====== Shell Command Line Parsing Sequence ======

  - Read the command line.
     * Split the command line. Identify "tokens":<code>
<space> <tab> <newline> ; () <> | & "words" and "keywords"; command sequences, command grouping, pipelines.
</code>
     * Perform a preliminary syntax check:<code>
quoting              : '...' "..." \...
i/o-redirection      : < file, > file, >> file, 2> file, 2>> file, 2>&1
conditional execution:   test,      [ ... ]     (Bourne)
                       [[ ... ]],  (( ... ))    (Korn/POSIX)
background execution : &
statement termination: (...) , {...}, if ... fi , case ... esac
for/while/until ... done
</code>
     * If the first "word" is a "non-opening keyword": issue a syntax error.<code>
fi, esac, then, else, do, done
</code>
     * If the command line is incomplete: display the "> " prompt ($PS2).\\ Then wait for the next return and read the command line again.
  - If the -v option is in effect: print the command line as it is.
  - Korn/POSIX:
    * If the first "word" is in the list of aliases: perform alias substitution.
    * If the value of the alias ends in space/tab, check the next "word" as well. If not, go on to the next step.
  - Korn/POSIX: Perform tilde substitution:<code>
~   ~+   ~-  ~username
</code>
  - Perform variable substitution:<code>
$varname,  ${varname}
</code>
  - Perform command substitution:<code>
`cmd` (Bourne),  $(cmd) (Korn/POSIX).
</code>
  - Perform I/O-redirection:<code>
<  file           : open file for reading via file descriptor 0
>  file,  >> file : open file for writing via file descriptor 1
2> file, 2>> file : open file for writing via file descriptor 2
</code>
  - Korn/POSIX:Evaluate arithmetic expressions:<code>
let "..", (( .. ))
</code>
  - Parse the command line into "words" via $IFS (space, tab, newline).
  - Perform filename generation: interpret<code>
 *  ?  [...]  [!...]
</code>
  - If the -x option is in effect: print the command line as it is now, preceding it with a + ($PS4).
  - If the first word is a shell built-in or a function(Korn/POSIX) execute the command in the current environment. \\ Else:
    - Locate the command via $PATH.
    - Determine the type of program. (UNIX command/shared executable or shell script)
    - Execute the command in a new environment(child process)


====== . ======
Copyright (C) 2000 Integrated Services; tux4u.nl\\
Authors: Ing.J.M.Waldorp, Drs.M.Waldorp-Bonk
sh_parse 20000809, 20110416