Provided by: tcl9.0-doc_9.0.1+dfsg-1_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 routine to carry out the command, and
the remaining words of the command are passed to that routine. The routine 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.
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(3tcl)