Tcl - Basic Syntax
Tcl is quite simple to learn and let's start creating our first Tcl program!
First Tcl Program
Let us write a simple Tcl program. All Tcl files will have an extension, i.e., .tcl. So, put the following source code in a test.tcl file.
#!/usr/bin/tclsh
puts "Hello, World!"
Assuming, Tcl environment is setup correctly; let's run the program after switching to file's directory and then execute the program using −
$ tclsh test.tcl
We will get the following output −
Hello, World!
Let us now see the basic structure of Tcl program, so that it will be easy for you to understand basic building blocks of the Tcl language. In Tcl, we use new line or semicolon to terminate the previous line of code. But semicolon is not necessary, if you are using newline for each command.
Comments
Comments are like helping text in your Tcl program and the interpreter ignores them. Comments can be written using a hash_(#) sign in the beginning.
#!/usr/bin/tclsh
# my first program in Tcl
puts "Hello World!"
When the above code is executed, it produces the following result −
Hello World!
Multiline or block comment is written using 'if' with condition '0'. An example is shown below.
#!/usr/bin/tclsh
if 0 {
my first program in Tcl program
Its very simple
}
puts "Hello World!"
When the above code is executed, it produces the following result −
Hello World!
Inline comments use ;#. An example is given below.
#!/usr/bin/tclsh
puts "Hello World!" ;# my first print in Tcl program
When the above code is executed, it produces the following result −
Hello World!
Identifiers
A Tcl identifier is a name used to identify a variable, function, or any other user-defined item. An identifier starts with a letter A to Z or a to z or an underscore (_) followed by zero or more letters, underscores, dollars ($) , and digits (0 to 9).
Tcl does not allow punctuation characters such as @, and % within identifiers. Tcl is a case sensitive_ language. Thus Manpower and manpower are two different identifiers in Tcl. Here are some of the examples of acceptable identifiers −
mohd zara abc move_name a_123
myname50 _temp j a23b9 retVal
Reserved Words
The following list shows a few of the reserved words in Tcl. These reserved words may not be used as constant or variable or any other identifier names.
| after |
append |
array |
auto_execok |
| auto_import |
auto_load |
auto_load_index |
auto_qualify |
| binary |
Bgerror |
break |
catch |
| cd |
Clock |
close |
concat |
| continue |
Dde |
default |
else |
| elseif |
Encoding |
eof |
error |
| eval |
Exec |
exit |
expr |
| fblocked |
Fconfigure |
fcopy |
file |
| fileevent |
Flush |
for |
foreach |
| format |
Gets |
glob |
global |
| history |
If |
info |
interp |
| join |
Lappend |
lindex |
linsert |
| list |
Llength |
load |
lrange |
| lreplace |
Lsearch |
lsort |
namespace |
| open |
Package |
pid |
pkg_mkIndex |
| proc |
Puts |
pwd |
read |
| regexp |
Regsub |
rename |
resource |
| return |
Scan |
seek |
set |
| socket |
Source |
split |
string |
| subst |
Switch |
tclLog |
tell |
| time |
Trace |
unknown |
unset |
| update |
Uplevel |
upvar |
variable |
| vwait |
While |
|
|
Whitespace in Tcl
A line containing only whitespace, possibly with a comment, is known as a blank line, and a Tcl interpreter totally ignores it.
Whitespace is the term used in Tcl to describe blanks, tabs, newline characters, and comments. Whitespace separates one part of a statement from another and enables the interpreter to identify where one element in a statement, such as puts, ends and the next element begins. Therefore, in the following statement −
#!/usr/bin/tclsh
puts "Hello World!"
There must be at least one whitespace character (usually a space) between “puts” and "Hello World!" for the interpreter to be able to distinguish them. On the other hand, in the following statement −
#!/usr/bin/tclsh
puts [expr 3 + 2] ;# print sum of the 3 and 2
When the above code is executed, it produces the following result −
5
No whitespace characters are necessary between 3 and +, or between + and 2; although, you are free to include some if you wish for the readability purpose.
