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ActiveTcl User Guide
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- NAME
- Tcl - Tool Command Language
- SYNOPSIS
- Summary of Tcl language syntax.
- DESCRIPTION
- [1] Commands.
- [2] Evaluation.
- [3] Words.
- [4] Double
quotes.
- [5] Braces.
- [6] Command
substitution.
- [7] Variable
substitution.
- $name
- $name(index)
- ${name}
- [8] Backslash
substitution.
- \a
- \b
- \f
- \n
- \r
- \t
- \v
- \<newline>whiteSpace
- \\
- \ooo
- \xhh
- \uhhhh
- [9] Comments.
- [10] Order of
substitution.
- [11] Substitution and word
boundaries.
Tcl - Tool Command Language
Summary of Tcl language syntax.
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 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 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.
- [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 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.
- [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-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.
- [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 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 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 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.
- [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 processing. 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 Unicode 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 hexadecimal value for the Unicode
character that will be inserted.
Backslash substitution is not performed on words enclosed 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 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 variable's
value contains spaces.
Copyright © 1993 The Regents of the University of California.
Copyright © 1994-1996 Sun Microsystems, Inc.
Copyright © 1995-1997 Roger E. Critchlow Jr.