Tcl(n)

✨ Tcl(n) ✨ — Tool Command Language

Tcl Built-In Commands 💖

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🎀 NAME 🎀

Tcl - Tool Command Language

🌸 SYNOPSIS 🌸

Summary of Tcl language syntax. ✨

💖 DESCRIPTION 💖

Okay bestie, buckle up — these rules define the syntax and semantics of the Tcl language. No cap. 🫶

[1] Commands 💅

A Tcl script is literally a string containing one or more commands. Semicolons and newlines are command separators unless quoted as described below. Close brackets are command terminators during command substitution (see below) unless quoted. Periodt. 💅

[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 routine to carry out the command, and the remaining words of the command are passed to that routine. The routine 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. It's giving flexible, ngl. 💅

[3] Words 🦋

Words of a command are separated by white space (except for newlines, which are command separators). Lowkey simple.

[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 semicolons, 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. So the quotes ate, but they don't stay. 😌

[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}. Obsessed with this little expansion moment. ✨

[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. Braces are the "do not touch" zone, fr fr. 🙈

[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. Nested chaos, but make it cute. 🌈

[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). Iykyk. 💅

[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 an 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.

Backslash substitution is not performed on words enclosed in braces, except for backslash-newline as described above. That's the tea. 🍵

• [10] Comments. — bestie, if a hash character (#) pops up at a spot where Tcl is expecting the first character of the first word of a command, then that # and every character that follows it, all the way through the next newline, get treated as a comment and just… ignored 💅. and like, real talk: the comment character only has significance when it appears at the beginning of a command. anywhere else it's just a regular character doing its little thing.
• [11] Order of substitution. — okay so here's the tea: each character is processed exactly once by the Tcl interpreter as part of creating the words of a command. so for example, if variable substitution happens, then no further substitutions are performed on the value of the variable — that value gets inserted into the word verbatim, no extra passes, no cap 🫶. and if command substitution happens, 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. it's giving ✨one and done✨.

substitutions take place from left to right, and each substitution is evaluated completely before even attempting to evaluate the next one. so a sequence like

set y [set x 0][incr x][incr x]

will always set the variable y to the value, 012. periodt. 💖

• [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. one word, spaces included, that's the vibe 🌸.

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✨ KEYWORDS ✨

backslash, command, comment, script, substitution, variable

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and that's a wrap, bestie 🎀 you just read the whole Tcl manual page like a literal icon 👑 this is Tcl 8.6, manual page Tcl(n) — go forth and script, you've got this 💜