expr confusion FC3

[FrankBlourtango]

I wanted to start some script programming. Check out the last one trying to multiply. * is mulitiply no? Why the syntax error?

[me@myhost ~]$ expr 24 + 4
28
[me@myhost ~]$ expr 24 - 4
20
[me@myhost ~]$ expr 24 / 4
6
[me@myhost ~]$ expr 24 * 4
expr: syntax error

[bs_texas]

Found this from a google:

http://howtos.linux.com/guides/abs-guide/moreadv.shtml

expr

All-purpose expression evaluator:
Concatenates and evaluates the arguments according to the operation given (arguments must be separated by spaces).
Operations may be arithmetic, comparison, string, or logical.

expr 3 + 5

returns 8
expr 5 % 3

returns 2
expr 5 \* 3

returns 15

The multiplication operator must be escaped when used in an arithmetic expression with expr.
y=`expr $y + 1`

Increment a variable, with the same effect as let y=y+1 and y=$(($y+1)). This is an example of arithmetic expansion.
z=`expr substr $string $position $length`

Extract substring of $length characters, starting at $position.

[FrankBlourtango]

Interesting. That does work. Now, why didn't I google that?

Another rung in my long steep climb learning Linux.

Thanks!

[bwkaz]

For a bit of explanation: The shell interprets wildcards in Unix, not programs (in DOS, for example, programs had to all duplicate the code to interpret wildcards). When you type "*" into the shell, it does pathname expansion, replacing it with the name of each file (except the ones that start with a period) in the current directory. The expr program sees these names between two numbers, and gives you the syntax error.

With the backslash, the shell does not do pathname expansion on the * character.

If you don't want to have to escape it, and if you use bash, then you can use bash's arithmetic expansion instead:

echo $((5*3))

for example, will print 15. bash does the arithmetic, and replaces the expression with the result. However, it only does integer math, and it may not be arbitrary-precision either (it might overflow after the numbers get large).