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markom |
'\"
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'\" Copyright (c) 1993 The Regents of the University of California.
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'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
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'\"
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'\" See the file "license.terms" for information on usage and redistribution
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'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
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'\"
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'\" RCS: @(#) $Id: Tcl.n,v 1.1.1.1 2002-01-16 10:25:24 markom Exp $
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'
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.so man.macros
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.TH Tcl n "" Tcl "Tcl Built-In Commands"
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.BS
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.SH NAME
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Tcl \- Summary of Tcl language syntax.
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.BE
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.SH DESCRIPTION
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.PP
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The following rules define the syntax and semantics of the Tcl language:
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.IP [1]
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A Tcl script is a string containing one or more commands.
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Semi-colons and newlines are command separators unless quoted as
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described below.
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Close brackets are command terminators during command substitution
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(see below) unless quoted.
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.IP [2]
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A command is evaluated in two steps.
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First, the Tcl interpreter breaks the command into \fIwords\fR
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and performs substitutions as described below.
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These substitutions are performed in the same way for all
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commands.
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The first word is used to locate a command procedure to
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carry out the command, then all of the words of the command are
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passed to the command procedure.
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The command procedure is free to interpret each of its words
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in any way it likes, such as an integer, variable name, list,
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or Tcl script.
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Different commands interpret their words differently.
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.IP [3]
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Words of a command are separated by white space (except for
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newlines, which are command separators).
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.IP [4]
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If the first character of a word is double-quote (``"'') then
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the word is terminated by the next double-quote character.
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If semi-colons, close brackets, or white space characters
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(including newlines) appear between the quotes then they are treated
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as ordinary characters and included in the word.
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Command substitution, variable substitution, and backslash substitution
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are performed on the characters between the quotes as described below.
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The double-quotes are not retained as part of the word.
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.IP [5]
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If the first character of a word is an open brace (``{'') then
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the word is terminated by the matching close brace (``}'').
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Braces nest within the word: for each additional open
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brace there must be an additional close brace (however,
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if an open brace or close brace within the word is
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quoted with a backslash then it is not counted in locating the
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matching close brace).
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No substitutions are performed on the characters between the
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braces except for backslash-newline substitutions described
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below, nor do semi-colons, newlines, close brackets,
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or white space receive any special interpretation.
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The word will consist of exactly the characters between the
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outer braces, not including the braces themselves.
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.IP [6]
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If a word contains an open bracket (``['') then Tcl performs
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\fIcommand substitution\fR.
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To do this it invokes the Tcl interpreter recursively to process
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the characters following the open bracket as a Tcl script.
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The script may contain any number of commands and must be terminated
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by a close bracket (``]'').
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The result of the script (i.e. the result of its last command) is
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substituted into the word in place of the brackets and all of the
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characters between them.
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There may be any number of command substitutions in a single word.
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Command substitution is not performed on words enclosed in braces.
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.IP [7]
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If a word contains a dollar-sign (``$'') then Tcl performs \fIvariable
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substitution\fR: the dollar-sign and the following characters are
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replaced in the word by the value of a variable.
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Variable substitution may take any of the following forms:
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.RS
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.TP 15
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\fB$\fIname\fR
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\fIName\fR is the name of a scalar variable; the name is terminated
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by any character that isn't a letter, digit, or underscore.
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.TP 15
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\fB$\fIname\fB(\fIindex\fB)\fR
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\fIName\fR gives the name of an array variable and \fIindex\fR gives
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the name of an element within that array.
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\fIName\fR must contain only letters, digits, and underscores.
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Command substitutions, variable substitutions, and backslash
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substitutions are performed on the characters of \fIindex\fR.
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.TP 15
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\fB${\fIname\fB}\fR
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\fIName\fR is the name of a scalar variable. It may contain any
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characters whatsoever except for close braces.
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.LP
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There may be any number of variable substitutions in a single word.
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Variable substitution is not performed on words enclosed in braces.
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.RE
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.IP [8]
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If a backslash (``\e'') appears within a word then
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\fIbackslash substitution\fR occurs.
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In all cases but those described below the backslash is dropped and
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the following character is treated as an ordinary
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character and included in the word.
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This allows characters such as double quotes, close brackets,
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and dollar signs to be included in words without triggering
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special processing.
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The following table lists the backslash sequences that are
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handled specially, along with the value that replaces each sequence.
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.RS
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.TP 6
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\e\fBa\fR
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Audible alert (bell) (0x7).
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.TP 6
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\e\fBb\fR
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Backspace (0x8).
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.TP 6
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\e\fBf\fR
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Form feed (0xc).
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.TP 6
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\e\fBn\fR
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Newline (0xa).
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.TP 6
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\e\fBr\fR
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Carriage-return (0xd).
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.TP 6
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\e\fBt\fR
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Tab (0x9).
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.TP 6
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\e\fBv\fR
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Vertical tab (0xb).
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.TP 6
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\e\fB\fIwhiteSpace\fR
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A single space character replaces the backslash, newline, and all
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spaces and tabs after the newline.
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This backslash sequence is unique in that it is replaced in a separate
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pre-pass before the command is actually parsed.
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This means that it will be replaced even when it occurs between
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braces, and the resulting space will be treated as a word separator
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if it isn't in braces or quotes.
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.TP 6
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\e\e
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Backslash (``\e'').
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.TP 6
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\e\fIooo\fR
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The digits \fIooo\fR (one, two, or three of them) give the octal value of
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the character.
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.TP 6
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\e\fBx\fIhh\fR
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The hexadecimal digits \fIhh\fR give the hexadecimal value of
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the character. Any number of digits may be present.
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.LP
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Backslash substitution is not performed on words enclosed in braces,
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except for backslash-newline as described above.
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.RE
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.IP [9]
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If a hash character (``#'') appears at a point where Tcl is
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expecting the first character of the first word of a command,
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then the hash character and the characters that follow it, up
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through the next newline, are treated as a comment and ignored.
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The comment character only has significance when it appears
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at the beginning of a command.
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.IP [10]
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Each character is processed exactly once by the Tcl interpreter
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as part of creating the words of a command.
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For example, if variable substitution occurs then no further
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substitutions are performed on the value of the variable; the
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value is inserted into the word verbatim.
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If command substitution occurs then the nested command is
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processed entirely by the recursive call to the Tcl interpreter;
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no substitutions are performed before making the recursive
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call and no additional substitutions are performed on the result
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of the nested script.
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.IP [11]
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Substitutions do not affect the word boundaries of a command.
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For example, during variable substitution the entire value of
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the variable becomes part of a single word, even if the variable's
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value contains spaces.
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