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Read multiple arguments in for loop each time

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# 1  
Old 01-15-2010
Read multiple arguments in for loop each time
Hi, Guys

I am new to shell programming and just get stuck with one simple question. please kindly help.

According to the tutorial here, we can do something like
Code:
   for NODE in "ABC 10" "EFG 20"
   do
      set -- $NODE
      echo "letter is $1, number is $2"
   done

And the result will be
Code:
   letter is ABC, number is 10
   letter is EFG, number is 20

But if I want to use a variable instead of hard-coded values like below, it doesn't work
Code:
   NODES='"ABC 10" "EFG 20"'
   for NODE in $NODES
   do
      set -- $NODE
      echo "letter is $1, number is $2"
   done

The result becomes
Code:
letter is "ABC, number is
letter is 10", number is
letter is "EFG, number is
letter is 20", number is

Can somebody please explain what is wrong here? and how to correct the error or point me to the right direction?
I am using Bourne Shell.

Many Thanks!
Last edited by pludi; 01-15-2010 at 09:55 AM..
# 2  
Old 01-15-2010
You're using the Bourne shell, or a Bourne-type shell?

For the former you could use something like this:

Code:
set -- "ABC 10" "EFG 20"
   
for NODE in "$@"; do
    set -- $NODE
    echo letter is $1, number is $2
done

Be aware that with the first line you loose the previously defined positional parametrs/arguments.

Some Bourne-type shells support arrays,
this is bash:

Code:
nodes=( "ABC 10" "EFG 20" )

for NODE in "${nodes[@]}"; do
  printf 'letter is %s, number is %s\n' $NODE
done


Anyway, using the shell word splitting mechanism this way is rather limited,
based on your real data, you may need to consider a different approach.
Last edited by radoulov; 01-15-2010 at 10:17 AM..
# 3  
Old 01-15-2010
The problem you are facing is the fact that IFS (Input Field Seperator) is set equal to a space. Therefore everytime a space is encountered in the variable's contents, the FOR loop treats it as a new value. You got around this problem in the first example by placing double quotes around the values you wanted:

Code:
for NODE in "ABC 10" "EFG 20"

The quotes allowed the space to be included as part of the value.

One way around this behavior is to change IFS to something else. Check this out:

Code:
export IFS="|"
NODES="ABC 10|EFG 20"
for NODE in $NODES
   do
      export IFS=" "
      set -- $NODE
      echo "letter is $1, number is $2"
   done

This results in :
Code:
letter is ABC, number is 10
letter is EFG, number is 20

But as you can see, I had to manipulate the IFS setting INSIDE the FOR loop to make it work.

What you need to get a better understanding on is how IFS impacts the parsing behavior of your script.

FYI - Unix in a Nutshell is a great deskside companion to have when shell scripting. Better than man pages...

Hope this helps!
# 4  
Old 01-26-2010
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