Linux shell command to reverse the field order of varying length text records

Question

I would like a command line incantation to reverse the field order of arbritrary length text records. Solutions provided in Rearrange columns using cut and Elegant way to reverse column order don't solve this issue since they assume a fixed amount of fields, though maybe they would with minor changes.

Sort of like the tac command that exhibits reverse cat functionality. I'd like what the ohce command would do (if it existed) to reverse echo functinality.

For example:

a b c d
e f
g h i

Should be transformed to

d c b a
f e
i h g

score 3 · Answer 1 · answered Feb 21 '14 at 20:50

3

Using awk:

awk '{for (i=NF; i>1; i--) printf "%s%s", $i, FS; print $i }' file
d c b a
f e
i h g

answered Feb 21 '14 at 20:50

anubhava

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Gilles Quénot · Accepted Answer · 2014-02-24T17:23:53.150

1

There's a command to do it, it's named rev from util-linux :

$ rev file
d c b a
f e
i h g

or using perl :

$ perl -lane 'print join " ", reverse @F' file
d c b a
f e
i h g

But like you explain in the comments, if you want the 3 latest columns, you can use awk :

awk '{print $(NF-2), $(NF-1), $NF}' file

edited Feb 24 '14 at 17:23

answered Feb 21 '14 at 20:55

Gilles Quénot

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3

`rev` reverses characters, so a line like `abc def` would appear as `fed cba`. The question is ambiguous, so @chiam will have to clarify. – glenn jackman Feb 21 '14 at 21:09
With this two solutions, OP have the choice, they treat both cases. – Gilles Quénot Feb 21 '14 at 21:15
Fixed perl version by adding newlines for each lines – Gilles Quénot Feb 22 '14 at 17:12
Sorry for the ambiguity, but the fact that rev reverses all the characters doesn't affect its usefulness for me, just apply it again. `echo` give me the last three fields | `rev` | (`read` a b c ignore the rest; `echo` $a $b $c) | `rev` – Chaim Geretz Feb 24 '14 at 16:18
1

See my `awk` solution if you want the 3 latest columns. – Gilles Quénot Feb 24 '14 at 16:26

score 1 · Answer 3 · answered Feb 21 '14 at 21:21

1

with bash:

while read -ra words; do 
    for ((i=${#words[@]}-1; i>=0; i--)); do 
        printf "%s " "${words[i]}"
    done
    echo
done < file

answered Feb 21 '14 at 21:21

glenn jackman

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score 0 · Answer 4 · answered May 27 '17 at 13:32

0

Using datamash:

echo 'a b c d
e f          
g h i' | datamash --no-strict -t' ' reverse

Output:

d c b a
f e
i h g

Unlike rev, it doesn't reverse words:

echo 'abc xyz' | datamash --no-strict -t' ' reverse

Output:

xyz abc

answered May 27 '17 at 13:32

agc

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Linux shell command to reverse the field order of varying length text records

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