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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.  Secondly, 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).  Letters and  digits  are
                             only the standard ASCII ones (09, AZ and az).

              $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  (09,
                             AZ  and  az).   Command substitutions, variable substitutions, and
                             backslash substitutions are performed on the characters of index.

              ${name}        Name is the name of a scalar variable  or  array  element.   It  may
                             contain  any  characters  whatsoever  except  for  close braces.  It
                             indicates an array element if name is in the form “arrayName(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 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) (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 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 a eight-bit octal value for
                     the Unicode character that will be inserted, in the range 000377 (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  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+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 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.

KEYWORDS

       backslash, command, comment, script, substitution, variable