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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 commands 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] Argument expansion.                                                                    │
              If a word starts with the string “{*}” followed by a non-whitespace 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 surrounded 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 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, 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  command  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 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.

       [9] Backslash substitution.
              If a backslash (“\”) appears within a word then backslash substitution 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 processing.  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 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 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  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.

       [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  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.

       [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.