The while command evaluates test as an expression (in the same way that expr evaluates its argument). The value of the expression must a proper boolean value; if it is a true value then body is executed by passing it to the Tcl interpreter. Once body has been executed then test is evaluated again, and the process repeats until eventually test evaluates to a false boolean value. Continue commands may be executed inside body to terminate the current iteration of the loop, and break commands may be executed inside body to cause immediate termination of the while command. The while command always returns an empty string.
Note: test should almost always be enclosed in braces. If not, variable
substitutions will be made before the while command starts executing, which
means that variable changes made by the loop body will not be considered
in the expression. This is likely to result in an infinite loop. If test
is enclosed in braces, variable substitutions are delayed until the expression
is evaluated (before each loop iteration), so changes in the variables
will be visible. For an example, try the following script with and without
the braces around $x<10:
set x 0
Example
Read lines from a channel until we get to the end of the stream,
and print them out with a line-number prepended:
while {$x<10} {
puts "x is $x"
incr x
}
set lineCount 0
while {[gets $chan line] >= 0} {
puts "[incr lineCount]: $line"
}