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           Tcl - Tool Command Language


           Summary of Tcl language syntax.




           The  following  rules  define  the syntax and semantics of the Tcl lan-
           [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 expan-                                                     |
           sion.                                                 |                 |
                  If a word starts with the string "{*}" followed by a non-whites- |
                  pace  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 {*}{$e f "g h"}" is equivalent to "cmd  a  b |
                  outer braces, not including the braces themselves.
           [7] Command substitution.
                  If a word contains an open bracket ("[") then Tcl performs  com-
                  mand  substitution.   To  do this it invokes the Tcl interpreter
                  recursively to process the characters following the open bracket
                  as  a Tcl script.  The script may contain any number of commands
                  and must be terminated by a close bracket ("]").  The result  of
                  the  script (i.e. the result of its last command) is substituted
                  into the word in place of the brackets and all of the characters
                  between  them.  There may be any number of command substitutions
                  in a single word.  Command  substitution  is  not  performed  on
                  words enclosed in braces.
           [8] Variable substitution.
                  If  a  word  contains a dollar-sign ("$") 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).
                  $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  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
                         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  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 char-
                         acter 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.
                  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

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