✎ soc:2009:lynusvaz:notes:scripting_doc:features_added

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=====Scripting Features=====
The features mentioned here are NOT YET part of mainline gPXE, and are still under development. The modified code can be found in my git repository, in the [[http://git.etherboot.org/?p=people/lynusvaz/gpxe.git;a=shortlog;h=refs/heads/expt|expt]] branch.

  - Identifiers
  - Arithmetic evaluator
  - Quoting
  - Branches
  - Return code
  - Loops

====Identifiers====
See the Identifiers section at: [[http://etherboot.org/wiki/commandline]], for the basic syntax of an identifier. The new code allows identifiers to be 'nested', like:
  set i 0
  echo ${net${i}/ip}
will print the IP address of the net0 interface. Arithmetic operations (see 2) can also be performed within the ${}:
E.g.:
  set i 0
  echo ${net$(${i}+1)/ip}
will print the IP address of the net1 interface (if it exists).

====Arithmetic Evaluation====
Arithmetic expressions can be evaluated by placing them within $().
The following operators are supported (in order of decreasing precedence):
  - !, ~          (logical NOT and bitwise negation)
  - *, /, %       (multiplication, division, and modulo)
  - +, -          (addition, subtraction)
  - <<, >>        (left- and right-shift)
  - <, <=, >, >=  (inequality)
  - !=, ==        (equal, not equal)
  - &             (bitwise AND)
  - |             (bitwise OR)
  - ^             (bitwise EX-OR)
  - &&            (logical AND)
  - ||            (logical OR)
The == and != operators also act on strings. Identifiers are expanded by placing them within ${}.
E.g.:
  echo $(1 + 2)
  set a 15
  echo $(${a} * 3 + 5)
  echo $( ${net0/ip} != "" )
Output:
  3
  50
  1

====Quoting====
The \ is used as an escape character. The following sequences are recognised:
  * \<space> Treats the space as part of the command-line argument
  * \<tab> Ditto
  * \<newline> Concatenates the next line to the current line. Both the \ and the newline character are removed
  * \<any other character> Removes the special meaning of the character (if any)
Within single-quotes, all characters lose their special meaning. Within double-quotes, the \ and $ retain their special meaning. Single- and double-quotes allow you to use a newline character in a command-line argument.
E.g.:
  set message "Hello World"
  echo '${message} = '${message}
  set message Hello\ \ World
  echo ${message}
  set message Hello\ World\ \#1	#'Hello World #1' is treated as a single argument
  echo ${message}
  echo 'Hello
  World'  # Will introduce a newline between Hello and World
  echo Hello \
  World
  echo It\'s good to see you!
Output:
  ${message} = Hello World
  Hello  World
  Hello World #1
  Hello
  World
  Hello World
  It's good to see you!

====Branches====
The keywords if, else and fi are used to branch command execution:
  if <condition>
      <statements>
  fi

  if <condition>
      <statements>
  else		#optional
      <statements>
  fi

A try-catch block is a special kind of branch.
  try
      <crucial statements>	#Call this statment sequence A
  catch
      <backup statements>	#Call this statement sequence B
  done
The statements in sequence A are executed one by one. If any of them fails, execution branches immediately to sequence B. If all the statements in sequence A are executed successfully, execution skips sequence B, and continue after the done statement.

E.g.:
  if $( ${filename} != "" && ${server} != "")
  	chain tftp://${server}//${filename}
  else
  	echo "No filename"
  fi
will first check that neither filename and server are not empty, before attempting to boot. If either is empty, display an error message.

  try
  	kernel tftp://${server}//${kernel}
  	initrd tftp://${server}//${initrd}
  	boot
  catch
  	echo "Oops: ${rc}"
  done
will attempt to boot using the given kernel and initrd. If any of the three commands fail, it displays a message.

====Return Code====
The return code of the previous statement can be checked using the ${rc} variable.
A value of 0 means that the command completed successfully, while any other value means the command was not successful.

E.g.:
  dhcp net0
  if $( ${rc} != 0 )
      echo "DHCP failed"
  else
      chain http://etherboot.org/gtest/gtest.gpxe # (Tom's Root Boot disk)
  fi

====Loops====
You can use while and for loops as:
  while <condition>
  do
  	<statements>
  done
The while loop executes the statement block while the condition is true.

  for <variable> in <value list>
  do
  	<statements>
  done
The variable takes on each value in the value list one by one.

E.g.:
  set i 0
  while $( ${net${i}/mac} != "" )
  do
  	dhcp net${i}
  	if $(${rc} == 0)
  		chain tftp://${server}//${filename}
  	fi
  	set i $( ${i} + 1 )
  done
will attempt to get a dhcp connection on each valid interface, and if it is successful, boot using a given file.

  for i in 0 1 2 3 5 $(6*3)
  do
        echo 'i =' ${i}
  done
displays:
  i = 0
  i = 1
  i = 2
  i = 3
  i = 5
  i = 18
A break statement will exit a loop, and a continue statement will start the next iteration of the loop.
  for i in 1 2 3 4 5 6
  do
        if $(${i} == 3)
              continue
        fi
        echo 'i =' ${i}
        if $(${i} == 4)
              break      #This will exit the for loop, not just the if branch
        fi
  done
displays:
  i = 1
  i = 2
  i = 4
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