MATLAB, 44

@(x) strjoin(fliplr(strsplit(x,'\n')),'\n');

Splits the string at new lines, flips the resulting array, then rejoins with new line characters.

Answer 29

Julia, 65 bytes

open(s->print(join(reverse([l for l=readlines(s)]),"")),ARGS[1])

This takes a file as a command line argument and prints its lines in reverse order. Trailing newlines are moved to the front, unlike tac, which is legit.

Ungolfed:

function p(s::Stream)
    # Create a vector of the lines of the input stream
    L = [l for l in readlines(s)]

    # Reverse the vector and join it back into a string
    j = join(reverse(L), "")

    # Print the string to STDOUT
    print(j)
end

# Open the file specified in the first command line argument
# and apply the function p to its contents
open(p, ARGS[1])

Answer 30

Pip, 3 + 2 = 5 bytes

Uses the r and n flags; reads from stdin.

RVg

The r flag reads stdin and stores it as a list of lines in g (which is normally a list of command-line args). We then reverse that list, and it is auto-printed. The n flag causes lists to be output with newline as a separator.