How to perform the reverse of `xargs`? - linux

Question

I have a list of numbers that I want to reverse.
They are already sorted.
35 53 102 342
I want this:
342 102 53 35
So I thought of this:
echo $NUMBERS | ??? | tac | xargs
What's the ???
It should turn a space separated list into a line separated list.
I'd like to avoid having to set IFS.
Maybe I can use bash arrays, but I was hoping there's a command whose purpose in life is to do the opposite of xargs (maybe xargs is more than a one trick pony as well!!)

Answer 1

You can use printf for that. For example:
$ printf "%s\n" 35 53 102 342
35
53
102
342
$ printf "%s\n" 35 53 102 342|tac
342
102
53
35

Answer 2

Another answer (easy to remember but not as fast as the printf method):
$ xargs -n 1 echo
e.g.
$ NUMBERS="35 53 102 342"
$ echo $NUMBERS | xargs -n 1 echo | tac | xargs
342 102 53 35
Here is the xargs manual for -n option:
-n number
Set the maximum number of arguments taken from standard input for
each invocation of utility. An invocation of utility will use less
than number standard input arguments if the number of bytes accumu-
lated (see the -s option) exceeds the specified size or there are
fewer than number arguments remaining for the last invocation of
utility. The current default value for number is 5000.

Answer 3

awk one-liner without tac:
awk '{NF++;while(NF-->1)print $NF}'
for example:
kent$ echo "35 53 102 342"|awk '{NF++;while(NF-->1)print $NF}'
342
102
53
35

Answer 4

Another option is to use Bash string manipulation
$ numbers="35 53 102 342"
$ echo "${numbers// /$'\n'}"
35
53
102
342
$ echo "${numbers// /$'\n'}" | tac
342
102
53
35

Answer 5

Well, you could write:
echo $(printf '%s\n' $NUMBERS | tac)
where printf '%s\n' ... prints each of ..., with a newline after each one, and $( ... ) is a built-in feature that makes xargs almost superfluous.
However, I don't think you should avoid using arrays, IFS, and so on; they make scripts more robust in the face of bugs and/or unexpected input.

Answer 6

There's a lot of answers using tac, but in case you'd like to use sort, it's almost the same:
printf "%s\n" 1 2 3 4 5 10 12 | sort -rn
n is important as it makes it sort numerically. r is reverse.

Answer 7

If you have sorted your list with sort, you might considered the -r reversed option

Answer 8

Another way to change space into newlines and the other way round is with tr :
echo 35 53 102 342|tr ' ' '\n'|tac|tr '\n' ' '
If data is not sorted, replace tac by sort -rn.