setenv: not found

For example when I type:

setenv bootargs 'console=ttyS2,115200n8 console=tty0 root=/dev/
mmcblk0p2 rootwait rootfstype=ext3 video=omapfb:vram:2M,vram:4M,mode:
640x480@60' ; saveenv

I get:

-sh: setenv: not found

Some other commands work, for example:

mplayer /sample_video.avi

I am using a rev C4 board, with images from

'setenv' is a U-Boot command

savenev is a u-boot command, not a Linux one. You usually need to interrupt u-boot by pressing a key before the kernel starts booting in order to get its prompt.

After a reset or power up, you need to stop the beagleboard before it
boots to set and save the environment, there are not OS commands.