1) Download s3cmd package from http://s3tools.org/download
2) extract it, and you should get a folder called s3cmd-master
3) put the folder to anywhere you want (at here I move it to ~/)
4) create a script called s3cmd under dir /usr/local/bin/ and put the following lines as its content.
#!/bin/sh
~/s3cmd-master/s3cmd $*
5) make the created script executable: sudo chmod +x /usr/local/bin/s3cmd
DONE and you should able to use s3cmd command without specify the full path.