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

______________________________________________________________________________

NAME
       Tcl - Tool Command Language

SYNOPSIS
       Summary of Tcl language syntax.
______________________________________________________________________________

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

       [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.  Secondly, the first word is used  to  lo-
	      cate a routine to	carry out the command, and the remaining words
	      of the command are passed	to that	routine.  The routine 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).  Letters and	digits
			     are only the standard ASCII ones  (0-9,  A-Z  and
			     a-z).

	      $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.  Letters and digits are only the
			     standard ASCII ones (0-9, A-Z and a-z).   Command
			     substitutions,  variable substitutions, and back-
			     slash substitutions are performed on the  charac-
			     ters of index.

	      ${name}	     Name  is  the  name of a scalar variable or array
			     element.  It may contain any  characters  whatso-
			     ever  except  for	close braces.  It indicates an
			     array element if name  is	in  the	 form  "array-
			     Name(index)" where	arrayName does not contain any
			     open parenthesis characters, "(", or close	 brace
			     characters, "}", and index	can be any sequence of
			     characters	except for close brace characters.  No
			     further  substitutions  are  performed during the
			     parsing of	name.

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

	      Note that	variables may contain character	sequences  other  than
	      those  listed  above,  but in that case other mechanisms must be
	      used to access them (e.g., via the set command's single-argument
	      form).

       [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
	      sequence.

	      \a     Audible alert (bell) (Unicode U+000007).

	      \b     Backspace (Unicode	U+000008).

	      \f     Form feed (Unicode	U+00000C).

	      \n     Newline (Unicode U+00000A).

	      \r     Carriage-return (Unicode U+00000D).

	      \t     Tab (Unicode U+000009).

	      \v     Vertical tab (Unicode U+00000B).

	      \<newline>whiteSpace
		     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 a	eight-
		     bit octal value for the Unicode character	that  will  be
		     inserted,	 in   the   range  000-377  (i.e.,  the	 range
		     U+000000-U+0000FF).  The parser  will  stop  just	before
		     this range	overflows, or when the maximum of three	digits
		     is	reached.  The upper bits of the	Unicode	character will
		     be	0.

	      \xhh   The  hexadecimal  digits  hh (one or two of them) give an
		     eight-bit hexadecimal value  for  the  Unicode  character
		     that  will	 be  inserted.	 The upper bits	of the Unicode
		     character will be 0 (i.e.,	the character will be  in  the
		     range U+000000-U+0000FF).

	      \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.  The upper bits of
		     the Unicode character will	be 0 (i.e., the	character will
		     be	in the range U+000000-U+00FFFF).

	      \Uhhhhhhhh
		     The hexadecimal digits hhhhhhhh (one up to	eight of them)
		     give a twenty-one-bit hexadecimal value for  the  Unicode
		     character	 that	will   be   inserted,	in  the	 range
		     U+000000-U+10FFFF.	 The parser will stop just before this
		     range  overflows,	or when	the maximum of eight digits is
		     reached.  The upper bits of the Unicode character will be
		     0.

		     The range U+010000-U+10FFFD is reserved for the future.

	      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.

KEYWORDS
       backslash, command, comment, script, substitution, variable

Tcl				      8.6				Tcl(n)

NAME | SYNOPSIS | DESCRIPTION | KEYWORDS

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