busybox

2008-07-14

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    Command parsing

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Command parsing results in a list of "pipe" structures.

This list correspond not only to usual "pipe1 || pipe2 && pipe3"

lists, but it also controls execution of if, while, etc statements.

Every such statement is a list for hush. List consists of pipes.

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struct pipe fields:

  smallint res_word - "none" for normal commands,

                      "if" for if condition etc

  struct child_prog progs[] - array of commands in pipe

  smallint followup - how this pipe is related to next: is it

                      "pipe; pipe", "pipe & pipe" "pipe && pipe",

                      "pipe || pipe"?

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Blocks of commands { pipe; pipe; } and (pipe; pipe) are represented

as one pipe struct with one progs[0] element which is a "group" -

struct child_prog can contain a list of pipes. Sometimes these

"groups" are created implicitly, e.g. every control

statement (if, while, etc) sits inside its own group.

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res_word controls statement execution. Examples:

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"echo Hello" -

pipe 0 res_word=NONE followup=SEQ prog[0] 'echo' 'Hello'

pipe 1 res_word=NONE followup=SEQ

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"echo foo || echo bar" -

pipe 0 res_word=NONE followup=OR  prog[0] 'echo' 'foo'

pipe 1 res_word=NONE followup=SEQ prog[0] 'echo' 'bar'

pipe 2 res_word=NONE followup=SEQ

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"if true; then echo Hello; true; fi" -

res_word=NONE followup=SEQ

 prog 0 group {}:

  pipe 0 res_word=IF followup=SEQ prog[0] 'true'

  pipe 1 res_word=THEN followup=SEQ prog[0] 'echo' 'Hello'

  pipe 2 res_word=THEN followup=SEQ prog[0] 'true'

  pipe 3 res_word=FI followup=SEQ

  pipe 4 res_word=NONE followup=(null)

pipe 1 res_word=NONE followup=SEQ

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Above you see that if is a list, and it sits in a {} group

implicitly created by hush. Also note two THEN res_word's -

it is explained below.

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"if true; then { echo Hello; true; }; fi" -

pipe 0 res_word=NONE followup=SEQ

 prog 0 group {}:

  pipe 0 res_word=IF followup=SEQ prog[0] 'true'

  pipe 1 res_word=THEN followup=SEQ

   prog 0 group {}:

    pipe 0 res_word=NONE followup=SEQ prog[0] 'echo' 'Hello'

    pipe 1 res_word=NONE followup=SEQ prog[0] 'true'

    pipe 2 res_word=NONE followup=SEQ

  pipe 2 res_word=NONE followup=(null)

pipe 1 res_word=NONE followup=SEQ

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"for v in a b; do echo $v; true; done" -

pipe 0 res_word=NONE followup=SEQ

 prog 0 group {}: