1Tcl(n)                       Tcl Built-In Commands                      Tcl(n)
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NAME

8       Tcl - Tool Command Language
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SYNOPSIS

11       Summary of Tcl language syntax.
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DESCRIPTION

15       The  following  rules  define  the syntax and semantics of the Tcl lan‐
16       guage:
17
18       [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
24       [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
36       [3] Words.
37              Words of a command are separated by white space (except for new‐
38              lines, which are command separators).
39
40       [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
50       [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
62       [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, close brackets,
72              or white space receive any  special  interpretation.   The  word
73              will consist of exactly the characters between the outer braces,
74              not including the braces themselves.
75
76       [7] Command substitution.
77              If a word contains an open bracket (“[”) then Tcl performs  com‐
78              mand  substitution.   To  do this it invokes the Tcl interpreter
79              recursively to process the characters following the open bracket
80              as  a Tcl script.  The script may contain any number of commands
81              and must be terminated by a close bracket (“]”).  The result  of
82              the  script (i.e. the result of its last command) is substituted
83              into the word in place of the brackets and all of the characters
84              between  them.  There may be any number of command substitutions
85              in a single word.  Command  substitution  is  not  performed  on
86              words enclosed in braces.
87
88       [8] Variable substitution.
89              If  a  word  contains a dollar-sign (“$”) followed by one of the
90              forms described below, then Tcl performs variable  substitution:
91              the dollar-sign and the following characters are replaced in the
92              word by the value of a variable.  Variable substitution may take
93              any of the following forms:
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95              $name          Name  is the name of a scalar variable;  the name
96                             is a sequence of one or more characters that  are
97                             a letter, digit, underscore, or namespace separa‐
98                             tors (two or more colons).   Letters  and  digits
99                             are  only  the  standard ASCII ones (09, AZ and
100                             az).
101
102              $name(index)   Name gives the name  of  an  array  variable  and
103                             index  gives  the  name of an element within that
104                             array.  Name must contain only  letters,  digits,
105                             underscores, and namespace separators, and may be
106                             an empty string.  Letters and digits are only the
107                             standard  ASCII ones (09, AZ and az).  Command
108                             substitutions, variable substitutions, and  back‐
109                             slash  substitutions are performed on the charac‐
110                             ters of index.
111
112              ${name}        Name is the name of a scalar  variable  or  array
113                             element.   It  may contain any characters whatso‐
114                             ever except for close braces.   It  indicates  an
115                             array  element  if  name  is  in the form “array‐
116                             Name(index)” where arrayName does not contain any
117                             open  parenthesis characters, “(”, or close brace
118                             characters, “}”, and index can be any sequence of
119                             characters except for close brace characters.  No
120                             further substitutions are  performed  during  the
121                             parsing of name.
122
123              There  may  be  any number of variable substitutions in a single
124              word.  Variable substitution is not performed on words  enclosed
125              in braces.
126
127              Note  that  variables may contain character sequences other than
128              those listed above, but in that case other  mechanisms  must  be
129              used to access them (e.g., via the set command's single-argument
130              form).
131
132       [9] Backslash substitution.
133              If a backslash (“\”) appears within a word then  backslash  sub‐
134              stitution  occurs.   In  all cases but those described below the
135              backslash is dropped and the following character is  treated  as
136              an  ordinary  character  and  included in the word.  This allows
137              characters such as double quotes,  close  brackets,  and  dollar
138              signs  to  be  included in words without triggering special pro‐
139              cessing.  The following table lists the backslash sequences that
140              are  handled  specially, along with the value that replaces each
141              sequence.
142
143              \a     Audible alert (bell) (Unicode U+000007).
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145              \b     Backspace (Unicode U+000008).
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147              \f     Form feed (Unicode U+00000C).
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149              \n     Newline (Unicode U+00000A).
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151              \r     Carriage-return (Unicode U+00000D).
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153              \t     Tab (Unicode U+000009).
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155              \v     Vertical tab (Unicode U+00000B).
156
157              \<newline>whiteSpace
158                     A single space character replaces the backslash, newline,
159                     and  all  spaces  and tabs after the newline.  This back‐
160                     slash sequence is unique in that it is replaced in a sep‐
161                     arate  pre-pass  before  the  command is actually parsed.
162                     This means that it will be replaced even when  it  occurs
163                     between  braces,  and the resulting space will be treated
164                     as a word separator if it is not in braces or quotes.
165
166              \\     Backslash (“\”).
167
168              \ooo   The digits ooo (one, two, or three of them) give a eight-
169                     bit  octal  value  for the Unicode character that will be
170                     inserted,  in  the  range  000377   (i.e.,   the   range
171                     U+000000–U+0000FF).   The  parser  will  stop just before
172                     this range overflows, or when the maximum of three digits
173                     is reached.  The upper bits of the Unicode character will
174                     be 0.
175
176              \xhh   The hexadecimal digits hh (one or two of  them)  give  an
177                     eight-bit  hexadecimal  value  for  the Unicode character
178                     that will be inserted.  The upper  bits  of  the  Unicode
179                     character  will  be 0 (i.e., the character will be in the
180                     range U+000000–U+0000FF).
181
182              \uhhhh The hexadecimal digits hhhh (one, two, three, or four  of
183                     them)  give  a sixteen-bit hexadecimal value for the Uni‐
184                     code character that will be inserted.  The upper bits  of
185                     the Unicode character will be 0 (i.e., the character will
186                     be in the range U+000000–U+00FFFF).
187
188              \Uhhhhhhhh
189                     The hexadecimal digits hhhhhhhh (one up to eight of them)
190                     give  a  twenty-one-bit hexadecimal value for the Unicode
191                     character  that  will   be   inserted,   in   the   range
192                     U+000000–U+10FFFF.  The parser will stop just before this
193                     range overflows, or when the maximum of eight  digits  is
194                     reached.  The upper bits of the Unicode character will be
195                     0.
196
197                     The range U+010000–U+10FFFD is reserved for the future.
198
199              Backslash substitution is not performed  on  words  enclosed  in
200              braces, except for backslash-newline as described above.
201
202       [10] Comments.
203              If  a  hash  character  (“#”)  appears  at  a point where Tcl is
204              expecting the first character of the first word  of  a  command,
205              then  the  hash  character and the characters that follow it, up
206              through the next newline, are treated as a comment and  ignored.
207              The  comment  character only has significance when it appears at
208              the beginning of a command.
209
210       [11] Order of substitution.
211              Each character is processed exactly once by the Tcl  interpreter
212              as  part  of  creating  the words of a command.  For example, if
213              variable substitution occurs then no further  substitutions  are
214              performed  on  the value of the variable;  the value is inserted
215              into the word verbatim.  If command substitution occurs then the
216              nested  command  is  processed entirely by the recursive call to
217              the Tcl interpreter; no substitutions are performed before  mak‐
218              ing  the recursive call and no additional substitutions are per‐
219              formed on the result of the nested script.
220
221              Substitutions take place from left to right, and each  substitu‐
222              tion  is  evaluated completely before attempting to evaluate the
223              next.  Thus, a sequence like
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225                     set y [set x 0][incr x][incr x]
226
227              will always set the variable y to the value, 012.
228
229       [12] Substitution and word boundaries.
230              Substitutions do not affect the word boundaries  of  a  command,
231              except  for  argument  expansion  as specified in rule [5].  For
232              example, during variable substitution the entire  value  of  the
233              variable  becomes  part of a single word, even if the variable's
234              value contains spaces.
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KEYWORDS

237       backslash, command, comment, script, substitution, variable
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241Tcl                                   8.6                               Tcl(n)
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