U-boot: how to evaluate one environment variable inside another

Question

In U-Boot, I have two environment variables:

filepath=myimages kernelfile=${filepath}/uImage.bin 

When I run this command:

 echo ${kernelfile} 

I get this output:

 ${filepath}/uImage.bin 

However, I want him to evaluate the filepath variable instead as follows:

 myimages/uImage.bin 

How can this be achieved?

Answer 1

In its current form, this cannot be done with the u-boot echo and shell right now. This is because replacing the macro that you see with echo is done by the u-boot command line interpreter before running the echo command. The entire echo command basically prints an array of strings passed to it.

In particular, if you look at common/cli_simple.c from the current (7/29/15) git u-boot repository, you will find the cli_simple_run_command function. This function is passed the cmd string, which may contain several commands separated by ; . If you look inside the loop by dividing this line, you will find cli_simple_process_macros , after which you will find the call cli_simple_parse_line followed by cmd_process . Skipping cli_simple_process_macros at the moment, cli_simple_parse_line basically takes a string and breaks it into an array of strings, similar to how the shell was set, giving you argv and argc , which it passes to cmd_process , which executes the command that is in argv[0] .

The interesting part is cli_simple_process_macros . This takes the input string as the first argument and the output string as the second. This function is a simple state machine that looks for u-boot environment variables (or macros, as the name of the function suggests) and replaces them with the value of the environment variable. This is visible with a call to getenv and then a copy to the output line. If you look at how cli_simple_process_macros works, you will notice that it skips only one, that is, if the environment variable contains another environment variable, it does not process the second environment variable, but simply copies the string value.

Finally, if you look at the source for the echo command, you will see that it is very simple by simply going through argv and printing each line with the corresponding spaces.

Basically, if you want the desired behavior, you need to either change cli_simple_process_macros to an iterative environment variable, or change echo to view the environment variables. You can also modify cli_simple_run_command to call cli_simple_process_macros several times so that all nested environment variables are expanded.

Answer 2

Or as a lazy way to do something like this:

 filepath=myimages set_kernelfile= setenv kernelfile ${filepath}/uImage.bin 

then do:

 run set_kernelfile; echo ${kernelfile}