Tcl

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.  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



Tcl                                   8.6                               Tcl(n)