Tcl(n) Tcl Built-In Commands Tcl(n)
____________________________________________________________________________________________________________
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 com-mand command
mand separators unless quoted as described below. Close brackets are command terminators dur-ing during
ing 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 com-mand command
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.
Words of a command are separated by white space (except for newlines, which are command sepa-rators). separators).
rators).
[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 substi-tution substitution
tution are performed on the characters between the quotes as described below. The double-quotes doublequotes
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-new-line backslash-newline
line 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] 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.
[7] Variable substitution.
If a word contains a dollar-sign (``$'') then Tcl performs variable substitution: the dollar-sign dollarsign
sign and the following characters are replaced in the word by the value of a variable. Vari-able Variable
able 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, vari-able variable
able 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 whatso-ever whatsoever
ever 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 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 process-ing. processing.
ing. 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 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 Uni- |
code 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 hexa- |
decimal value for the Unicode character that will be inserted.
Backslash substitution is not performed on words enclosed in braces, except for backslash-new-line backslash-newline
line 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 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.
[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 variable's
able's value contains spaces.
Tcl 8.1 Tcl(n)
|