How to reverse column order of a file in Linux from the command line

Question

I have a file named ip-list with two columns:

IP1  <TAB>  Server1
IP2  <TAB>  Server2

And I want to produce:

Server1  <TAB>  IP1
Server2  <TAB>  IP2

What's the most elegant, shortest Linux command line tool to do it?

Answer 1

Use awk:

awk '{print $2,$1}' ip-list

That should give you what you want.

Answer 2

More than two columns

printf "\
1 2 3 4
5 6 7 8
" | awk '{for(i=NF;i>1;i--)printf "%s ",$i;printf "%s",$1;print ""}'

Output:

4 3 2 1
8 7 6 5

See also: https://unix.stackexchange.com/questions/46275/swapping-an-unlimited-number-of-columns

Tested in GNU Awk 4.0.1.

Answer 3

The simplest solution is:

awk '{print $2 "\t" $1}'

However, there are some issues. If there may be white space in either of the fields, you need to do one of: (depending on if your awk supports -v)

awk -v FS='\t' '{print $2 "\t" $1}'
awk 'BEGIN{ FS="\t" } {print $2 "\t" $1}'

Alternatively, you can do one of:

awk -v OFS='\t' '{print $2,$1}'
awk 'BEGIN{ OFS="\t" } {print $2,$1}'
awk -v FS='\t' -v OFS='\t' '{print $2,$1}' # if allowing spaces in fields

One of the comments asks, 'where does the filename go'? awk is used as a filter, so it would typically appear as:

$ some-cmd | awk ... | other-cmd

with no filename given. Or, a filename can be given as an argument after all commands:

$ awk ... filename