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Full Discussion: Infinite while loop
Top Forums Shell Programming and Scripting Infinite while loop Post 302370109 by atoponce on Tuesday 10th of November 2009 11:48:56 AM
Old 11-10-2009
Code:
$ true
$ echo $?
0
$ false
$ echo $?
1

That shows the exit code of "true" and "false". As you can see, they both do nothing. One returns a passing exit code, while the other returns a failing exit code. The "while" function wants an argument. It will continue parsing code inside the loop until the argument fails.

Code:
i="0"
while [ $i -lt 4 ]; do
  xterm &
  i=$[$i+1]
done

This piece of code will open four xterm windows. It will only open four, because we set our variable 'i' to 0 and increment it once per loop iteration. When it reaches the value of four, the while condition fails, and the code block will no longer be executed.

So, because "true" does nothing successfully, if you had "while true" in that code block, it would continue and continue to open xterm windows until you ran out of physical resources, and the box could no longer function.

Because "false" does nothing unsuccessfully, if you had "while false" in that code block, it would not even initialize the loop, and immediately exit without opening a single xterm window.
 

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TEST(1) 						      General Commands Manual							   TEST(1)

NAME
test, [ - test for a condition SYNOPSIS
test expr [ expr ] OPTIONS
(none) EXAMPLES
test -r file # See if file is readable DESCRIPTION
Test checks to see if files exist, are readable, etc. and returns an exit status of zero if true and nonzero if false. The legal operators are -r file true if the file is readable -w file true if the file is writable -x file true if the file is executable -f file true if the file is not a directory -d file true if the file is a directory -s file true if the file exists and has a size > 0 -t fd true if file descriptor fd (default 1) is a terminal -z s true if the string s has zero length -n s true if the string s has nonzero length s1 = s2 true if the strings s1 and s2 are identical s1 != s2 true if the strings s1 and s2 are different m -eq m true if the integers m and n are numerically equal The operators -gt, -ge, -ne, -le, and -lt may be used as well. These operands may be combined with -a (Boolean and), -o (Boolean or), ! (negation). The priority of -a is higher than that of -o. Parentheses are permitted, but must be escaped to keep the shell from trying to interpret them. SEE ALSO
expr(1), sh(1). TEST(1)
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