Rearrange columns using cut

Question

For the cut(1) man page:

Use one, and only one of -b, -c or -f. Each LIST is made up of one range, or many ranges separated by commas. Selected input is written in the same order that it is read, and is written exactly once.

It reaches field 1 first, so that is printed, followed by field 2.

Use awk instead:

awk '{ print $2 " " $1}' file.txt

You may also combine cut and paste:

paste <(cut -f2 file.txt) <(cut -f1 file.txt)

via comments: It's possible to avoid bashisms and remove one instance of cut by doing:

paste file.txt file.txt | cut -f2,3

using just the shell,

while read -r col1 col2
do
  echo $col2 $col1
done <"file"

You can use Perl for that:

perl -ane 'print "$F[1] $F[0]\n"' < file.txt

• -e option means execute the command after it
• -n means read line by line (open the file, in this case STDOUT, and loop over lines)
• -a means split such lines to a vector called @F ("F" - like Field). Perl indexes vectors starting from 0 unlike cut which indexes fields starting form 1.
• You can add -F pattern (with no space between -F and pattern) to use pattern as a field separator when reading the file instead of the default whitespace

The advantage of running perl is that (if you know Perl) you can do much more computation on F than rearranging columns.

Using join:

join -t $'\t' -o 1.2,1.1 file.txt file.txt

---

Notes:

• -t $'\t' In GNU join the more intuitive -t '\t' without the $ fails, (coreutils v8.28 and earlier?); it's probably a bug that a workaround like $ should be necessary. See: unix join separator char.

• join needs two filenames, even though there's just one file being worked on. Using the same name twice tricks join into performing the desired action.

• For systems with low resources join offers a smaller footprint than some of the tools used in other answers:

  wc -c $(realpath `which cut join sed awk perl`) | head -n -1
    43224 /usr/bin/cut
    47320 /usr/bin/join
   109840 /bin/sed
   658072 /usr/bin/gawk
  2093624 /usr/bin/perl