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Old 04-24-2005, 11:25 PM   #1
CowboyJ
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Registered: Nov 2003
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bash shell script expression


What does this expression do?

[ "$1 != "" ] && variable=$1

I understand the test expression return a zero exit status if $1 equals something. Then the result of logical ANDing the exit status with variable is assigned to $1. I do not understand using a zero or nonzero exit status in a logical AND with a variable. Thank you for your help.
 
Old 04-25-2005, 12:33 AM   #2
ilikejam
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Hi.

It's a replacement for an if then (kind of). The && means 'only evaluate the second part if the first part is true' (as opposed to & which will evaluate both parts no matter what), so if '$1 == ""', then 'variable=$1' will be executed. If the first part is true, and if the assignment part is successful, then the whole expression will be true.

If, though, '$1 != ""', then the assignment part will not be executed, and the whole expression will be false.

Dave
 
Old 04-25-2005, 12:33 AM   #3
randyding
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Most languages use "short circuit" conditions to save computation time when evaluating expressions. If you have some "A and B" expression, and A is false, then B will not be evaluated because the whole expression is going to be false regardless of what B evaulates to.
So in your example the variable assignment on the right will never happen if the expression on the left evaluates false.
Really its just a cryptic shorthand for this
if [ "$1" != "" ]; then variable=$1; fi
You can do similar things with binary OR operator instead of AND, but in this case the right side argument is evaluated only when the left argument is false.
 
  


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