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Special IF construct syntax in UNIX

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# 1  
Old 05-22-2015
HP Special IF construct syntax in UNIX
Hi,

I don't understand && and || in this context. I thought && is for logical 'AND' and || is for logical 'OR'.

Code:
[ ! -z "$var" ] && echo "Not empty" || echo "Empty"

Please help


Thank You
# 2  
Old 05-22-2015
Yes, it is AND and OR. They connect pipelines and execute them conditionally. man bash:
Quote:
AND and OR lists are sequences of one of more pipelines separated by the && and || control operators, respectively. AND and OR lists are executed with left associa‐
tivity. An AND list has the form

command1 && command2

command2 is executed if, and only if, command1 returns an exit status of zero.

An OR list has the form

command1 || command2

command2 is executed if and only if command1 returns a non-zero exit status. The return status of AND and OR lists is the exit status of the last command executed in
the list.
So - if the test result is TRUE, execute the first command list, if FALSE, the second. That's why it acts as a short if - then - else version. It works because the shell uses "Short Cut evaluation", which means that as soon as the program can determine that the expression is false No Further Evaluation takes place.
This User Gave Thanks to RudiC For This Post:
TomG
# 3  
Old 05-22-2015
this is very tricky..I like it.
# 4  
Old 05-22-2015
Unfortunatelly this sequence is not a perfect if-then-else replacement. It works as expected in situations like:
Code:
 $ true && echo true || echo false
true
 $ false && echo true || echo false
false

But consider this:
Code:
 $ true && false || echo false
false

Ouch :-) Allthough the first expression ist true, both the "then" and the "else" part of the sequence is executed, because the "then" part evaluates to false.
This User Gave Thanks to hergp For This Post:
TomG
# 5  
Old 05-22-2015
Code:
[[ true || ! false ]] && echo yay || echo oh no

Smilie
# 6  
Old 05-23-2015
Quote:
Originally Posted by hergp
.
.
.
Code:
 $ true && false || echo false
false

Ouch :-) Allthough the first expression ist true, both the "then" and the "else" part of the sequence is executed, because the "then" part evaluates to false.
Absolutely. This might circumvent the trap (not sure if it opens others):
Code:
true && { false || :;  } || echo bad

@ sea: || ! false will never be executed.
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