When I run chmod +w filename it doesn't give write permission to other, it just gives write permission to user and group.
After executing this command
chmod +w testfile.txt running ls -l testfile.txt prints
-rw-rw-r-- 1 ravi ravi 20 Mar 10 18:09 testfile.txt but in case of +r and +x it works properly.
I don't want to use chmod ugo+w filename.
In your specific situation, we can guess that your current umask is 002 (this is a common default value) and this explains your surprise.
In that specific situation where umask value is 002 (all numbers octal).
+r means ugo+r because 002 & 444 is 000, which lets all bits to be set+x means ugo+x because 002 & 111 is 000, which lets all bits to be set+w means ug+w because 002 & 222 is 002, which prevents the "o" bit to be set.umask 022 +w would mean u+w.umask 007 +rwx would mean ug+rwx.umask 077 +rwx would mean u+rwx.When you change umask to 000, by executing
umask 000 in your terminal, then
chmod +w file will set permissions to ugo+w.
As suggested by ilkkachu, note that umask 000 doesn't mean that everybody can read and write all your files.
But umask 000 means everyone that has some kind of access to any user account on your machine (which may include programs running server services ofc) can read and write all the files you make with that mask active and don't change (if the containing chain of directories up to the root also allows them).
r bit for those parties where the umask allows it. This is clearly stated in the manuals of e.g. GNU chmod and FreeBSD chmod, as well as the standard. Same for +x. For +w you're right, for the case of that particular umask. 0 doesn't mean that everybody can read and write all your files, since many applications create files that are particularly private with mode 0600, meaning that the group and others don't get any access, regardless of the umask. umask is an important part of how +r behaves, not a sidenote. Besides, even assuming umask 002, chmod +r doesn't mean chmod ugo=r, it means chmod ugo+r With:
chmod +<perms> the perms are added to user, group and other but with the umask still applying. It makes sure the file is not granted more permission than a newly created file would.
If you want to add the perms to user, groups and other regardless of the umask, use
chmod a+<perms> which is short for
chmod ugo+<perms> 
You need to specify to whom you are giving the permissions to, such as other, by using chmod o+w testfile.txt
+w gives permission to all( user, group and other). chmod a+w testfile.txt. Use u for user, g for group, o for other, and a for all. chmod a+w filename, chmod +w filename and chmod ugo+w filename are alternative to each other then why not just use +w
ugo, usea.+wnot working.askubuntuanswer that you reference above that "This behavior is POSIX-mandated, and so, not a bug". Please be advised that POSIX Application Usage text is non-normative and is therefore not mandated by POSIX.