Bash Awk, Reverse order of records

Question

Hey everyone I am making an awk bash script that will take an input text file such as:

1111 Joe Brown
2222 Charlie Rogers
3333 Chris Williams
4444 Rob Black

And simply reverse the order of the rows, so output would be:

4444 Rob Black
3333 Chris Williams
2222 Charlie Rogers
1111 Joe Brown

I am getting a syntax error saying that there is an error near "(" and also that I have an extra "{" I cannot figure out what is happening here is my code:

#!/bin/bash
awk '{a[NR]=$0} END (for(i=NR; i>=1; i--)) printf("%s\n",a[i])}'

The simplest way to reverse the lines is `tac filename`. But that won't teach you any awk. — Mike Sherrill 'Cat Recall', Mar 11 '14 at 14:41
You're missing the `{` bracket after `END` and there's a superfluous pair of `(` `)` around the `for` clause. — Henk Langeveld, Mar 11 '14 at 14:58
Possible duplicate of [How can I reverse the order of lines in a file?](http://stackoverflow.com/questions/742466/how-can-i-reverse-the-order-of-lines-in-a-file) — Henk Langeveld, Mar 11 '14 at 15:02

score 5 · Answer 1 · answered Mar 11 '14 at 14:38

5

You can probably use just sort(1) command.

sort -nr < input.txt

Sort simply takes the input file and tries to sort it, taking the whitespace as a separator (which is our case), so it sorts the input by first column. If you need non-alphabetical sorting of the first column, the -n switch is needed (to sort numerically). The -r switch just reverses the output (ascending/descending).

answered Mar 11 '14 at 14:38

tvm

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If you simply need to reverse the content of the file, you can use tac(1), which is present on the most unix systems, as @Dinesh mentioned. – tvm Mar 11 '14 at 14:44
I m trying to use this -r to reverse my sort. I am trying to sort by two columns simultaneously, ie names and logintime. i want to sort names in alphabetical order, but login time is to be latest login time first. using -r reverses both the sorts for each column. – Tan Yu Hau Sean Jul 31 '22 at 05:29

score 4 · Answer 2 · answered Mar 11 '14 at 14:38

4

You have two extra brackets there. Correcting it:

awk '{a[NR]=$0} END {for(i=NR; i>=1; i--) printf("%s\n",a[i]);}' file

If you don't have to use awk, you can do easily with: tac file

answered Mar 11 '14 at 14:38

P.P

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score 2 · Answer 3 · answered Mar 11 '14 at 14:40

2

If it is unix machine tac can be used.

answered Mar 11 '14 at 14:40

Dinesh Reddy

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`tac` is not a standard unix command, but is part of the Gnu coreutils (present in most linux distros). – Henk Langeveld Mar 11 '14 at 15:01

Jotne · Answer 4 · 2014-03-20T12:23:47.780

2

Here is an awk variation:

awk '{a[i++]=$0} END {while(i--) print a[i]}' file
4444 Rob Black
3333 Chris Williams
2222 Charlie Rogers
1111 Joe Brown

edited Mar 20 '14 at 12:23

answered Mar 11 '14 at 15:44

Jotne

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score 0 · Answer 5 · answered Mar 12 '14 at 02:22

0

perl solution:

$ perl -e 'print reverse <>' file
4444 Rob Black
3333 Chris Williams
2222 Charlie Rogers
1111 Joe Brown

or sed

$ sed '1!G;h;$!d' file
4444 Rob Black
3333 Chris Williams
2222 Charlie Rogers
1111 Joe Brown

answered Mar 12 '14 at 02:22

jaypal singh

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Bash Awk, Reverse order of records

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