Positional Parameters

ericelysia · #1 (**permalink**) 10-21-2005

Hello,

I am using the Bourne shell.

I am trying to understand the concept of positional parameters.

I do understand that positional parameters:

1. Are initialized by shell
2. Have a max of 9 parameters ($1 to $9)
3. Have no limit on the number of arguments
4. Can be rearranged with the shift command

Can someone please give me a simple example to help me better understand how to use positional parameters?

Thanks,
Eric

vino · #2 (**permalink**) 10-22-2005

Code:

1. Are initialized by shell
They are not initialized by the shell. Rather they pick up the values from the arguments passed to a shell script.

2. Have a max of 9 parameters ($1 to $9)
Not true. It can go beyond 9. After $9, the next positional parameter will be accessed as ${10}, ${11} .. et al.

3. Have no limit on the number of arguments
This is true. Bit this statement is contradicting to what you mentioned in point 2. 

4. Can be rearranged with the shift command
The postional arguments gets shifted one place to the left when you make one shift call. i.e. $1 will get the value that was held by $2, $2 will get the value that was held by $3.. so on and so forth..
$0 never gets shifted.

And positional parameters cant be assigned new values unless you use set

Read the man bash

vino

ericelysia · #3 (**permalink**) 10-22-2005

Code:

1. Are initialized by shell
They are not initialized by the shell. Rather they pick up the values from the arguments passed to a shell script.

I am not sure I completely understand this.

For example, if I do the following:

Code:

$> var1=one
$> var2=two
$> var3=three
$> echo $var1
one
$> echo $var2
two
$> echo $var3
three

How am I able to use these variables from a file within a different directory?
Is this a good example that will help me understand the concept? I am trying to write a few small examples to help me see exactly what is happening.

Code:

2. Have a max of 9 parameters ($1 to $9)
Not true. It can go beyond 9. After $9, the next positional parameter will be accessed as ${10}, ${11} .. et al.

What about addressable parameters? Is it a max of 9 addressable parameters? I am not sure where I read this. I wrote it down as I was trying to learn more about it.

Code:

3. Have no limit on the number of arguments
This is true. Bit this statement is contradicting to what you mentioned in point 2.

I understand that this does contradict what I originally posted for #2.

Code:

4. Can be rearranged with the shift command
The postional arguments gets shifted one place to the left when you make one shift call. i.e. $1 will get the value that was held by $2, $2 will get the value that was held by $3.. so on and so forth..
$0 never gets shifted.

I do understand how shift works now.

Thank you,
Eric

vino · #4 (**permalink**) 10-22-2005

There are a few examples on the use of positional parameters - Positional Parameters

Perderabo · #5 (**permalink**) 10-22-2005

The Bourne shell does not support ${10}. Here is an example where I tried it:

Code:

$ set one two three four five six seven eight nine ten eleven
$ echo $1
one
$ echo ${1}
one
$ echo $9
nine
$ echo ${10}
bad substitution
$ shift
$ echo $9
ten
$

The Korn shell and Bash both do support it. But not the old Bourne shell. The Bourne shell is very old. Replacements have been available since at least 1988. I don't understand why people would use it for new scripts. The Posix standard mandates a newer shell. Any version of Unix that does not have a modern shell available is in violation of the standard.

ericelysia · #6 (**permalink**) 10-22-2005

That's good to know.

Thanks for the clarification.

Eric

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