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

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NAME

       Tcl - Tool Command Language

SYNOPSIS

       Summary of Tcl language syntax.
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DESCRIPTION

       The following rules define the syntax and semantics of the Tcl language:

       [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 command
	      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 com-
	      mands interpret their words differently.

       [3] Words.
	      Words of a command are separated by white space (except for newlines, 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 substitution, variable substitution, and backslash substitution are performed  on  the
	      characters between the quotes as described below.  The double-quotes are not retained as part of the word.

       [5] Braces.
	      If  the  first character of a word is an open brace (``{'') then the word is terminated by the matching close brace (``}'').  Braces
	      nest within the word: for each additional open brace there must be an additional close brace (however, 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,  new-
	      lines,  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.

       [6] Command substitution.
	      If a word contains an open bracket (``['') then Tcl performs command substitution.  To do this it invokes the Tcl interpreter recur-
	      sively 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.

       [7] Variable substitution.
	      If a word contains a dollar-sign (``$'') 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 separators (two or more colons).

	      $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.  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.

       [8] Backslash substitution.
	      If  a  backslash (``'') appears within a word then backslash substitution occurs.  In all cases but those described below the back-
	      slash 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 processing.  The follow-
	      ing table lists the backslash sequences that are handled specially, along with the value that replaces each sequence.

	      a     Audible alert (bell) (0x7).

	           Backspace (0x8).

	      f     Form feed (0xc).

	      
     Newline (0xa).

	      
     Carriage-return (0xd).

	      	     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 backslash sequence
		     is  unique  in  that it is replaced in a separate 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  isn't	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 character will be 0.										   |

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

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

       [9] 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.

       [10] 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 vari-
	      able 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 inter-
	      preter; no substitutions are performed before making the recursive call and no additional substitutions are performed on the  result
	      of the nested script.

	      Substitutions  take  place from left to right, and each substitution 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.

       [11] Substitution and word boundaries.
	      Substitutions do not affect the word boundaries of a command.  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.

Tcl									8.1								    Tcl(n)