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Tcl(n) - Tcl, Tcl - Tool Command Language - man n Tcl

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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(3,n) 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(1,8) two steps.  First, the Tcl interpreter
              breaks the command into  words  and  performs  substitutions  as
              described  below.  These substitutions are performed in(1,8) 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(1,8) 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(2,7,n) brackets, or white space characters  (includ-
              ing newlines) appear between the quotes then they are treated as
              ordinary characters and included in(1,8) 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] Braces.
              If  the  first character of a word is an open(2,3,n) brace (``{'') then
              the word is terminated by  the  matching  close(2,7,n)  brace  (``}'').
              Braces  nest  within  the  word:  for each additional open(2,3,n) brace
              there must be an additional close(2,7,n) brace  (however,  if(3,n)  an  open(2,3,n)
              brace  or close(2,7,n) brace within the word is quoted with a backslash
              then it is not counted in(1,8) locating the  matching  close(2,7,n)  brace).
              No  substitutions  are  performed  on the characters between the
              braces  except  for  backslash-newline  substitutions  described
              below,  nor  do  semi-colons, newlines, close(2,7,n) brackets, or white
              space receive any special interpretation.  The word will consist
              of  exactly the characters between the outer braces, not includ-
              ing the braces themselves.

       [6] Command substitution.
              If a word contains an open(2,3,n) bracket  (``['')  then  Tcl  performs
              command substitution.  To do this it invokes the Tcl interpreter
              recursively to process the characters following the open(2,3,n) bracket
              as  a Tcl script.  The script may contain any number of commands
              and must be terminated by a close(2,7,n) bracket (``]'').   The  result
              of  the  script (i.e. the result of its last command) is substi-
              tuted into the word in(1,8) place of the  brackets  and  all  of  the
              characters  between  them.   There  may be any number of command
              substitutions in(1,8) a single word.   Command  substitution  is  not
              performed on words enclosed in(1,8) braces.

       [7] Variable substitution.
              If a word contains a dollar-sign (``$'') then Tcl performs vari-
              able substitution:  the dollar-sign and the following characters
              are  replaced  in(1,8) 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(2,7,n) braces.

              There  may  be  any number of variable substitutions in(1,8) a single
              word.  Variable substitution is not performed on words  enclosed
              in(1,8) braces.

       [8] 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(1,8)  the  word.   This  allows
              characters  such  as  double  quotes, close(2,7,n) brackets, and dollar
              signs to be included in(1,8) 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(1,8) that it is replaced in(1,8) 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(3,n) it isn't in(1,8) 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(1,8)
              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(3,n)
              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(7,n,1 builtins) y [set(7,n,1 builtins) x 0][incr x][incr x]
              will always set(7,n,1 builtins) 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(3,n)  the  vari-
              able's value contains spaces.



Tcl                                   8.1                               Tcl(n)

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