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 lan-
guage:
[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 com-
mand 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 new-
lines, 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 (includ-
ing newlines) appear between the quotes then they are treated as
ordinary characters and included in the word. Command substitu-
tion, variable substitution, and backslash substitution are per-
formed on the characters between the quotes as described below.
The double-quotes are not retained as part of the word.
[5] Braces.
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 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 includ-
ing the braces themselves.
[6] 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 substi-
tuted 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] Variable substitution.
If a word contains a dollar-sign (``$'') then Tcl performs vari-
able 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 separa-
tors (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.
[8] Backslash substitution.
If a backslash (``\'') appears within a word then backslash sub-
stitution 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 pro-
cessing. 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 back-
slash sequence is unique in that it is replaced in a sep-
arate 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 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 char- |
acter will be 0. |
\uhhhh ||
The hexadecimal digits hhhh (one, two, three, or four of |
them) give a sixteen-bit hexadecimal value for the Uni- |
code character that will be inserted.
Backslash substitution is not performed on words enclosed in
braces, except for backslash-newline as described above.
[9] 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.
[10] 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 mak-
ing the recursive call and no additional substitutions are per-
formed on the result of the nested script.
Substitutions take place from left to right, and each substitu-
tion 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.
[11] Substitution and word boundaries.
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 vari-
able's value contains spaces.
Tcl 8.1 Tcl(n)
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