Manual Pages for UNIX Darwin command on man Tcl

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

NAME

Tcl - Tool Command Language

SYNOPSIS

Summary of Tcl language syntax.

DESCRIPTION

The following rules define the syntax and semantics of the Tcl lan-

guage: [1] CCoommmmaannddss..

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] EEvvaalluuaattiioonn..

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 com-

mand 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] WWoorrddss..

Words of a command are separated by white space (except for new-

lines, which are command separators). [4] DDoouubbllee qquuootteess..

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 (includ-

ing newlines) appear between the quotes then they are treated as

ordinary characters and included in the word. Command substitu-

tion, variable substitution, and backslash substitution are per-

formed on the characters between the quotes as described below.

The double-quotes are not retained as part of the word.

[5] BBrraacceess.. 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 includ-

ing the braces themselves. [6] CCoommmmaanndd ssuubbssttiittuuttiioonn..

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 substi-

tuted 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] VVaarriiaabbllee ssuubbssttiittuuttiioonn..

If a word contains a dollar-sign (``$'') then Tcl performs vari-

able 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 separa-

tors (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] BBaacckkssllaasshh ssuubbssttiittuuttiioonn..

If a backslash (``\'') appears within a word then backslash sub-

stitution 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 pro-

cessing. The following table lists the backslash sequences that are handled specially, along with the value that replaces each sequence. \aa Audible alert (bell) (0x7). \bb Backspace (0x8). \ff Form feed (0xc). \nn Newline (0xa).

\rr Carriage-return (0xd).

\tt Tab (0x9). \vv Vertical tab (0xb). \<>whiteSpace A single space character replaces the backslash, newline,

and all spaces and tabs after the newline. This back-

slash sequence is unique in that it is replaced in a sep-

arate 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. | \xxhh ||

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 char- |

acter will be 0. | \uuhhhh || The hexadecimal digits hhhh (one, two, three, or four of |

them) give a sixteen-bit hexadecimal value for the Uni- |

code character that will be inserted. Backslash substitution is not performed on words enclosed in

braces, except for backslash-newline as described above.

[9] CCoommmmeennttss..

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] OOrrddeerr ooff ssuubbssttiittuuttiioonn..

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 mak-

ing the recursive call and no additional substitutions are per-

formed on the result of the nested script.

Substitutions take place from left to right, and each substitu-

tion 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] SSuubbssttiittuuttiioonn aanndd wwoorrdd bboouunnddaarriieess.. 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 vari-

able's value contains spaces.