How to multiply with expr? ( expr 3 * 4 doesn't work, though expr 3 + 4 does)

lumix · 03-27-2008, 12:49 PM

I've tried many combinations and googled this ad nauseam, but I keep getting a syntax error on multiply only. I've also tried using a bash shell.

Ultimately, I want to assign a number to $x, then multiply $x * 25, for example. Can't do it.

colucix · 03-27-2008, 01:00 PM

Multiplication with expr is done by \*, e.g.

Code:

y=$(expr $x \* 5)

lumix · 03-27-2008, 01:43 PM

Wow...okay. Huge thanks.

Why didn't that come up at ALL in dozens of google search results? Don't get me wrong, it worked (in bash), but it seems strange that it didn't come up.

colucix · 03-27-2008, 02:11 PM

Indeed it is poorly documented, but the first place to look at is the man page. Here is an excerpt from 'man expr'

Quote:

Beware that many operators need to be escaped or quoted for shells.

The second place to look at when facing shell issues is the Advanced Bash Scripting Guide, where in the description of the expr command you find

Quote:

expr 5 \* 3 returns 15
The multiplication operator must be escaped when used in an arithmetic expression with expr.

aeronaut · 02-25-2009, 09:30 AM

Colucix,

Thanks. I also had problems with this "feature".

Why don't the man pages spell this out in an example. +, -, / and % all work as expected, so the idea that we need to use \* to multiply doesn't come across well at all. Both the supercomputer at ANL and my own mac have man pages for expr, slightly different, and neither makes this clear. A simple

prompt>expr 2 /* 3
6

would make this quite transparent.

Regards,
aeronaut

AlucardZero · 02-25-2009, 05:31 PM

* is expanded by your shell. You have to pass the * to the program instead of your shell, escaping it with \ does that.