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.
Secondly, 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] Argument expansion.
- If a word starts with the string “{*}” followed by a
non-whitespace character, then the leading “{*}” 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, “cmd a {*}{b [c]} d {*}{$e f {g
h}}” is equivalent to “cmd a b {[c]} d {$e} f {g
h}”.
- [6] Braces.
- If the first character of a word is an open brace (“{”) and
rule [5] does not apply, 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.
- [7] 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.
- [8] 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). Letters and digits are only the
standard ASCII ones (0–9, A–Z
and a–z).
- $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. Letters and digits are only the standard ASCII ones
(0–9, A–Z and
a–z). Command substitutions, variable substitutions,
and backslash substitutions are performed on the characters of
index.
- ${name}
- Name 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 name is in the form
“arrayName(index)” where
arrayName does not contain any open parenthesis characters,
“(”, or close brace characters,
“}”, and index can be any sequence of
characters except for close brace characters. No further substitutions are
performed during the parsing of name.
There may be any number of variable substitutions in a single
word. Variable substitution is not performed on words enclosed in
braces.
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 set command's single-argument form).
- [9] 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) (Unicode U+000007).
- \b
- Backspace (Unicode U+000008).
- \f
- Form feed (Unicode U+00000C).
- \n
- Newline (Unicode U+00000A).
- \r
- Carriage-return (Unicode U+00000D).
- \t
- Tab (Unicode U+000009).
- \v
- Vertical tab (Unicode U+00000B).
- \<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 is not
in braces or quotes.
- \\
- Backslash (“\”).
- \ooo
- The digits ooo (one, two, or three of them) give a eight-bit octal
value for the Unicode character that will be inserted, in the range
000–377 (i.e., the range U+000000–U+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.
- \xhh
- The hexadecimal digits hh (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–U+0000FF).
- \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. The upper bits of the Unicode character will be 0 (i.e., the
character will be in the range U+000000–U+00FFFF).
- \Uhhhhhhhh
- The hexadecimal digits hhhhhhhh (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–U+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.
The range U+010000–U+10FFFD is reserved for the future.
Backslash substitution is not performed on words enclosed in
braces, except for backslash-newline as described above.
- [10] 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.
- [11] 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.
- [12] Substitution and word boundaries.
- 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.
backslash, command, comment, script, substitution, variable