platform_bootable_recovery/edify

edify

Update scripts (from donut onwards) are written in a new little scripting language ("edify") that is superficially somewhat similar to the old one ("amend"). This is a brief overview of the new language.

• The entire script is a single expression.

• All expressions are string-valued.

• String literals appear in double quotes. \n, \t, ", and \ are understood, as are hexadecimal escapes like \x4a.

• String literals consisting of only letters, numbers, colons, underscores, slashes, and periods don't need to be in double quotes.

• The following words are reserved:

   if    then    else   endif
  

  They have special meaning when unquoted. (In quotes, they are just string literals.)

• When used as a boolean, the empty string is "false" and all other strings are "true".

• All functions are actually macros (in the Lisp sense); the body of the function can control which (if any) of the arguments are evaluated. This means that functions can act as control structures.

• Operators (like "&&" and "||") are just syntactic sugar for builtin functions, so they can act as control structures as well.

• ";" is a binary operator; evaluating it just means to first evaluate the left side, then the right. It can also appear after any expression.

• Comments start with "#" and run to the end of the line.

Some examples:

• There's no distinction between quoted and unquoted strings; the quotes are only needed if you want characters like whitespace to appear in the string. The following expressions all evaluate to the same string.

  "a b" a + " " + b "a" + " " + "b" "a\x20b" a + "\x20b" concat(a, " ", "b") "concat"(a, " ", "b")

  As shown in the last example, function names are just strings, too. They must be string literals, however. This is not legal:

  ("con" + "cat")(a, " ", b) # syntax error!

• The ifelse() builtin takes three arguments: it evaluates exactly one of the second and third, depending on whether the first one is true. There is also some syntactic sugar to make expressions that look like if/else statements:

  these are all equivalent

  ifelse(something(), "yes", "no") if something() then yes else no endif if something() then "yes" else "no" endif

  The else part is optional.

  if something() then "yes" endif # if something() is false, # evaluates to false

  ifelse(condition(), "", abort()) # abort() only called if # condition() is false

  The last example is equivalent to:

  assert(condition())

• The && and || operators can be used similarly; they evaluate their second argument only if it's needed to determine the truth of the expression. Their value is the value of the last-evaluated argument:

  file_exists("/data/system/bad") && delete("/data/system/bad")

  file_exists("/data/system/missing") || create("/data/system/missing")

  get_it() || "xxx" # returns value of get_it() if that value is # true, otherwise returns "xxx"

• The purpose of ";" is to simulate imperative statements, of course, but the operator can be used anywhere. Its value is the value of its right side:

  concat(a;b;c, d, e;f) # evaluates to "cdf"

  A more useful example might be something like:

  ifelse(condition(), (first_step(); second_step();), # second ; is optional alternative_procedure())