Rearrange columns using cut

Question

I am having a file in the following format

Column1    Column2
str1       1
str2       2
str3       3

I want the columns to be rearranged. I tried below

Answer 1

using just the shell,

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

Answer 2

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