Tcl Basics
The following provides a very brief summary of the Tcl language syntax.
The user is strongly advised to refer to the
man page for Tcl (note the
uppercase T).
A Tcl comment line begins with #. More generally, any occurrence of the # sign
in a command line means that text to the right of it is considered as comment
and is ignored.
The command set is used to assign variables, and a $ sign
is used to obtain the value of a variable. All variables in
Tcl are stored as strings.
set a 1
set b $a
sets both variables a and b to the string "1".
Global Variables
A variable may be declared global using the global command. In
this case all procedures including the declaration can access and
modify the variable.
The return code of Tcl command is a string, which may used
as an argument to another function by surrounding the command with square
brackets [ ].
set a [ prog arg1 arg2 ]
The concept of a list is used in Tcl to allow a variable
to be composed of a number of other variables, in a certain
order. The curly braces { } and double quotes (" ") are
used to define lists.
You will notice a lot of braces in Tcl scripts;
sometimes this will reflect explicit construction of a list
for the use by the script, more usually they group together
items to form complex arguments to Tcl operators like
for and if.
set a { 1 2 3 }
The distinction between the use of braces and quotes is
significant when the list contains references to Tcl
variables. These are substituted by the variable values when quoted,
but not when in braces. For example,
set b 2
set t { 1 $b 3 }
puts stdout $t
Will output
1 $b 3
whereas
set b 2
set t " 1 $b 3 "
puts stdout $t
generates
1 2 3
The backslash may be used to continue a Tcl command on
another line. For example,
set a \
b
The command puts is used to write a string, and is the
simplest way of writing output, either to a file (see Tcl
documentation on how to open files) or to stdout.
set a [ myprog arg1 arg2 ]
puts stdout "the answer from myprog was $a"
for { set i 0 } { $i < 5 } { incr i } {
...
}
if { "$name" == "paul" } then {
...
} elseif { $code == 0 } then {
...
} else {
..
}
The Tcl command list will return a list formed from its
arguments. It is often useful when constructing complex list
structures, especially when the output of a Tcl command is incorporated
into the list.
set a [ list b c d [ expr $e + $f ] ]
The command proc is used to declare a procedure. The name of
the procedure may then be used as a Tcl command.
proc myproc { arg1 arg2 } {
.. procedure body...
}
myproc a b
The Tcl command exec will run an external program, the
return value is the output (on stdout) from the program.
set tmp [ exec myprog ]
puts stdout $tmp
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