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Tcl(n)			     Tcl Built-In Commands			Tcl(n)


       Tcl - Tool Command Language

       Summary of Tcl language syntax.

       The  following  rules  define  the syntax and semantics of the Tcl lan-

       [1] Commands.
	      A	Tcl script is a	string containing one or more commands.	 Semi-
	      colons  and newlines are command separators unless quoted	as de-
	      scribed below.  Close brackets are  command  terminators	during
	      command substitution (see	below) unless quoted.

       [2] Evaluation.
	      A	command	is evaluated in	two steps.  First, the Tcl interpreter
	      breaks the command into words and	performs substitutions as  de-
	      scribed  below.	These  substitutions are performed in the same
	      way for all commands.  The first word is used to locate  a  com-
	      mand  procedure  to carry	out the	command, then all of the words
	      of the command are passed	to the command procedure.  The command
	      procedure	 is  free to interpret each of its words in any	way it
	      likes, such as an	integer, variable name,	list, or  Tcl  script.
	      Different	commands interpret their words differently.

       [3] Words.
	      Words of a command are separated by white	space (except for new-
	      lines, which are command separators).

       [4] Double quotes.
	      If the first character of	a word is double-quote (""") then  the
	      word is terminated by the	next double-quote character.  If semi-
	      colons, close brackets, or  white	 space	characters  (including
	      newlines)	appear between the quotes then they are	treated	as or-
	      dinary characters	and included in	the word.   Command  substitu-
	      tion, variable substitution, and backslash substitution are per-
	      formed on	the characters between the quotes as described	below.
	      The double-quotes	are not	retained as part of the	word.

       [5] Argument expansion.						       |
	      If  a word starts	with the string	"{*}" followed by a non-white- |
	      space character, then the	leading	"{*}" is removed and the  rest |
	      of  the  word is parsed and substituted as any other word. After |
	      substitution, the	word is	parsed as a list (without  command  or |
	      variable substitutions; backslash	substitutions are performed as |
	      is normal	for a list and individual internal words may  be  sur- |
	      rounded  by  either  braces or double-quote characters), and its |
	      words are	added to the command being substituted.	For  instance, |
	      "cmd  a  {*}{b [c]} d {*}{$e f "g	h"}" is	equivalent to "cmd a b |
	      {[c]} d {$e} f "g	h"".

       [6] Braces.
	      If the first character of	a word is an open brace	("{") and rule
	      [5]  does	not apply, then	the word is terminated by the matching
	      close brace ("}").  Braces nest within the word: for each	 addi-
	      tional  open brace there must be an additional close brace (how-
	      ever, if an open brace or	close brace within the word is	quoted
	      with a backslash then it is not counted in locating the matching
	      close brace).  No	substitutions are performed on the  characters
	      between  the  braces  except for backslash-newline substitutions
	      described	below, nor do semi-colons, newlines,  close  brackets,
	      or  white	 space	receive	 any special interpretation.  The word
	      will consist of exactly the characters between the outer braces,
	      not including the	braces themselves.

       [7] Command substitution.
	      If  a word contains an open bracket ("[")	then Tcl performs com-
	      mand substitution.  To do	this it	invokes	 the  Tcl  interpreter
	      recursively to process the characters following the open bracket
	      as a Tcl script.	The script may contain any number of  commands
	      and  must	be terminated by a close bracket ("]").	 The result of
	      the script (i.e. the result of its last command) is  substituted
	      into the word in place of	the brackets and all of	the characters
	      between them.  There may be any number of	command	 substitutions
	      in  a  single  word.   Command  substitution is not performed on
	      words enclosed in	braces.

       [8] Variable substitution.
	      If a word	contains a dollar-sign ("$") followed by  one  of  the
	      forms  described below, then Tcl performs	variable substitution:
	      the dollar-sign and the following	characters are replaced	in the
	      word by the value	of a variable.	Variable substitution may take
	      any of the following forms:

	      $name	     Name is the name of a scalar variable;  the  name
			     is	 a sequence of one or more characters that are
			     a letter, digit, underscore, or namespace separa-
			     tors (two or more colons).

	      $name(index)   Name  gives the name of an	array variable and in-
			     dex gives the name	of an element within that  ar-
			     ray.  Name	must contain only letters, digits, un-
			     derscores,	and namespace separators, and  may  be
			     an	empty string.  Command substitutions, variable
			     substitutions, and	 backslash  substitutions  are
			     performed on the characters of index.

	      ${name}	     Name  is  the  name of a scalar variable.	It may
			     contain  any  characters  whatsoever  except  for
			     close braces.

	      There  may  be  any number of variable substitutions in a	single
	      word.  Variable substitution is not performed on words  enclosed
	      in braces.

       [9] Backslash substitution.
	      If  a  backslash ("\") appears within a word then	backslash sub-
	      stitution	occurs.	 In all	cases but those	 described  below  the
	      backslash	 is  dropped and the following character is treated as
	      an ordinary character and	included in  the  word.	  This	allows
	      characters  such	as  double  quotes, close brackets, and	dollar
	      signs to be included in words without  triggering	 special  pro-
	      cessing.	The following table lists the backslash	sequences that
	      are handled specially, along with	the value that	replaces  each

	      \a     Audible alert (bell) (0x7).

	      \b     Backspace (0x8).

	      \f     Form feed (0xc).

	      \n     Newline (0xa).

	      \r     Carriage-return (0xd).

	      \t     Tab (0x9).

	      \v     Vertical tab (0xb).

		     A single space character replaces the backslash, newline,
		     and all spaces and	tabs after the	newline.   This	 back-
		     slash sequence is unique in that it is replaced in	a sep-
		     arate pre-pass before the	command	 is  actually  parsed.
		     This  means  that it will be replaced even	when it	occurs
		     between braces, and the resulting space will  be  treated
		     as	a word separator if it is not in braces	or quotes.

	      \\     Backslash ("\").

	      \ooo   The  digits  ooo  (one,  two,  or	three of them) give an
		     eight-bit octal value for the Unicode character that will
		     be	 inserted.   The  upper	 bits of the Unicode character
		     will be 0.

	      \xhh   The hexadecimal digits hh give an	eight-bit  hexadecimal
		     value  for	 the  Unicode character	that will be inserted.
		     Any number	of hexadecimal digits may be present; however,
		     all  but the last two are ignored (the result is always a
		     one-byte quantity).  The upper bits of the	Unicode	 char-
		     acter will	be 0.

	      \uhhhh The  hexadecimal digits hhhh (one,	two, three, or four of
		     them) give	a sixteen-bit hexadecimal value	for  the  Uni-
		     code character that will be inserted.

	      Backslash	 substitution  is  not	performed on words enclosed in
	      braces, except for backslash-newline as described	above.

       [10] Comments.
	      If a hash	character ("#")	appears	at a point where  Tcl  is  ex-
	      pecting the first	character of the first word of a command, then
	      the hash character and the characters that follow	it, up through
	      the  next	 newline,  are	treated	as a comment and ignored.  The
	      comment character	only has significance when it appears  at  the
	      beginning	of a command.

       [11] Order of substitution.
	      Each  character is processed exactly once	by the Tcl interpreter
	      as part of creating the words of a  command.   For  example,  if
	      variable	substitution  occurs then no further substitutions are
	      performed	on the value of	the variable;  the value  is  inserted
	      into the word verbatim.  If command substitution occurs then the
	      nested command is	processed entirely by the  recursive  call  to
	      the  Tcl interpreter; no substitutions are performed before mak-
	      ing the recursive	call and no additional substitutions are  per-
	      formed on	the result of the nested script.

	      Substitutions  take place	from left to right, and	each substitu-
	      tion is evaluated	completely before attempting to	 evaluate  the
	      next.  Thus, a sequence like
		     set y [set	x 0][incr x][incr x]
	      will always set the variable y to	the value, 012.

       [12] Substitution and word boundaries.
	      Substitutions  do	 not  affect the word boundaries of a command,
	      except for argument expansion as specified in rule [5].  For ex-
	      ample,  during  variable	substitution  the  entire value	of the
	      variable becomes part of a single	word, even if  the  variable's
	      value contains spaces.

Tcl				      8.5				Tcl(n)


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