how to make "if" and comparison statement in uboot?

i'm newbie in uboot and tftp programing

based on this url, there is how to make if statement like this if imi $addr; then echo Image OK; else echo Image corrupted!!; fi

and this is my "if" :

开发者_开发技巧

=> setenv a true
=> printenv a
a=true
=> setenv b true
=> printenv b
b=true
=> if a b; then echo 'same';fi
Unknown command 'a' - try 'help'
=> if $a $b; then echo 'same';fi
Unknown command 'true' - try 'help'
=> if ${a} ${b}; then echo 'same';fi
Unknown command 'true' - try 'help'
=>

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I am not sure if it is in all u-boot versions or not, but there should be a test command for comparison. Can you try:

if test "${a}" = "${b}"; then echo "same"; fi

Unfortunately I don't have access to u-boot, so this is all from memory.