Tcl Built-In Commands
NAME - Tcl - Summary of Tcl language syntax.
DESCRIPTION
The following rules define the syntax and semantics of the Tcl language:
- 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.
- 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.
- Words of a command are separated by white space (except for newlines,
which are command separators).
- 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.
- 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.
- 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.
- 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 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
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 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.
- 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 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.
- 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.
- 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 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 contains spaces.