How to reverse a string made of digit in bash?

Question

What is the best way to turn out digits in number?

E.g.

$ echo 123 | hook
321
$ echo 12358 | hook
85321 

Answer 1

:~> echo 123456 | rev
654321

It should work. The usage is pretty simple. from man:

NAME
rev - reverse lines of a file or files

SYNOPSIS
rev [file ...]

DESCRIPTION
The rev utility copies the specified files to the standard output, reversing the order of characters in every line. If no files are specified, the standard input is read.

If not available you should also be able to use awk:

:~> echo 123456 | awk '{ for(i=length;i!=0;i--)x=x substr($0,i,1);}END{print x}'
654321

Source the unix school

If you eventually need to completly invert a file you could add tac to rev command line like this:

tac myfile | rev 

Answer 2

For those not limited to bash, with zsh:

forward=123
backward=${(j::)${(Oas::)forward}}

Those are variable expansion flags:

• s::: split on the empty string, so one element per character
• Oa: reverse the resulting array (O for reverse order, a for array subscript order).
• j::: join the elements with the empty string.

Answer 3

Are you looking for rev?

echo 123|rev
321

Answer 4

#!/bin/bash
#set -xv

string="$1"
reverse_string=""
string_length="${#string}"

while (( "$string_length">=1 )) 
do
     string_length=$(( $string_length - 1 ))
     reverse_string="$reverse_string${string:$string_length:1}"
done

echo "$reverse_string"

make the file executable

$ chmod +x reverse_string
$ ./reverse_string 123456789    
98765432    

Answer 5

rev is the obvious and probably the best answer. But rev is not always installed by default. If you want to do this purely with coreutils you can use this longer command:

echo 123 | fold -w1 | tac | paste -d '' -s -

• fold -w1 puts each digit on a line
• tac is a reverse cat, i.e. reverses the order of the lines
• paste -d '' -s - serially pastes the digit from each line back onto one line

Answer 6

Try Perl:

echo 123 | perl -nle 'print scalar reverse $_'

Source: Perl one-liners

Answer 7

Given:

$ echo "$s"
12345
6789

Here is a Ruby that reverses each line of input:

$ echo "$s" | ruby -lpe '$_=$_.reverse'
54321
9876

Answer 8

s=""
for i in $*
do
    s="$i $s";
done
for j in $s
do
    echo $j
done