1Tcl(n) Tcl Built-In Commands Tcl(n)
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8 Tcl - Tool Command Language
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11 Summary of Tcl language syntax.
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15 The following rules define the syntax and semantics of the Tcl lan‐
16 guage:
17 [1] Commands.
19 A Tcl script is a string containing one or more commands. Semi-
20 colons and newlines are command separators unless quoted as
21 described below. Close brackets are command terminators during
22 command substitution (see below) unless quoted.
23 [2] Evaluation.
25 A command is evaluated in two steps. First, the Tcl interpreter
26 breaks the command into words and performs substitutions as
27 described below. These substitutions are performed in the same
28 way for all commands. Secondly, the first word is used to
29 locate a command procedure to carry out the command, then all of
30 the words of the command are passed to the command procedure.
31 The command procedure is free to interpret each of its words in
32 any way it likes, such as an integer, variable name, list, or
33 Tcl script. Different commands interpret their words differ‐
34 ently.
35 [3] Words.
37 Words of a command are separated by white space (except for new‐
38 lines, which are command separators).
39 [4] Double quotes.
41 If the first character of a word is double-quote (“"”) then the
42 word is terminated by the next double-quote character. If semi-
43 colons, close brackets, or white space characters (including
44 newlines) appear between the quotes then they are treated as
45 ordinary characters and included in the word. Command substitu‐
46 tion, variable substitution, and backslash substitution are per‐
47 formed on the characters between the quotes as described below.
48 The double-quotes are not retained as part of the word.
49 [5] Argument expansion.
51 If a word starts with the string “{*}” followed by a non-white‐
52 space character, then the leading “{*}” is removed and the rest
53 of the word is parsed and substituted as any other word. After
54 substitution, the word is parsed as a list (without command or
55 variable substitutions; backslash substitutions are performed as
56 is normal for a list and individual internal words may be sur‐
57 rounded by either braces or double-quote characters), and its
58 words are added to the command being substituted. For instance,
59 “cmd a {*}{b [c]} d {*}{$e f {g h}}” is equivalent to “cmd a b
60 {[c]} d {$e} f {g h}”.
61 [6] Braces.
63 If the first character of a word is an open brace (“{”) and rule
64 [5] does not apply, then the word is terminated by the matching
65 close brace (“}”). Braces nest within the word: for each addi‐
66 tional open brace there must be an additional close brace (how‐
67 ever, if an open brace or close brace within the word is quoted
68 with a backslash then it is not counted in locating the matching
69 close brace). No substitutions are performed on the characters
70 between the braces except for backslash-newline substitutions
71 described below, nor do semi-colons, newlines,