The first line:
#!/usr/bin
should be #!/usr/bin/sh or #!/usr/bin/ksh, or which ever shell you use.
When run as a script (./script_name), it tries to use /usr/bin as an interpreter, which of course a directory can not do.
When you source it ( . ./script_name ), it takes the # to mean it's a comment, so ignores it and executes the rest.