Reset your PATH variable
Solution 1:
I have the following in my ~/.profile:
PATH=/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/X11/bin
PATH=$PATH:~/bin
The first PATH declaration overrides any existing PATH setting (it doesn't include $PATH). The path used is the OS X default.
The second line appends ~/bin to the PATH and it is a personal preference, not a default.
After edit, restart your session — or use command source ~/.profile to activate the changes immediately.