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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
	      described 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
	      described	 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
	      ordinary 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
			     index gives the name of an	 element  within  that
			     array.   Name  must contain only letters, digits,
			     underscores, 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) (0x7).

	      \b     Backspace (0x8).

	      \f     Form feed (0xc).

	      \n     Newline (0xa).

	      \r     Carriage-return (0xd).

	      \t     Tab (0x9).

	      \v     Vertical tab (0xb).

	      \<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.  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.

	      \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.

	      \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+0000..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
	      expecting 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
	      example, 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)
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