Simple conditional unexpectedly fails in bash if statement - bash

Question

I've spent an embarrassingly long time trying to understand why the second conditional in the "foo" script below fails but the first one succeeds.
Please note:
The current directory contains two files: bar and foo.
All three strings $s1, $s2 and $s3 are equal according to hexdump.
Thanks in advance for any help.
Session: (Running on a Centos7 host):
>ls
bar foo
>cat foo
#!/bin/bash
s1="bar foo"
s2="bar foo"
s3=`ls`
echo -n $s1 | hexdump -C
echo -n $s2 | hexdump -C
echo -n $s3 | hexdump -C
if [ "$s1" = "$s2" ]; then # True
echo s1 = s2
fi
if [ "$s1" = "$s3" ]; then # NOT true! Why?
echo s1 = s3
fi
>foo
00000000 62 61 72 20 66 6f 6f |bar foo|
00000007
00000000 62 61 72 20 66 6f 6f |bar foo|
00000007
00000000 62 61 72 20 66 6f 6f |bar foo|
00000007
s1 = s2
>

Answer 1

Quote the variables when echoing.
echo -n "$s3" | hexdump -C
You'll see a newline between the file names, as ls uses -1 when the output is redirected.

Answer 2

Your demo would be more convincing with echo -n "$s1" etc. That would show that there's a newline in the middle of s3 where there's a space in s1 and s2. The echo without the double quotes mangles the newline into a space (and generally each sequence of one or more white space characters in the string into a single space).
Given:
#!/bin/bash
s1="bar foo"
s2="bar foo"
s3=`ls`
echo -n "$s1" | hexdump -C
echo -n "$s2" | hexdump -C
echo -n "$s3" | hexdump -C
if [ "$s1" = "$s2" ]; then # True
echo s1 = s2
fi
if [ "$s1" = "$s3" ]; then # NOT true because s3 contains a newline!
echo s1 = s3
fi
I get:
$ sh foo
00000000 2d 6e 20 62 61 72 20 66 6f 6f 0a |-n bar foo.|
0000000b
00000000 2d 6e 20 62 61 72 20 66 6f 6f 0a |-n bar foo.|
0000000b
00000000 2d 6e 20 62 61 72 0a 66 6f 6f 0a |-n bar.foo.|
0000000b
s1 = s2
$ bash foo
00000000 62 61 72 20 66 6f 6f |bar foo|
00000007
00000000 62 61 72 20 66 6f 6f |bar foo|
00000007
00000000 62 61 72 0a 66 6f 6f |bar.foo|
00000007
s1 = s2
$