一般情况下，U-Boot 启动后、Linux 内核启动倒计时结束前，通过按下任何按键即可中断 Linux 内核的启动，并进入 U-Boot 的命令模式。我们可以通过输入一些命令对 U-Boot 执行一些操作。
在 U-Boot 命令模式下，我们可以通过输入 help 或者 ? 查看 U-Boot 支持的所有命令以及这些命令的主要功能：
=> help
? - alias for 'help'
base - print or set address offset
bdinfo - print Board Info structure
bmode - sd1|sd2|qspi1|normal|usb|sata|ecspi1:0|ecspi1:1|ecspi1:2|ecspi1:3|esdhc1|esdhc2|esdhc3|esdhc4 [noreset]
bmp - manipulate BMP image data
boot - boot default, i.e., run 'bootcmd'
bootd - boot default, i.e., run 'bootcmd'
bootelf - Boot from an ELF image in memory
bootm - boot application image from memory
bootp - boot image via network using BOOTP/TFTP protocol
bootvx - Boot vxWorks from an ELF image
bootz - boot Linux zImage image from memory
clocks - display clocks
clrlogo - fill the boot logo area with black
cmp - memory compare
coninfo - print console devices and information
cp - memory copy
crc32 - checksum calculation
dcache - enable or disable data cache
dhcp - boot image via network using DHCP/TFTP protocol
dm - Driver model low level access
echo - echo args to console
editenv - edit environment variable
env - environment handling commands
erase - erase FLASH memory
exit - exit script
ext2load- load binary file from a Ext2 filesystem
ext2ls - list files in a directory (default /)
ext4load- load binary file from a Ext4 filesystem
ext4ls - list files in a directory (default /)
ext4size- determine a file's size
ext4write- create a file in the root directory
false - do nothing, unsuccessfully
fatinfo - print information about filesystem
fatload - load binary file from a dos filesystem
fatls - list files in a directory (default /)
fatsize - determine a file's size
fdt - flattened device tree utility commands
flinfo - print FLASH memory information
forlinx - forlinx params menu
fstype - Look up a filesystem type
fuse - Fuse sub-system
go - start application at address 'addr'
gpio - query and control gpio pins
help - print command description/usage
i2c - I2C sub-system
icache - enable or disable instruction cache
iminfo - print header information for application image
imxtract- extract a part of a multi-image
itest - return true/false on integer compare
load - load binary file from a filesystem
loadb - load binary file over serial line (kermit mode)
loads - load S-Record file over serial line
loadx - load binary file over serial line (xmodem mode)
loady - load binary file over serial line (ymodem mode)
loop - infinite loop on address range
ls - list files in a directory (default /)
md - memory display
mdio - MDIO utility commands
mii - MII utility commands
mm - memory modify (auto-incrementing address)
mmc - MMC sub system
mmcinfo - display MMC info
mtest - simple RAM read/write test
mw - memory write (fill)
nfs - boot image via network using NFS protocol
nm - memory modify (constant address)
ping - send ICMP ECHO_REQUEST to network host
pmic - PMIC
printenv- print environment variables
protect - enable or disable FLASH write protection
reset - Perform RESET of the CPU
run - run commands in an environment variable
save - save file to a filesystem
saveenv - save environment variables to persistent storage
setenv - set environment variables
setexpr - set environment variable as the result of eval expression
sf - SPI flash sub-system
showvar - print local hushshell variables
size - determine a file's size
sleep - delay execution for some time
source - run script from memory
test - minimal test like /bin/sh
tftpboot- boot image via network using TFTP protocol
true - do nothing, successfully
usb - USB sub-system
usbboot - boot from USB device
version - print monitor, compiler and linker version
查询命令
- 输入 bdinfo 查看开发板信息：
- 输入 printenv 查看 U-Boot 环境变量，如当前串口的波特率为 115200，Linux 内核启动倒计时为 3 秒：
- 输入 version 查看 U-Boot 版本
环境变量操作命令
环境变量的操作涉及到两个命令：setenv 和 saveenv，命令 setenv 用于设置或者修改环境变量的值，命令 saveenv 用于保存修改后的环境变量。一般环境变量是存放在外部 Flash 中的，U-Boot 启动时会将环境变量从 Flash 读取到 DRAM 中。使用命令 setenv 修改的是 DRAM 中的环境变量值，修改后要使用 saveenv 命令将修改后的环境变量保存到 Flash 中，否则 U-Boot 下一次重启会继续使用以前的环境变量值。
=> setenv
setenv - set environment variables
Usage:
setenv [-f] name value ...
- [forcibly] set environment variable 'name' to 'value ...'
setenv [-f] name
- [forcibly] delete environment variable 'name'
=> saveenv
Saving Environment to MMC...
Writing to MMC(1)... done
修改环境变量
- 以修改 Linux 内核启动倒计时为例：
- 修改倒计时为 5 秒：
setenv bootdelay 5
saveenv
- 输入 printenv 查看环境变量：
- 重启开发板，5 秒倒计时：
新建环境变量
- 新建一个 author 环境变量
setenv author alpha
saveenv
- 查看新建的环境变量：
删除环境变量，
- 只要将环境变量设为空即可：
setenv author
saveenv
- 查看 author 环境变量已删除：
