[SOLVED] using small bracket and its effect

naushi · 01-28-2012, 02:28 PM

When I runt he following program it give me a result of 10

y=10
(x=$y*3)
y=x
echo $y

then answer is 10

when I run the following

y=10
x=$y*3
y=x
echo $y

i get the answer 30

what is the explanation for the difference that the bracket is causing

thanks in advance

Doc CPU · 01-28-2012, 03:11 PM

Hi there,

Quote:

Originally Posted by naushi

When I runt he following program it give me a result of 10

what language is your code sample supposed to be?
It can't be C or C++, because the semicolons are missing, and the '$' is not a valid character in an identifier.
It can't be PHP, because the semicolons are missing, and y is something else than $y.
It can't be Javascript, because y is something else than $y.

So what is it?

[X] Doc CPU

naushi · 01-28-2012, 03:17 PM

language linux - redhat - bash command

Cedrik · 01-28-2012, 03:18 PM

Bash ?
echo $y should output 'x' in both cases, no ?

Tinkster · 01-28-2012, 04:03 PM

Cedrik is correct.

And for the record: those aren't "small brackets", they're commonly called
parentheses (specially in programming environments).

Code:

[] = brackets, square brackets
{} = braces, curly braces
() = parentheses, round brackets
<> = angle brackets

Cheers,
Tink

naushi · 01-28-2012, 07:23 PM

If I could find the answer I would not ask. I looked at differnt books and found out the command in parentheses are sub routines and they dont bring results itnot he main stream program. probably generally used to do some side calculation. Is that true? Then I wonder if the sub routine bring back value why cant the result be used in main stream program

grail · 01-29-2012, 07:38 AM

Think of a subshell as calling a separate program that is using your value. Once the subshell / separate program has finished you are then returned to your script which
continues with the values it had prior to the previous call as it is not aware of any changes made.

A more precise explanation can be found here: http://mywiki.wooledge.org/SubShell

David the H. · 01-29-2012, 07:57 AM

Please use [code][/code] tags around your code and data, to preserve formatting and to improve readability. Please do not use quote tags, colors, or other fancy formatting.

Neither of the commands you posted should work the way you describe them (edit: unless you've previously declared the variables to be integer only. But the next-to-last line still also needs "$x" instead of "x"). I'm going to add comments to explain exactly what's happening, then show you how it should work.

Code:

y=10		#set variable "y" to 10
(x=$y*3)	#set the variable "x" to the literal string "10*3".  But
		#the parens set it to operate in a subshell, which closes
		#after the command is run, and all changes are lost.
		#If you echoed "$x" after this, you'd get nothing. 
y=x		#set the variable "y" to the literal string "x"
echo $y		#echo the value of "y", which should give you "x"

###

y=10		#set variable "y" to 10
x=$y*3		#set the variable "x" to the literal string "10*3".
		#now you don't have a subshell, so the value is global.
		#If you echoed "$x" after this, you'd get "10*3".
y=x		#set the variable "y" to the literal string "x"
echo $y		#echo the value of "y", which should give you "x"

Your big problem above is that at no time are you operating in an arithmetic context, so all values are treated as strings. To run an arithmetic context, you should use either ((..)) or the let keyword.

Code:

y=10		#set variable "y" to 10
((x=y*3))	#inside an arithmetic context, set "x" to the value of y*3; meaning 
		#you get "x=30".  Notice that you don't need "$" in front of a
		#variable when in an arithmetic context, as anything other than a
		#digit or an operator is automatically assumed to be a variable.
y=$x		#set the variable "y" to the current value of "x" (which is 30).
echo "$y"	#echo the value of "y", which should now be "30".

See here for more on how to use arithmetic operators in bash:

http://mywiki.wooledge.org/ArithmeticExpression

naushi · 01-29-2012, 08:09 AM

thanks everyone for a great explanation