How to remove all white spaces from a given text file

I want to remove all the white spaces from a given text file. Is there any shell command available for this ? Or, how to use sed for this purpose.

I want something like below:

$ cat hello.txt | sed ....

I tried this : cat hello.txt | sed 's/ //g' .But it removes only spaces, not tabs.

$ man tr
NAME
    tr - translate or delete characters

SYNOPSIS
    tr [OPTION]... SET1 [SET2]

DESCRIPTION
   Translate, squeeze, and/or delete characters from standard 
   input, writing to standard output.

In order to wipe all whitespace including newlines you can try:

cat file.txt | tr -d " \t\n\r" 

You can also use the character classes defined by tr (credits to htompkins comment):

cat file.txt | tr -d "[:space:]"

For example, in order to wipe just horizontal white space:

cat file.txt | tr -d "[:blank:]"

Much simpler to my opinion:

sed -r 's/\s+//g' filename

I think you may use sed to wipe out the space while not losing some infomation like changing to another line.

cat hello.txt | sed '/^$/d;s/[[:blank:]]//g'