Reversing a variable's contents by words

For a short list, and in a variable, the shell itself is the fastest solution:

var="10 20 30 40 50 60 70 80 90 100" set -- $var; unset a for ((i=$#;i>0;i--)); do printf '%s%s' "${a:+ }" "${!i}" a=1 done echo 

Output:

100 90 80 70 60 50 40 30 20 10 

Note: the split operation in set -- $var is safe for the exact string used here, but will not be so if the string contains wildcard characters (*, ? or []). It may be avoided with set -f before the split, if needed. The same note is also valid for the following solutions (except where noted).

Or, if you want to set some other variable with the reverse list:

var="10 20 30 40 50 60 70 80 90 100" set -- $var for i; do out="$i${out:+ }$out"; done echo "$out" 

Or using only the positional parameters.

var="10 20 30 40 50 60 70 80 90 100" set -- $var a='' for i do set -- "$i${a:+ $@}" a=1 done echo "$1" 

Or using only one variable (which may be var itself):
Note: This solution is not affected by globing nor IFS.

var="10 20 30 40 50 60 70 80 90 100" a=" $var" while [[ $a ]]; do printf '<%s>' "${a##* }" var="${a% *}" done 

awk to the rescue

$ var='10 20 30 40 50 60 70 80 90 100' $ echo "$var" | awk '{for(i=NF; i>0; i--) printf i==1 ? $i"\n" : $i" "}' 100 90 80 70 60 50 40 30 20 10 

with perl, courtesy How to read an IP address backwards? shared by @steeldriver

$ echo "$var" | perl -lane '$,=" "; print reverse @F' 100 90 80 70 60 50 40 30 20 10 

Or, with bash itself by converting the string to array

$ arr=($var) $ for ((i=${#arr[@]}-1; i>=0; i--)); do printf "${arr[i]} "; done; echo 100 90 80 70 60 50 40 30 20 10 

You can use bash features such as arrays to do it entirely in-process:

#!/bin/bash VAR="10 20 30 40 50 60 70 80 90 100" a=($VAR); rev_a=() for ((i=${#a[@]}-1;i>=0;i--)); do rev_a+=( ${a[$i]} ); done; RVAR=${rev_a[*]}; 

This should be about the fastest way to do it provided that $VAR isn't very very large.

zsh:

print -r -- ${(Oa)=VAR} 

$=VAR splits $VAR on $IFS. (Oa) orders the resulting list in reverse array order. print -r -- (like in ksh), same as echo -E - (zsh specific) is a reliable versions of echo: prints its arguments as-is separated by space, terminated by newline.

If you want to split on space only, and not on whatever $IFS contains (space, tab, newline, nul by default), either assign space to $IFS, or use an explicit splitting like:

print -r -- ${(Oas: :)VAR} 

To sort in reverse numerical order:

$ VAR='50 10 20 90 100 30 60 40 70 80' $ print -r -- ${(nOn)=VAR} 100 90 80 70 60 50 40 30 20 10 

POSIXly (so would also work with bash):

With shell builtin (except for printf in some shells) mechanisms only (better for variables with a short value):

unset -v IFS # restore IFS to its default value of spc, tab, nl set -o noglob # disable glob set -- $VAR # use the split+glob operator to assign the words to $1, $2... reversed_VAR= sep= for i do reversed_VAR=$i$sep$reversed_VAR sep=' ' done printf '%s\n' "$reversed_VAR" 

With awk (better for large variables, especially with bash, but up to the limit of the size of the arguments (or of a single argument)):

 awk ' BEGIN { n = split(ARGV[1], a); while (n) {printf "%s", sep a[n--]; sep = " "} print "" }' "$VAR" 

printf "%s\n" $var | tac | paste -sd' ' 100 90 80 70 60 50 40 30 20 10 

Use a bit of Python magic here:

bash-4.3$ VAR='10 20 30 40 50 60 70 80 90 100' bash-4.3$ python -c 'import sys;print " ".join(sys.argv[1].split()[::-1])' "$VAR" 100 90 80 70 60 50 40 30 20 10 

The way this works is fairly simple:

• we refer to "$VAR" as second command line paramenter sys.argv[1] ( first parameter with -c option is always just that - -c itself. Notice the quoting around "$VAR" to prevent splitting the variable itself into individual words ( which hisi done by shell , not python )
• Next we use .split() method, to break it into a list of strings
• [::-1] part is just extended slice syntax to get reverse of the list
• finally we join everything together back into one string with " ".join() and print out

But those who aren't big Python fans, we could use AWK to do essentially same thing: split, and print array in reverse:

 echo "$VAR" | awk '{split($0,a);x=length(a);while(x>-1){printf "%s ",a[x];x--};print ""}' 100 90 80 70 60 50 40 30 20 10 

Inelegant POSIX software tools one-liner:

echo `echo $VAR | tr ' ' '\n' | sort -nr`