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Top Forums Shell Programming and Scripting Help on eval please
# 1  
Old 10-18-2012
Help on eval please
Hello All,

Since my variables are nested I use eval to populate the data.
I have an ambiguity here when eval is used along with &

say I have the below variable

Code:
url="www.unix.com"

, this come from function call as argument.

I want to take this into another variable say
Code:
home_url="http://$url/newthread.php?do=newthread&f=30"

All I want is, home_url should have the value https://www.unix.com/newthread.php?do=newthread&f=30

When eval echo is used, it considers f=30 as another variable.

My piece of code is as below

Code:
home_url=$(sed 's/$1/$url/' filename)
home_url=$(eval echo $home_url)


Where the filename stores, http://$1/newthread.php?do=newthread&f=30

if I do
Code:
echo $home_url

it says https://www.unix.com/newthread.php?do=newthread

PS: I am having a real hard time to explain this Smilie


Please help me to come out of this confusion.
# 2  
Old 10-18-2012
The shell puts the command line up to the "&" char into background execution, then, assigns 30 to the variable f; you would need to escapethe "&" somehow. If you use double quotes for the sed expression, it will work from the beginning:
Code:
home_url=$(sed "s/\$1/$url/" filename)

Make sure the first "$" sign is escaped.
# 3  
Old 10-18-2012
Hello RudiC,

I have even changed the $1 pattern in the file "filename" to XYZ, even then its not working.
Code:
home_url=$(sed "s/XYZ/$url/" filename)
home_url=$(eval echo $home_url)

The below is my debugging output

Code:
+ sed s/XYZ/$url/ filename
+ home_url=http://$url/newthread.php?do=newthread&f=30
+ eval echo http://$url/newthread.php?do=newthread&f=30
+ f=30
+ echo https://www.unix.com/newthread.php?do=newthread
+ home_url=https://www.unix.com/newthread.php?do=newthread

# 4  
Old 10-18-2012
You're making this much too hard on yourself. When you set a variable in the shell, parameter expansion occurs in double quoted strings and that is all you need here.
Adding the eval also causes the entire string to be executed which is causing the & to be evaluated as a command separator. All you need is:
Code:
url="www.unix.com"
home_url="http://$url/newthread.php?do=newthread&f=30"

This User Gave Thanks to Don Cragun For This Post:
sathyaonnuix
# 5  
Old 10-18-2012
you are wonderful... Yes, i thought the double quotes might create an issue whilst using wget, but it didn't. Its working now.. TA
# 6  
Old 10-18-2012
@sathyaonnuix: The one single statement in my post will set home_url correctly, at least on my linux/bash. You don't need the second using eval. In fact, eval will do what I said in my previous post (and what you showed in your debug output).
@Don Cragun: The requestor wanted to read the extension from a file; that's what the sed construct was for.
Last edited by RudiC; 10-19-2012 at 04:55 AM.. Reason: typos
# 7  
Old 10-18-2012
If you find yourself using eval, odds are good you've taken a wrong turn somewhere. The first rule of using eval is, don't. Smilie
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