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Operator test in bash

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# 1  
Old 05-23-2013
Operator test in bash
Hello, can you please help me because I am totally confused with a simple script:
#!/bin/bash
Code:
[[ ((5 > 9)) ]] || [[ ((7 > 10)) ]] && echo "Good Morning" 
[[ ((4 < 1)) ]] || [[ ((2 < 1)) ]] && echo "Good Night"

For me, these two strings are indentical: false || false and there is no point to execute echo command.

But the run result is confusing me:
It is displayed just one string "Good Morning".
Why is it displayed? Smilie

The used OS: Ubuntu 12.04 LTS
Thank you in advance, Best Regards
Last edited by Scott; 05-23-2013 at 06:35 PM.. Reason: Please use code tags
# 2  
Old 05-23-2013
I am not used to the [[ ]] stuff in ksh and bash,
and it seems to behave different from the obvious.
The classic way works
Code:
[ 5 -gt 9 ] || [ 7 -gt 10 ] && echo "Good Morning"
[ 4 -lt 1 ] || [ 2 -lt 1 ] && echo "Good Night"

# 3  
Old 05-23-2013
The < and > sign do a string compare: char 7 is greater than char 1, so the term is true, and it prints "Good Morning".
As MadeInGermany says, use -lt or -gt for numeric compares, and it flies.
Last edited by RudiC; 05-23-2013 at 06:37 PM.. Reason: typo
# 4  
Old 05-23-2013
You can use multiple expressions (Logical AND && , Logical OR ||) within ((...)) conditional expression for Arithmetic Evaluation.

So I guess the correct syntax would be:
Code:
(( 5 > 9 || 7 > 10 )) && echo "Good Morning"
(( 4 < 1 || 2 < 1  )) && echo "Good Night"

The reason why it was considered a string comparison because the OP used [[...]] compound command along with > < operators instead of -gt -lt operators.

But ((...)) conditional expression evaluates an arithmetic expression with > < operators.
# 5  
Old 05-24-2013
The problem has nothing to do with string comparisons. For your (overcomplicated) extended test ("[[...]]") example to work correctly, you need to use the results of the arithmetic expressions "((..))"

Once you do this, your example will work:
Code:
[[ $((5 > 9)) ]] || [[ $((7 > 10)) ]] && echo "Good Morning"
[[ $((4 < 1)) ]] || [[ $((2 < 1)) ]] && echo "Good Night"

# 6  
Old 05-24-2013
Quote:
Originally Posted by fpmurphy
The problem has nothing to do with string comparisons.
If the problem has nothing to do with string comparison, then can you please explain why "Good Morning" is getting printed while running OP's code?
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