busybox/shell/hush_doc.txt

2008-07-14

	Command parsing

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.

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"?

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.

res_word controls statement execution. Examples:

"echo Hello" -
pipe 0 res_word=NONE followup=SEQ prog[0] 'echo' 'Hello'
pipe 1 res_word=NONE followup=SEQ

"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

"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

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.

"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

"for v in a b; do echo $v; true; done" -
pipe 0 res_word=NONE followup=SEQ
 prog 0 group {}:
  pipe 0 res_word=FOR followup=SEQ prog[0] 'v'
  pipe 1 res_word=IN followup=SEQ prog[0] 'a' 'b'
  pipe 2 res_word=DO followup=SEQ prog[0] 'echo' '$v'
  pipe 3 res_word=DO followup=SEQ prog[0] 'true'
  pipe 4 res_word=DONE followup=SEQ
  pipe 5 res_word=NONE followup=(null)
pipe 1 res_word=NONE followup=SEQ

Note how "THEN" and "DO" does not just mark the first pipe,
it "sticks" to all pipes in the body. This is used when
hush executes parsed pipes.

Dummy trailing pipes with no commands are artifacts of imperfect
parsing algorithm - done_pipe() appends new pipe struct beforehand
and last one ends up empty and unused.

"for" and "case" statements (ab)use progs[] to keep their data
instead of argv vector progs[] usually do. "for" keyword is forcing
pipe termination after first word, which makes hush see
"for v in..." as "for v; in...". "case" keyword does the same.
Other judiciuosly placed hacks make hush see
"case word in a) cmd1;; b) cmd2;; esac" as if it was
"case word; match a; cmd; match b; cmd2; esac"
("match" is a fictitious keyword here):

"case word in a) cmd1;; b) cmd2; esac" -
pipe 0 res_word=NONE followup=1 SEQ
 prog 0 group {}:
  pipe 0 res_word=CASE followup=SEQ prog[0] 'word'
  pipe 1 res_word=MATCH followup=SEQ prog[0] 'a'
  pipe 2 res_word=CASEI followup=SEQ prog[0] 'cmd1'
  pipe 3 res_word=MATCH followup=SEQ prog[0] 'b'
  pipe 4 res_word=CASEI followup=SEQ prog[0] 'cmd2'
  pipe 5 res_word=CASEI followup=SEQ prog[0] 'cmd3'
  pipe 6 res_word=ESAC followup=SEQ
  pipe 7 res_word=NONE followup=(null)
pipe 1 res_word=NONE followup=SEQ


2008-01

	Command execution

/* callsite: process_command_subs */
generate_stream_from_list(struct pipe *head) - handles `cmds`
  create UNIX pipe
  [v]fork
  child:
  redirect pipe output to stdout
  _exit(run_list(head));   /* leaks memory */
  parent:
  return UNIX pipe's output fd
  /* head is freed by the caller */

/* callsite: parse_and_run_stream */
run_and_free_list(struct pipe *)
  run_list(struct pipe *)
  free_pipe_list(struct pipe *)

/* callsites: generate_stream_from_list, run_and_free_list, pseudo_exec, run_pipe */
run_list(struct pipe *) - handles "cmd; cmd2 && cmd3", while/for/do loops
  run_pipe - for every pipe in list

/* callsite: run_list */
run_pipe - runs "cmd1 | cmd2 | cmd3 [&]"
  run_list - used if only one cmd and it is of the form "{cmds;}"
  forks for every cmd if more than one cmd or if & is there
  pseudo_exec - runs each "cmdN" (handles builtins etc)

/* callsite: run_pipe */
pseudo_exec - runs "cmd" (handles builtins etc)
  exec - execs external programs
  run_list - used if cmdN is "(cmds)" or "{cmds;}"
  /* problem: putenv's malloced strings into environ -
  ** with vfork they will leak into parent process
  */
  /* problem with ENABLE_FEATURE_SH_STANDALONE:
  ** run_applet_no_and_exit(a, argv) uses exit - this can interfere
  ** with vfork - switch to _exit there?
  */