When do I need `expr ......` and when not? $(($aaa - $bbb)) simplyfiable?
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From: Wolfgang Meister on 15 Jun 2008 02:48 As I learned it is not always necessary to use `expr ....` (with backticks) to calculate mathematical expressions. When I do for example the following firstsec=345 secondsec=723 diffsec=$((secondsec - firstsec)) echo diff=$diffsec then it works although I did not used "expr" for calculate the difference. So when do I have to use expr and when not? By the way: When can I write just diffsec=$(secondsec - firstsec) and when diffsec=$((secondsec - firstsec)) and when diffsec=$(($secondsec - $firstsec)) Wolfgang
From: Wayne on 15 Jun 2008 14:38 Stephane CHAZELAS wrote: > > The ":" operator of "expr" can still be uselful nowadays. Indeed, expr can be used with if to test if some value is a number, something that test alone can't do. And while sed can be used to return a testable value when matching some string against a regular expression, it is much easier with expr. > > You may still want to use expr when dealing with 0 padded > decimal numbers. For $((...)), 010 is 8 (octal 010) while for > expr, it's 10. Of course you can also easily strip off leading zeros from numbers in shell variables in POSIX shells, using ${NUM##0}: $(( ${aaa##0} - ${bbb##0} )) Stephane, I still say you should write a book. Your knowledge and experience of shells is invaluable. -Wayne
From: Jignesh on 12 Jul 2008 14:50
On Jun 15, 11:38 pm, Wayne <nos...(a)all4me.invalid> wrote: > Stephane CHAZELAS wrote: > > > The ":" operator of "expr" can still be uselful nowadays. > > Indeed, expr can be used with if to test if some value is > Stephane, I still say you should write a book. Your > knowledge and experience of shells is invaluable. > > -Wayne Yes, Stephane, You should :). I have read your tips on 'Advanced Bash scripting Guide' at TLDP. Jignesh |