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 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 (``$'') followed by one of
the forms described below, 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.