Tcl(n)TclTcl(n)
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NAME
Tcl - Summary of Tcl language syntax.
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DESCRIPTION
The following rules define the syntax and semantics of the
Tcl language:
[1] 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] 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 commands
interpret their words differently.
[3] Words of a command are separated by white space (except
for newlines, which are command separators).
[4] 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] If the first character of a word is an open brace
(``{'') 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
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Tcl(n)TclTcl(n)
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.
[6] 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.
[7] If a word contains a dollar-sign (``$'') 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 terminated by any character
that isn't a letter, digit, or
underscore.
$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, and underscores.
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.
[8] 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
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Tcl(n)TclTcl(n)
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).
\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 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 isn't in braces or
quotes.
\\ Backslash (``\'').
\ooo The digits ooo (one, two, or three of them) give
the octal value of the character.
\xhh The hexadecimal digits hh give the hexadecimal
value of the character. Any number of digits may
be present.
Backslash substitution is not performed on words
enclosed in braces, except for backslash-newline as
described above.
[9] 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.
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Tcl(n)TclTcl(n)
[10] 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.
[11] 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 the variable's value contains spaces.
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