How the Command Line Parsing Works
There are two different command line parsers available with U-Boot: the old “simple” one, and the much more powerful “hush” shell:
Old, simple command line parser
supports environment variables (through setenv / saveenv commands)
several commands on one line, separated by ‘;’
variable substitution using “… ${_variablename_} …” syntax
ALERT! NOTE: Older versions of U-Boot used “$(…)” for variable substitution. Support for this syntax is still present in current versions, but will be removed soon. Please use “${…}” instead, which has the additional benefit that your environment definitions are compatible with the Hush shell, too.
special characters (‘$’, ‘;’) can be escaped by prefixing with ‘\’, for example:
setenv bootcmd bootm \${address}
You can also escape text by enclosing in single apostrophes, for example:
setenv addip ‘setenv bootargs ${bootargs} ip=${ipaddr}:${serverip}:${gatewayip}:${netmask}:${hostname}:${netdev}:off’
Hush shell
similar to Bourne shell, with control structures like if…then…else…fi, for…do…done, while…do…done, until…do…done, …
supports environment (“global”) variables (through setenv / saveenv commands) and local shell variables (through standard shell syntax name=value ); only environment variables can be used with the run command, especially as the variable to run (i. e. the first argument).
In the current implementation, the local variables space and global environment variables space are separated. Local variables are those you define by simply typing like name=value. To access a local variable later on, you have to write ‘$name’ or ‘${name}’; to execute the contents of a variable directly you can type ‘$name’ at the command prompt. Note that local variables can only be used for simple commands, not for compound commands etc.
Global environment variables are those you can set and print using setenv and printenv. To run a command stored in such a variable, you need to use the run command, and you must not use the ‘$’ sign to access them.
To store commands and special characters in a variable, use single quotation marks surrounding the whole text of the variable, instead of the backslashes before semicolons and special symbols.
Be careful when using the hash (‘#’) character – like with a “real” Bourne shell it is the comment character, so you have to escape it when you use it in the value of a variable.
Examples:
setenv bootcmd bootm \$address
setenv addip ‘setenv bootargs $bootargs ip=$ipaddr:$serverip:$gatewayip:$netmask:$hostname:$netdev:off’
Hush shell scripts
Here are a few examples for the use of the advanced capabilities of the hush shell in U-Boot environment variables or scripts:
Example:
=> setenv check ‘if imi $addr; then echo Image OK; else echo Image corrupted!!; fi’
=> print check
check=if imi $addr; then echo Image OK; else echo Image corrupted!!; fi
=> addr=0 ; run check
## Checking Image at 00000000 …
Bad Magic Number
Image corrupted!!
=> addr=40000 ;run check
## Checking Image at 00040000 …
Image Name: ARM Linux-2.4.18
Created: 2003-06-02 14:10:54 UTC
Image Type: ARM Linux Kernel Image (gzip compressed)
Data Size: 801609 Bytes = 782.8 kB
Load Address: 0c008000
Entry Point: 0c008000
Verifying Checksum … OK
Image OK
Instead of “echo Image OK” there could be a command (sequence) to boot or otherwise deal with the correct image; instead of the “echo Image corrupted!!” there could be a command (sequence) to (load and) boot an alternative image, etc.
Example:
=> addr1=0
=> addr2=10
=> bootm $addr1 || bootm $addr2 || tftpboot $loadaddr $loadfile && bootm
## Booting image at 00000000 …
Bad Magic Number
## Booting image at 00000010 …
Bad Magic Number
TFTP from server 192.168.3.1; our IP address is 192.168.3.68
Filename ‘/tftpboot/TRAB/uImage’.
Load address: 0xc400000
Loading: #################################################################
#################################################################
###########################
done
Bytes transferred = 801673 (c3b89 hex)
## Booting image at 0c400000 …
Image Name: ARM Linux-2.4.18
This will check if the image at (flash?) address “addr1″ is ok and boot it; if the image is not ok, the alternative image at address “addr2″ will be checked and booted if it is found to be OK. If both images are missing or corrupted, a new image will be loaded over TFTP and booted.
14.2.17.4. General rules
If a command line (or an environment variable executed by a run command) contains several commands separated by semicolons, and one of these commands fails, the remaining commands will still be executed.
If you execute several variables with one call to run (i. e. calling run with a list of variables as arguments), any failing command will cause run to terminate, i. e. the remaining variables are not executed.
