Provided by: tcl8.6-doc_8.6.13+dfsg-2_all 
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 (0–9, A–Z and a–z).
$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 (0–9,
A–Z and a–z). 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 000–377 (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