path: root/doc/Tcl.n

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.TH Tcl n "8.6" Tcl "Tcl Built-In Commands"
.so man.macros
.BS
.SH NAME
Tcl \- Tool Command Language
.SH SYNOPSIS
Summary of Tcl language syntax.
.BE
.SH DESCRIPTION
.PP
The following rules define the syntax and semantics of the Tcl language:
.IP "[1] \fBCommands.\fR"
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.
.IP "[2] \fBEvaluation.\fR"
A command is evaluated in two steps.
First, the Tcl interpreter breaks the command into \fIwords\fR
and performs substitutions as described below.
These substitutions are performed in the same way for all
commands.
Secondly, the first word is used to locate a routine to
carry out the command, and the remaining words of the command are
passed to that routine.
The routine 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.
.IP "[3] \fBWords.\fR"
Words of a command are separated by white space (except for
newlines, which are command separators).
.IP "[4] \fBDouble quotes.\fR"
If the first character of a word is double-quote
.PQ \N'34'
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.
.IP "[5] \fBArgument expansion.\fR"
If a word starts with the string
.QW {*}
followed by a non-whitespace character, then the leading
.QW {*}
is removed and the rest of the word is parsed and substituted as any other
word. After substitution, the word is parsed as a list (without command or
variable substitutions; backslash substitutions are performed as is normal for
a list and individual internal words may be surrounded by either braces or
double-quote characters), and its words are added to the command being
substituted. For instance,
.QW "cmd a {*}{b [c]} d {*}{$e f {g h}}"
is equivalent to
.QW "cmd a b {[c]} d {$e} f {g h}" .
.IP "[6] \fBBraces.\fR"
If the first character of a word is an open brace
.PQ {
and rule [5] does not apply, then
the word is terminated by the matching close brace
.PQ } "" .
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.
.IP "[7] \fBCommand substitution.\fR"
If a word contains an open bracket
.PQ [
then Tcl performs \fIcommand substitution\fR.
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
.PQ ] "" .
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.
.IP "[8] \fBVariable substitution.\fR"
If a word contains a dollar-sign
.PQ $
followed by one of the forms
described below, then Tcl performs \fIvariable
substitution\fR:  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:
.RS
.TP 15
\fB$\fIname\fR
.
\fIName\fR 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).
Letters and digits are \fIonly\fR the standard ASCII ones (\fB0\fR\(en\fB9\fR,
\fBA\fR\(en\fBZ\fR and \fBa\fR\(en\fBz\fR).
.TP 15
\fB$\fIname\fB(\fIindex\fB)\fR
.
\fIName\fR gives the name of an array variable and \fIindex\fR gives
the name of an element within that array.
\fIName\fR must contain only letters, digits, underscores, and
namespace separators, and may be an empty string.
Letters and digits are \fIonly\fR the standard ASCII ones (\fB0\fR\(en\fB9\fR,
\fBA\fR\(en\fBZ\fR and \fBa\fR\(en\fBz\fR).
Command substitutions, variable substitutions, and backslash
substitutions are performed on the characters of \fIindex\fR.
.TP 15
\fB${\fIname\fB}\fR
.
\fIName\fR is the name of a scalar variable or array element.  It may contain
any characters whatsoever except for close braces.  It indicates an array
element if \fIname\fR is in the form
.QW \fIarrayName\fB(\fIindex\fB)\fR
where \fIarrayName\fR does not contain any open parenthesis characters,
.QW \fB(\fR ,
or close brace characters,
.QW \fB}\fR ,
and \fIindex\fR can be any sequence of characters except for close brace
characters.  No further
substitutions are performed during the parsing of \fIname\fR.
.PP
There may be any number of variable substitutions in a single word.
Variable substitution is not performed on words enclosed in braces.
.PP
Note that variables may contain character sequences other than those listed
above, but in that case other mechanisms must be used to access them (e.g.,
via the \fBset\fR command's single-argument form).
.RE
.IP "[9] \fBBackslash substitution.\fR"
If a backslash
.PQ \e
appears within a word then \fIbackslash substitution\fR 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 specially, along with the value that replaces each sequence.
.RS
.TP 7
\e\fBa\fR
Audible alert (bell) (Unicode U+000007).
.TP 7
\e\fBb\fR
Backspace (Unicode U+000008).
.TP 7
\e\fBf\fR
Form feed (Unicode U+00000C).
.TP 7
\e\fBn\fR
Newline (Unicode U+00000A).
.TP 7
\e\fBr\fR
Carriage-return (Unicode U+00000D).
.TP 7
\e\fBt\fR
Tab (Unicode U+000009).
.TP 7
\e\fBv\fR
Vertical tab (Unicode U+00000B).
.TP 7
\e\fB<newline>\fIwhiteSpace\fR
.
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 is not
in braces or quotes.
.TP 7
\e\e
Backslash
.PQ \e "" .
.TP 7
\e\fIooo\fR
.
The digits \fIooo\fR (one, two, or three of them) give a eight-bit octal
value for the Unicode character that will be inserted, in the range
\fI000\fR\(en\fI377\fR (i.e., the range U+000000\(enU+0000FF).
The parser will stop just before this range overflows, or when
the maximum of three digits is reached.  The upper bits of the Unicode
character will be 0.
.TP 7
\e\fBx\fIhh\fR
.
The hexadecimal digits \fIhh\fR (one or two of them) give an eight-bit
hexadecimal value for the Unicode character that will be inserted.  The upper
bits of the Unicode character will be 0 (i.e., the character will be in the
range U+000000\(enU+0000FF).
.TP 7
\e\fBu\fIhhhh\fR
.
The hexadecimal digits \fIhhhh\fR (one, two, three, or four of them) give a
sixteen-bit hexadecimal value for the Unicode character that will be
inserted.  The upper bits of the Unicode character will be 0 (i.e., the
character will be in the range U+000000\(enU+00FFFF).
.TP 7
\e\fBU\fIhhhhhhhh\fR
.
The hexadecimal digits \fIhhhhhhhh\fR (one up to eight of them) give a
twenty-one-bit hexadecimal value for the Unicode character that will be
inserted, in the range U+000000\(enU+10FFFF.  The parser will stop just
before this range overflows, or when the maximum of eight digits
is reached.  The upper bits of the Unicode character will be 0.
.RS
.PP
The range U+00D800\(enU+00DFFF is reserved for surrogates, which
are illegal on its own. Therefore, such sequences will result in
the replacement character U+FFFD. Surrogate pairs should be
encoded as single \e\fBU\fIhhhhhhhh\fR character.
.RE
.PP
Backslash substitution is not performed on words enclosed in braces,
except for backslash-newline as described above.
.RE
.IP "[10] \fBComments.\fR"
If a hash character
.PQ #
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.
.IP "[11] \fBOrder of substitution.\fR"
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.
.RS
.PP
Substitutions take place from left to right, and each substitution is
evaluated completely before attempting to evaluate the next.  Thus, a
sequence like
.PP
.CS
set y [set x 0][incr x][incr x]
.CE
.PP
will always set the variable \fIy\fR to the value, \fI012\fR.
.RE
.IP "[12] \fBSubstitution and word boundaries.\fR"
Substitutions do not affect the word boundaries of a command,
except for argument expansion as specified in rule [5].
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.
.SH KEYWORDS
backslash, command, comment, script, substitution, variable
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