Tcl(3tcl)
NAME
Tcl - Tool Command Language
SYNOPSIS
Summary of Tcl language syntax. ______________________________________________________________________________
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 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).
$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).
\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 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
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.
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