Lines 1,2,3,4,…,n-1, n to lines n,n-1, …,4,3,2,1

Question

I just realized that I could solve this problem by reversing line numbers from 1, 2, 3,...,n to n, n-1, ..., 3,2,1 and then use the same logic as earlier. So I want to know how can I reverse the order of lines?

Answer 1

For your particular problem, this is not the most efficient way to go about it, however, you can use any of the following to print a file with lines in reverse order, with varying degrees of portability (tac for example is not included by default on many Unixes):

• sed '1!G;h;$!d' [file]
• awk '{f[n=NR]=$0}END{for(i=n;i>0;i--)print f[i]}' [file]
• perl -e 'print reverse<>' [file]
• tac [file]

On my system the fastest is tac, as tested by the following:

$ printf '%s\n' {a..z}{a..z}{a..z} > foo
$ time sed '1!G;h;$!d' foo > /dev/null 2>&1

real    0m0.582s
user    0m0.544s
sys     0m0.012s

$ time awk '{f[n=NR]=$0}END{for(i=n;i>0;i--)print f[i]}' foo > /dev/null 2>&1

real    0m0.060s
user    0m0.052s
sys     0m0.008s

$ time perl -e 'print reverse<>' foo > /dev/null 2>&1

real    0m0.021s
user    0m0.016s
sys     0m0.004s

$ time tac foo > /dev/null 2>&1

real    0m0.003s
user    0m0.004s
sys     0m0.000s

... so if you have tac, use it, but otherwise, use perl or awk.

Answer 2

tac (cat backwards, and included in coreutils as well) will cat a file in reverse order:

$ cat /tmp/test
One
Two
Three

$ tac /tmp/test
Three
Two
One

_{(In the case of the particular problem you mentioned the solutions there are better than reversing the entire file just so you can append a file and reverse the whole file again)}

Answer 3

The tac utility reverses lines. It is cat in reverse.

Answer 4

GNU utilities (Linux, Cygwin) and BusyBox have the tac command, which reverses the order of the lines in a text file.

On systems that don't have tac, you can work it from standard commands. Chris Down's answer shows a few ways to do it by storing the file entirely in memory. For a very large file, a way that will work without thrashing on most unices is to make sort do the reversal. This is not as efficient for medium-sized files, but sort implementations can typically cope with files that are larger than the available memory.

nl | sort -nr | sed 's/.*\t//'

(Replace \t by a literal tab character.)

As already noted by Michael Mrozek, reversing lines is a poor way of prepending data to a file. The method is hard to understand, and performs a lot of extra work.