Tcl Built-In Commands Tcl(1T)
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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 theTcl language:
[1] Commands.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] 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 thecommand 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. Words of a command are separated by white space (except for newlines, which are command separators). [4] Double quotes.If the first character of a word is double-quote
(``"'') then the word is terminated by the nextdouble-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. Commandsubstitution, variable substitution, and backslash sub-
stitution are performed on the characters between thequotes 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 matchingTcl Last change: 8.1 1
Tcl Built-In Commands Tcl(1T)
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 betweenthe 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] Command substitution.If a word contains an open bracket (``['') then Tcl
performs command substitution. To do this it invokesthe 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 anynumber of command substitutions in a single word. Com-
mand substitution is not performed on words enclosed in braces. [7] Variable substitution.If a word contains a dollar-sign (``$'') followed by
one of the forms described below, then Tcl performs
variable substitution: the dollar-sign and the follow-
ing characters are replaced in the word by the value of a variable. Variable substitution may take any of the following forms:$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 separators (two or more colons).$name(index) Name gives the name of an array variable
and index gives the name of an element within that array. Name must contain only letters, digits, underscores, and namespace separators, and may be an empty string. Command substitutions, variable substitutions, and backslashsubstitutions are performed on the char-
acters of index.${name} Name is the name of a scalar variable.
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Tcl Built-In Commands Tcl(1T)
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 substitution occurs. In all cases but thosedescribed 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 tobe 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). \whiteSpace A single space character replaces the backslash, newline, and all spaces and tabs after the new-
line. This backslash sequence is unique in thatit 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 Unicode charac- |
ter that will be inserted. The upper bits of | the Unicode character will be 0. |Tcl Last change: 8.1 3
Tcl Built-In Commands Tcl(1T)
\xhh ||The hexadecimal digits hh give an eight-bit hex- |
adecimal 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 Unicode character that will be | inserted. Backslash substitution is not performed on wordsenclosed 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 charac-
ters that follow it, up through the next newline, aretreated as a comment and ignored. The comment charac-
ter only has significance when it appears at the begin-
ning 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 thevariable; the value is inserted into the word verba-
tim. If command substitution occurs then the nested command is processed entirely by the recursive call tothe 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. 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 variable's value contains spaces.Tcl Last change: 8.1 4
Tcl Built-In Commands Tcl(1T)
ATTRIBUTES
See attributes(5) for descriptions of the following attri-
butes:_______________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE|
|____________________|__________________|_
| Availability | runtime/tcl-8 |
|____________________|__________________|_
| Interface Stability| Uncommitted ||____________________|_________________|
NOTESSource for Tcl is available on http://opensolaris.org.
Tcl Last change: 8.1 5