Version 1 of Tcl Tutorial Lesson 1

Simple Text Output

Note: to replace the <code ...></code> tags properly, concatenate all the strings within a paragraph and then transform

The traditional starting place for a tutorial is the classic "Hello, World" program. Once you can print out a string, you're well on your way to using Tcl for fun and profit!

The command to output a string in Tcl is the <code class="keyword">puts</code> command.

A single unit of text after the <code class="keyword">puts</code> command will be printed to the standard output device. The default behavior is to print a newline character ("return") appropriate for the system after printing the text.

If the string has more than one word, you must enclose the string in double quotes or braces ({}). A set of words enclosed in quotes or braces is treated as a single unit, while words separated by whitespace are treated as multiple arguments to the command. Quotes and braces can both be used to group several words into a single unit. However, they actually behave differently. In the next lesson you'll start to learn some of the differences between their behaviors. <i>Note</i> that in Tcl, single quotes are not significant, as they are in other programming languages such as C, Perl and Python.

Many commands in Tcl (including <code class="keyword">puts</code>) can accept multiple arguments. If a string is not enclosed in quotes or braces, the Tcl interpreter will consider each word in the string as a separate argument, and pass each individually to the <code class="keyword">puts</code> command. The <code class="keyword">puts</code> command will try to evaluate the words as optional arguments. This will probably result in an error.

A command in Tcl is a list of words terminated by a newline or semicolon. Tcl comments are indicated by a \# at a position where Tcl expects a new command (i.e., following a newline or semicolon), and continue until the end of the line.

Example

puts "Hello, World - In quotes"    ;# This is a comment after the command.
# This is a comment at beginning of a line
puts {Hello, World - In Braces}
puts {Bad comment syntax example}   # *Error* - there is no semicolon!

puts "This is line 1"; puts "this is line 2"

puts "Hello, World; - With  a semicolon inside the quotes"

# Words don't need to be quoted unless they contain white space:
puts HelloWorld

Hello, World - In quotes
Hello, World - In Braces
wrong # args: should be "puts ?-nonewline? ?channelId? string"
    while executing
"puts {Bad comment syntax example}   # *Error* - there is no semicolon!"
    (file "xx.tcl" line 4)