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While-read and if-elif

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# 8  
Old 09-13-2012
Thanks guys! It's working well now! Thanks again.
# 9  
Old 09-13-2012
bash man page:
Quote:
CONDITIONAL EXPRESSIONS
Conditional expressions are used by the [[ compound command and the test and [ builtin commands to test file attributes and perform string and arithmetic comparisons.
...
string1 > string2
True if string1 sorts after string2 lexicographically.
arg1 OP arg2
OP is one of -eq, -ne, -lt, -le, -gt, or -ge. These arithmetic binary operators return true if arg1 is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal to arg2, respectively. Arg1 and arg2 may be positive or negative integers.
# 10  
Old 09-13-2012
Quote:
Originally Posted by pamu
Hi Rudic,

I am not able to understand how we can use < and > signs for string comparison....????
According to the standards, you can't with test... and [ ... ],
but in [[ ... ]], < and > are string comparison operators. The -lt and -gt operators perform numeric comparisons in all three forms of these testing methods.

In bash, the < and > are string comparisons in test ... and [ ... ]. In ksh, the < and > will be treated as redirection operators, not string comparisons. I.e., in ksh:
Code:
if [ 100 > 200 ]
then echo gt
else echo not gt
fi

will print gt because 100 is not an empty string and will create a file named 200 because > is a redirection operator. But in bash, this same code will print not gt and will not create a file. Both of these behaviors conform to the standards because the standards don't specify any meaning for < and > in these two cases.

=================
I take part of this back. Despite what the bash man page says, at least on Mac OS X, bash and ksh both treat < and > as redirection operators in test and [.
Last edited by Don Cragun; 09-13-2012 at 06:40 AM.. Reason: Update based on testing bash on OS X Lion
This User Gave Thanks to Don Cragun For This Post:
pamu
# 11  
Old 09-13-2012
Thanks Don Cragun and Rudic,

I have tried out few things. just see below..

For String Operation -
Code:
$ if [[ "b" -gt "a" ]]; then  echo "gt"; else  echo "not gt"; fi
not gt

$ if [[ "b" > "a" ]]; then  echo "gt"; else  echo "not gt"; fi
gt

$ if [[ "a" -gt "b" ]]; then  echo "gt"; else  echo "not gt"; fi
not gt

$ if [[ "a" > "b" ]]; then  echo "gt"; else  echo "not gt"; fi
not gt

$ if [[ a -gt b ]]; then  echo "gt"; else  echo "not gt"; fi
not gt

$ if [[ a > b ]]; then  echo "gt"; else  echo "not gt"; fi
not gt

$ if [[ b -gt a ]]; then  echo "gt"; else  echo "not gt"; fi
not gt

$ if [[ b > a ]]; then  echo "gt"; else  echo "not gt"; fi
gt

For numeric operation -
Code:
$ if [[ "1" -gt "2" ]]; then  echo "gt"; else  echo "not gt"; fi
not gt

$ if [[ "1" > "2" ]]; then  echo "gt"; else  echo "not gt"; fi
not gt

$ if [[ "2" -gt "1" ]]; then  echo "gt"; else  echo "not gt"; fi
gt

$ if [[ "2" > "1" ]]; then  echo "gt"; else  echo "not gt"; fi
gt

$ if [[ 1 -gt 2 ]]; then  echo "gt"; else  echo "not gt"; fi
not gt

$ if [[ 1 > 2 ]]; then  echo "gt"; else  echo "not gt"; fi
not gt

$ if [[ 2 -gt 1 ]]; then  echo "gt"; else  echo "not gt"; fi
gt

$ if [[ 2 > 1 ]]; then  echo "gt"; else  echo "not gt"; fi
gt

From this i came to conclusion that -

1) Never use -gt and -ltfor string comparison.
2) Use > and < for string comparison and -gt and -lt for numeric comparison..
3) String and numbers are compared in sorted order as
Number - 1 - 9 ( like 9 is greater than 1)
String - A,B,C...Z, a,b,c,....z ( like z is greater than a)

these are my few. you can add if you feel so.....Smilie
Last edited by pamu; 09-13-2012 at 07:33 AM.. Reason: added info
# 12  
Old 09-13-2012
@pamu: I think this would be a more evident example:
Code:
$ [[ 21 > 3 ]] && echo gt || echo lt
lt
$ [[ 21 -gt 3 ]] && echo gt || echo lt
gt

21 interpreted as a string compares less than 3, while as an integer, it compares greater. This is valid for bash; maybe someone can comment on other shells.
Last edited by RudiC; 09-13-2012 at 07:33 AM..
This User Gave Thanks to RudiC For This Post:
pamu
# 13  
Old 09-13-2012
Quote:
Originally Posted by RudiC
@pamu: I think this would be the most striking example:
Code:
$ [[ 21 > 3 ]] && echo gt || echo lt
lt
$ [[ 21 -gt 3 ]] && echo gt || echo lt
gt

21 interpreted as a string compares less than 3, while as an integer, it compares greater. This is valid for bash; maybe someone can comment on other shells.
That's good one.. it clears useless use of < and > while numeric comparison.. removed from numeric comparison...Smilie

As explained in my previous post < and > signs treats as string so they compare each digit by digit. such as..

Code:
$ [[ 211 > 12 ]] && echo gt || echo lt        # Here it compares 2 with 1 so 2 is greater than 1 as per sort pattern so gt.
gt

$ [[ 211 > 32 ]] && echo gt || echo lt         #Here it compares 2 with 3 so 3 is greater than 2 as per sort pattern so lt.
lt

Thanks Rudic...
Last edited by pamu; 09-13-2012 at 07:45 AM..
# 14  
Old 09-13-2012
A mnemonic: For numeric comparison, use alphabetic operators (eq, ne, gt, ge, lt, le). Makes perfect sense! Smilie

Regards,
Alister
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