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Old 11-01-2007, 12:25 AM   #1
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bash script arguments

I wrote a bash script that takes in an undefined number of arguments. As part of the script, I want to iterate through these variables. The problem is that I need the script to take the argument corresponding to the current iteration value. I tried "echo $$k" where $k is the iterator and received an answer equal to value stored in k, and tried "echo $"$k"" and received $ followed by variable stored in k, I also tried a bunch of other things that returned unsuccessful results, interestingly "echo $$$k" produces a large unexplainable number, a memory pointer???

So for example, say I have the following:
./script testing 123

#desired output: testing
echo $$k #output: 1
echo $"$k" #output: $1
echo \$"$k" #output: $1
I realize I could take $@ and cut it up into separate strings using space as divider, but I really want to know if my original way of doing it is possible? Thanks.
Old 11-01-2007, 01:49 AM   #2
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Cool question. Here's what I found.

Doing a "shift" in the script will remove the first argument in the argument list until none are left.


echo "Parameters: ${@}"
echo "P1 = $1"

echo ""
echo "Shifting..."
echo "Parameters: ${@}"
echo "P1 = $1"

echo ""
echo "Shifting..."
echo "Parameters: ${@}"
echo "P1 = $1"
So, you could technically just shift each time you go through and use $1 (if you don't care about accessing the args anymore). Or, do a for loop on $@.

But a direct answer to your question, I'm not sure.

Read a bit in here, maybe it'll say more...

Old 11-01-2007, 04:37 AM   #3
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edit: this isn't what you mean is it?

you aren't being very clear, have you a simple example?

use for

# "$@" preserves spaces
for thing in "$@"; do
    echo $thing
$ hello there old 'fellow me lad'
fellow me lad

Last edited by bigearsbilly; 11-01-2007 at 04:39 AM.
Old 11-01-2007, 06:04 AM   #4
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Originally Posted by R3N3G4D3 View Post
but I really want to know if my original way of doing it is possible? Thanks.
Yes. An example is by means of the eval command. You want to interpret $1 where 1 itself is a variable? You simply do
eval echo \$$k
the first $ sign is escaped to let literal interpretation by eval, therefore the command executed is exactly
echo $1
for k equal to 1. Another way can be using arrays, if available in your version of bash. Just assign arguments to array's elements and retrieve them with the index k, as in
echo ${args[$k]}
In this case, just take in mind that array elements are zero-based.

Last edited by colucix; 11-01-2007 at 06:17 AM.
Old 11-01-2007, 10:40 AM   #5
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Yet another way ("YAW" ?), perhaps the easiest, is indirect expansion:
for X in "$@"
  echo ${!X}   # echos the value of the variable X
For more info, search the bash man page for "exclamation point".

BTW & FWIW, combining the 2 techniques will finally give me double indirection:
$ K=k1 ;  A=K ;  Z=A
$ eval echo \$${!Z}
Old 11-01-2007, 11:16 AM   #6
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Thanks for the replies guys, the "eval echo \$$k" command did exactly what I needed. I'm sure other solutions work too, but that is the easiest modification to my script.

Last edited by R3N3G4D3; 11-01-2007 at 11:18 AM.


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