195 lines
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195 lines
9.0 KiB
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<HTML><HEAD><TITLE>Tcl Built-In Commands - Tcl manual page</TITLE></HEAD><BODY>
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<DL>
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<DD><A HREF="Tcl.htm#M2" NAME="L2">NAME</A>
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<DL><DD>Tcl - Summary of Tcl language syntax.</DL>
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<DD><A HREF="Tcl.htm#M3" NAME="L3">DESCRIPTION</A>
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<DL>
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<DL>
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<DD><A HREF="Tcl.htm#M4" NAME="L4"><B>$</B><I>name</I></A>
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<DD><A HREF="Tcl.htm#M5" NAME="L5"><B>$</B><I>name</I><B>(</B><I>index</I><B>)</B></A>
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<DD><A HREF="Tcl.htm#M6" NAME="L6"><B>${</B><I>name</I><B>}</B></A>
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</DL>
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<DL>
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<DD><A HREF="Tcl.htm#M7" NAME="L7">\<B>a</B></A>
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<DD><A HREF="Tcl.htm#M8" NAME="L8">\<B>b</B></A>
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<DD><A HREF="Tcl.htm#M9" NAME="L9">\<B>f</B></A>
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<DD><A HREF="Tcl.htm#M10" NAME="L10">\<B>n</B></A>
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<DD><A HREF="Tcl.htm#M11" NAME="L11">\<B>r</B></A>
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<DD><A HREF="Tcl.htm#M12" NAME="L12">\<B>t</B></A>
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<DD><A HREF="Tcl.htm#M13" NAME="L13">\<B>v</B></A>
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<DD><A HREF="Tcl.htm#M14" NAME="L14">\<B><newline></B><I>whiteSpace</I></A>
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<DD><A HREF="Tcl.htm#M15" NAME="L15">\\</A>
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<DD><A HREF="Tcl.htm#M16" NAME="L16">\<I>ooo</I></A>
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<DD><A HREF="Tcl.htm#M17" NAME="L17">\<B>x</B><I>hh</I></A>
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<DD><A HREF="Tcl.htm#M18" NAME="L18">\<B>u</B><I>hhhh</I></A>
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</DL>
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</DL>
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</DL><HR>
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<H3><A NAME="M2">NAME</A></H3>
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Tcl - Summary of Tcl language syntax.
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<H3><A NAME="M3">DESCRIPTION</A></H3>
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The following rules define the syntax and semantics of the Tcl language:
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<P>
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<DL>
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<P><DT>[1]<DD>
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A Tcl script is a string containing one or more commands.
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Semi-colons and newlines are command separators unless quoted as
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described below.
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Close brackets are command terminators during command substitution
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(see below) unless quoted.
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<P><DT>[2]<DD>
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A command is evaluated in two steps.
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First, the Tcl interpreter breaks the command into <I>words</I>
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and performs substitutions as described below.
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These substitutions are performed in the same way for all
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commands.
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The first word is used to locate a command procedure to
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carry out the command, then all of the words of the command are
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passed to the command procedure.
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The command procedure is free to interpret each of its words
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in any way it likes, such as an integer, variable name, list,
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or Tcl script.
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Different commands interpret their words differently.
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<P><DT>[3]<DD>
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Words of a command are separated by white space (except for
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newlines, which are command separators).
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<P><DT>[4]<DD>
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If the first character of a word is double-quote (``"'') then
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the word is terminated by the next double-quote character.
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If semi-colons, close brackets, or white space characters
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(including newlines) appear between the quotes then they are treated
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as ordinary characters and included in the word.
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Command substitution, variable substitution, and backslash substitution
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are performed on the characters between the quotes as described below.
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The double-quotes are not retained as part of the word.
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<P><DT>[5]<DD>
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If the first character of a word is an open brace (``{'') then
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the word is terminated by the matching close brace (``}'').
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Braces nest within the word: for each additional open
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brace there must be an additional close brace (however,
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if an open brace or close brace within the word is
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quoted with a backslash then it is not counted in locating the
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matching close brace).
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No substitutions are performed on the characters between the
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braces except for backslash-newline substitutions described
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below, nor do semi-colons, newlines, close brackets,
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or white space receive any special interpretation.
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The word will consist of exactly the characters between the
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outer braces, not including the braces themselves.
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<P><DT>[6]<DD>
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If a word contains an open bracket (``['') then Tcl performs
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<I>command substitution</I>.
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To do this it invokes the Tcl interpreter recursively to process
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the characters following the open bracket as a Tcl script.
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The script may contain any number of commands and must be terminated
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by a close bracket (``]'').
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The result of the script (i.e. the result of its last command) is
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substituted into the word in place of the brackets and all of the
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characters between them.
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There may be any number of command substitutions in a single word.
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Command substitution is not performed on words enclosed in braces.
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<P><DT>[7]<DD>
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If a word contains a dollar-sign (``$'') then Tcl performs <I>variable
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substitution</I>: the dollar-sign and the following characters are
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replaced in the word by the value of a variable.
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Variable substitution may take any of the following forms:
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<P>
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<DL>
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<P><DT><A NAME="M4"><B>$</B><I>name</I></A><DD>
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<I>Name</I> is the name of a scalar variable; the name is terminated
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by any character that isn't a letter, digit, or underscore.
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<P><DT><A NAME="M5"><B>$</B><I>name</I><B>(</B><I>index</I><B>)</B></A><DD>
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<I>Name</I> gives the name of an array variable and <I>index</I> gives
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the name of an element within that array.
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<I>Name</I> must contain only letters, digits, and underscores.
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Command substitutions, variable substitutions, and backslash
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substitutions are performed on the characters of <I>index</I>.
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<P><DT><A NAME="M6"><B>${</B><I>name</I><B>}</B></A><DD>
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<I>Name</I> is the name of a scalar variable. It may contain any
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characters whatsoever except for close braces.
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</DL><P>There may be any number of variable substitutions in a single word.
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Variable substitution is not performed on words enclosed in braces.<DL>
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<P></DL>
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<P><DT>[8]<DD>
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If a backslash (``\'') appears within a word then
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<I>backslash substitution</I> occurs.
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In all cases but those described below the backslash is dropped and
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the following character is treated as an ordinary
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character and included in the word.
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This allows characters such as double quotes, close brackets,
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and dollar signs to be included in words without triggering
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special processing.
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The following table lists the backslash sequences that are
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handled specially, along with the value that replaces each sequence.
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<P>
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<DL>
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<P><DT><A NAME="M7">\<B>a</B></A><DD>
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Audible alert (bell) (0x7).
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<P><DT><A NAME="M8">\<B>b</B></A><DD>
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Backspace (0x8).
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<P><DT><A NAME="M9">\<B>f</B></A><DD>
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Form feed (0xc).
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<P><DT><A NAME="M10">\<B>n</B></A><DD>
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Newline (0xa).
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<P><DT><A NAME="M11">\<B>r</B></A><DD>
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Carriage-return (0xd).
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<P><DT><A NAME="M12">\<B>t</B></A><DD>
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Tab (0x9).
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<P><DT><A NAME="M13">\<B>v</B></A><DD>
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Vertical tab (0xb).
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<P><DT><A NAME="M14">\<B><newline></B><I>whiteSpace</I></A><DD>
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A single space character replaces the backslash, newline, and all spaces
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and tabs after the newline. This backslash sequence is unique in that it
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is replaced in a separate pre-pass before the command is actually parsed.
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This means that it will be replaced even when it occurs between braces,
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and the resulting space will be treated as a word separator if it isn't
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in braces or quotes.
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<P><DT><A NAME="M15">\\</A><DD>
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Backslash (``\'').
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<P><DT><A NAME="M16">\<I>ooo</I></A><DD>
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The digits <I>ooo</I> (one, two, or three of them) give an eight-bit octal
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value for the Unicode character that will be inserted. The upper bits of the
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Unicode character will be 0.
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<P><DT><A NAME="M17">\<B>x</B><I>hh</I></A><DD>
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The hexadecimal digits <I>hh</I> give an eight-bit hexadecimal value for the
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Unicode character that will be inserted. Any number of hexadecimal digits
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may be present; however, all but the last two are ignored (the result is
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always a one-byte quantity). The upper bits of the Unicode character will
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be 0.
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<P><DT><A NAME="M18">\<B>u</B><I>hhhh</I></A><DD>
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The hexadecimal digits <I>hhhh</I> (one, two, three, or four of them) give a
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sixteen-bit hexadecimal value for the Unicode character that will be
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inserted.
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</DL><P>Backslash substitution is not performed on words enclosed in braces,
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except for backslash-newline as described above.<DL>
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<P></DL>
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<P><DT>[9]<DD>
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If a hash character (``#'') appears at a point where Tcl is
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expecting the first character of the first word of a command,
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then the hash character and the characters that follow it, up
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through the next newline, are treated as a comment and ignored.
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The comment character only has significance when it appears
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at the beginning of a command.
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<P><DT>[10]<DD>
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Each character is processed exactly once by the Tcl interpreter
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as part of creating the words of a command.
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For example, if variable substitution occurs then no further
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substitutions are performed on the value of the variable; the
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value is inserted into the word verbatim.
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If command substitution occurs then the nested command is
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processed entirely by the recursive call to the Tcl interpreter;
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no substitutions are performed before making the recursive
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call and no additional substitutions are performed on the result
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of the nested script.
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<P><DT>[11]<DD>
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Substitutions do not affect the word boundaries of a command.
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For example, during variable substitution the entire value of
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the variable becomes part of a single word, even if the variable's
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value contains spaces.
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<P></DL>
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<HR><PRE>
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<A HREF="../copyright.htm">Copyright</A> © 1993 The Regents of the University of California.
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<A HREF="../copyright.htm">Copyright</A> © 1994-1996 Sun Microsystems, Inc.
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<A HREF="../copyright.htm">Copyright</A> © 1995-1997 Roger E. Critchlow Jr.</PRE>
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