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    Server1
IP2    Server2

And I want to produce:

Answer 1

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