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] 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 again without substitutions, and its  words |
              are added to the command being substituted. For instance, "cmd a {*}{b c} d {*}{e f}" is equivalent to "cmd a b c d e f".

       [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  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.

       [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).

                   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 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  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.

       [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.

Tcl                                                                     8.5                                                                 Tcl(n)