11-16-2006
The following is a segment from that post.
"There is a command called [ and like many commands it takes parameters. It checks its last parameter and gets mad unless it is ]. If the last parameter is a ], it looks at everything else and tries to determine what you are trying to test. If you do:
lines=""
if [ $lines -eq 0 ]
you will have a problem. All that the [ command will see is the -eq and the 0 so it will complain. Because of these concerns, Dave Korn invented another syntax. Now you can do:
if [[ $lines = 0 ]]
and it won't get mixed up as easily."
'All that the [ command will see is the -eq and the 0'...why so? why wont it see the ']' ? isnt ']' the last argument for command '[' ? plz explain. and by 'it looks at everything else' , u mean that it looks inside the square brackets? if so, then i dont see, why the first format shudnt work. (it isnt working, obviously, but i cant get the reason)
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while(n) Tcl Built-In Commands while(n)
__________________________________________________________________________________________________________________________________________________
NAME
while - Execute script repeatedly as long as a condition is met
SYNOPSIS
while test body
_________________________________________________________________
DESCRIPTION
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 exe-
cuted inside body to terminate the current iteration of the loop, and break commands may be executed inside body to cause immediate termi-
nation 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 execut-
ing, 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 iter-
ation), so changes in the variables will be visible. For an example, try the following script with and without the braces around $x<10:
set x 0
while {$x<10} {
puts "x is $x"
incr x
}
EXAMPLE
Read lines from a channel until we get to the end of the stream, and print them out with a line-number prepended:
set lineCount 0
while {[gets $chan line] >= 0} {
puts "[incr lineCount]: $line"
}
SEE ALSO
break(n), continue(n), for(n), foreach(n)
KEYWORDS
boolean value, loop, test, while
Tcl while(n)