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Introduction to uEnv.txt File

The Essence of uEnv.txt File

uEnv.txt file is nothing but a collection of various environment variables which are initialized to a number of U-boot commands that automates the command execution.

Important Background Knowledge

$\to$ Boot BBB (e.g., Boot from eMMC) to U-boot prompt

U-boot Commands

help will display all the U-boot commands supported by the current version of U-boot.

Specify the command if you need to see only a specific command: help <command>


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=> help
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?       - alias for 'help'
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askenv  - get environment variables from stdin
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base    - print or set address offset
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bdinfo  - print Board Info structure
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boot    - boot default, i.e., run 'bootcmd'
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bootd   - boot default, i.e., run 'bootcmd'
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bootefi - Boots an EFI payload from memory
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bootelf - Boot from an ELF image in memory
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bootm   - boot application image from memory
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bootp   - boot image via network using BOOTP/TFTP protocol
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bootvx  - Boot vxWorks from an ELF image
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bootz   - boot Linux zImage image from memory
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cmp     - memory compare
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coninfo - print console devices and information
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cp      - memory copy
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crc32   - checksum calculation
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dfu     - Device Firmware Upgrade
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dhcp    - boot image via network using DHCP/TFTP protocol
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dm      - Driver model low level access
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echo    - echo args to console
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editenv - edit environment variable
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eeprom  - EEPROM sub-system
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env     - environment handling commands
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exit    - exit script
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ext2load- load binary file from a Ext2 filesystem
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ext2ls  - list files in a directory (default /)
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ext4load- load binary file from a Ext4 filesystem
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ext4ls  - list files in a directory (default /)
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ext4size- determine a file's size
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ext4write- create a file in the root directory
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false   - do nothing, unsuccessfully
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fastboot- use USB Fastboot protocol
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fatinfo - print information about filesystem
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fatload - load binary file from a dos filesystem
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fatls   - list files in a directory (default /)
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fatsize - determine a file's size
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fatwrite- write file into a dos filesystem
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fdt     - flattened device tree utility commands
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fstype  - Look up a filesystem type
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go      - start application at address 'addr'
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gpio    - query and control gpio pins
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gpt     - GUID Partition Table
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help    - print command description/usage
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i2c     - I2C sub-system
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iminfo  - print header information for application image
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imxtract- extract a part of a multi-image
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itest   - return true/false on integer compare
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load    - load binary file from a filesystem
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loadb   - load binary file over serial line (kermit mode)
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loads   - load S-Record file over serial line
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loadx   - load binary file over serial line (xmodem mode)
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loady   - load binary file over serial line (ymodem mode)
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loop    - infinite loop on address range
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ls      - list files in a directory (default /)
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md      - memory display
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mdio    - MDIO utility commands
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mii     - MII utility commands
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mm      - memory modify (auto-incrementing address)
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mmc     - MMC sub system
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mmcinfo - display MMC info
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mw      - memory write (fill)
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nfs     - boot image via network using NFS protocol
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nm      - memory modify (constant address)
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part    - disk partition related commands
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ping    - send ICMP ECHO_REQUEST to network host
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printenv- print environment variables
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pxe     - commands to get and boot from pxe files
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reset   - Perform RESET of the CPU
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run     - run commands in an environment variable
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save    - save file to a filesystem
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setenv  - set environment variables
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sf      - SPI flash sub-system
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showvar - print local hushshell variables
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size    - determine a file's size
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sleep   - delay execution for some time
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source  - run script from memory
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spl     - SPL configuration
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sspi    - SPI utility command
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sysboot - command to get and boot from syslinux files
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test    - minimal test like /bin/sh
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tftpboot- boot image via network using TFTP protocol
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true    - do nothing, successfully
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ums     - Use the UMS [USB Mass Storage]
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usb     - USB sub-system
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usbboot - boot from USB device
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version - print monitor, compiler and linker version

U-boot Environment Variables

Similar to Linux environment variables, U-boot also has a set of standard as well as user defined environmental variables which determine the behavior of the U-boot. This behavior can be altered by overriding or changing the environmental variable values.

Use printenv to display all environment variables.


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=> printenv soc
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soc=am33xx

Use setenv to set the value of a specified environment. (Creates on if the environment variable is not present.)


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=> setenv serverip 192.168.27.1
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=> printenv serverip
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serverip=192.168.27.1

You can also combine multiple U-boot commands delimited by ; in one envinronment variable. (They all will run in one go!)


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=> printenv bootcmd
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bootcmd=if test ${boot_fit} -eq 1; then run update_to_fit;fi;run findfdt; run init_console; run envboot; run disd

Use run to run a command.

Exercise

Create a user-defined environment variable.


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=> setenv my_mmc_list 'mmc list'
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=> printenv my_mmc_list 
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my_mmc_list=mmc list
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=> run my_mmc_list 
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OMAP SD/MMC: 0
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OMAP SD/MMC: 1

mmc list is a predefined U-boot command.
$\micro$ SD card.
MMC1 is the interface to eMMC.

References

Nayak, K. (2022). Embedded Linux Step by Step Using Beaglebone Black [Video file]. Retrieved from https://www.udemy.com/course/embedded-linux-step-by-step-using-beaglebone/