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The manual page on Terminal for echo -n is the following:

 -n    Do not print the trailing newline character.  This may also be
       achieved by appending `\c' to the end of the string, as is done by
       iBCS2 compatible systems.  Note that this option as well as the
       effect of `\c' are implementation-defined in IEEE Std 1003.1-2001
       (``POSIX.1'') as amended by Cor. 1-2002.  Applications aiming for
       maximum portability are strongly encouraged to use printf(1) to
       suppress the newline character.
 Some shells may provide a builtin echo command which is similar or iden-
 tical to this utility.  Most notably, the builtin echo in sh(1) does not
 accept the -n option.  Consult the builtin(1) manual page.
When I try to do generate an MD5 hash by:
echo "password" | md5
It returns 286755fad04869ca523320acce0dc6a4
When I do
echo -n "password"
It returns the value that online MD5 generators return: 5f4dcc3b5aa765d61d8327deb882cf99
What difference does the option -n do? I don't understand the entry in Terminal.
                You might also find the POSIX specification for echo at pubs.opengroup.org/onlinepubs/009604599/utilities/echo.html worth reading. Notably, it points out that echo -n is not portable across all UNIX systems (though this is better than echo -e, which cannot be implemented without breaking the standard outright!)
– Charles Duffy
                Jun 10, 2015 at 16:31
When you do echo "password" | md5, echo adds a newline to the string to be hashed, i.e. password\n. When you add the -n switch, it doesn't, so only the characters password are hashed.
Better to use printf, which does what you tell it to without needing any switches:
printf 'password' | md5
For cases where 'password' isn't just a literal string, you should use a format specifier instead:
printf '%s' "$pass" | md5
This means that escape characters within the password (e.g. \n, \t) aren't interpreted by printf and are printed literally.
                OK, that makes a lot more sense! I always thought first the string was hashed and then the newline was added; now I know it's the other way around.
– user4820905
                Jun 10, 2015 at 18:42
                The ironic thing is now that the hashes I generate are harder to be cracked because cracking programs have to add a newline to every entry, so using echo is better in a way! ;)
– user4820905
                Jun 11, 2015 at 15:22
                Do you know why break lines are ignored when comparing strings in bash, like this: [[ $(echo -e "a\n") = $(echo -n "a") ]] && echo yes why does that condition return true?
– Edgar Magallon
                Jan 27 at 8:13
                @EdgarMagallon all trailing newlines are removed from command substitutions, so those commands are both equivalent to a (try for yourself after running set -x in your shell).
– Tom Fenech
                Jan 29 at 1:41
  
Output the args, separated by spaces, followed by a newline. (...)
  If -n is specified, the trailing newline is suppressed.
Having this into account, it is always safer to use printf, which provides the same functionality as echo -n. That is, no default new line is added:
$ echo "password" | md5sum
286755fad04869ca523320acce0dc6a4  -
$ echo -n "password" | md5sum
5f4dcc3b5aa765d61d8327deb882cf99  -
$ printf "%s" "password" | md5sum
5f4dcc3b5aa765d61d8327deb882cf99  -   # same result as echo -n
See the superb answer in Why is printf better than echo? for more info.
And another example:
$ echo "hello" > a
$ cat a
hello
$ echo -n "hello" > a
$ cat a
hello$            # the new line is not present, so the prompt follows last line
                I might suggest printf %s "password", to work reliably even if the password is being expanded from a variable and may contain backslash escapes or the like.
– Charles Duffy
                Jun 10, 2015 at 16:30
        
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