tcl7.6 User Commands - Tcl
NAME
Tcl - Summary of Tcl language syntax.
DESCRIPTION
The following rules define the syntax and semantics of the
Tcl language:
[1] A Tcl script is a string containing one or more com-
mands. 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 com-
mand 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 brack-
ets, 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 sub-
stitution 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
described below, nor do semi-colons, newlines, close
brackets, or white space receive any special interpre-
tation. The word will consist of exactly the charac-
ters 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 charac-
ters 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. Com-
mand substitution is not performed on words enclosed in
braces.
[7] If a word contains a dollar-sign (``$'') then Tcl per-
forms 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 under-
score.
$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 substi-
tutions, 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 fol-
lowing 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 new-
line. 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 charac-
ters that follow it, up through the next newline, are
treated as a comment and ignored. The comment charac-
ter only has significance when it appears at the begin-
ning of a command.
[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 verba-
tim. 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 sub-
stitutions 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.