Tcl(n)
_________________________________________________________________
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
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] 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 com-
mand 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 of a command are separated by white space
(except for newlines, which are command separa-
tors).
[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
brackets, or white space characters (including new-
lines) 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] 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
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 characters following the open bracket as a Tcl
script. The script may contain any number of com-
mands 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 substitu-
tion is not performed on words enclosed in braces.
[7] If a word contains a dollar-sign (``$'') then Tcl
performs variable substitution: the dollar-sign
and the following characters are replaced in the
word by the value of a variable. Variable substi-
tution may take any of the following forms:
$name Name is the name of a scalar vari-
able; the name is terminated by any
character that isn't a letter,
digit, or underscore.
$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 substitu-
tions, variable substitutions, and
backslash substitutions are per-
formed on the characters of index.
${name} Name is the name of a scalar vari-
able. 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 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 spe-
cially, 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 back-
slash, 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 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 hexadeci-
mal 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 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] Each character is processed exactly once by the Tcl
interpreter as part of creating the words of a com-
mand. 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 sub-
stitutions are performed before making the recur-
sive call and no additional substitutions are per-
formed on the result of the nested script.
[11] Substitutions do not affect the word boundaries of
a command. For example, during variable substitu-
tion the entire value of the variable becomes part
of a single word, even if the variable's value con-
tains spaces.