Shell Programming and Scripting

Here's my problem: the $1 parameter is getting permanently set when you dot-run a script - and I cannot unset it.

Here's how to duplicate the problem:

1) Create a script called testme and put this simple command in the script:
echo 1=$1

Save the script

2) Make the testme script executable
chmod a+rx testme

3) Now dot-run the script:
. testme

Output:
1=

4)Now dot-run the script with a parameter:
. testme YOU

Output:
1=YOU

5) Now dot-run the script without a parameter:
. testme

Output:
1=YOU

HUH??? There's the problem!!! Why did the script remember the $1 parameter value?

The question is - how do I clear that $1 value???

edit by bakunin: while this is a very interesting question it is hardly related to AIX. I move it over to "Shell programming and scripting" therefore.

I used bash shell and it works exactly fine.the variable $1 is unset.Let me know which shell you use and I can try on it , if I have access to the same.

Thanks!
nua7

Well, you can thank the forum administrator for all the confusion. I originally posted this question in the AIX FORUM since this behavior seemed to be specific to AIX.

I am using KSH shell running on AIX 5.x

First off, I'd like to state for the record, that you should thank yourself instead of thanking me: you haven't made clear that this occurs only in AIX (to your knowledge) and among the many things i am incompetent in is reading minds. Furthermore it will become clear with the answer that i was correct moving the thread as it is indeed a shell-related problem, not a OS-related one. In fact it is a problem of understanding, not of software.

Further, and to clear up the confusion: This effect can probably be demonstrated in other implementations of the Korn Shell too, as it is no bug, but a feature. You execute a script IN THE CURRENT ENVIRONMENT and hence you can set any variable and it will stick to this value until being changed. The same is the case with system variables like $1, which you are setting here - in your current environment. You can query the value of $1 also in the current environment:

Code:

# echo $1

# . ./testme xxx
1=xxx
# echo $1
xxx

It may look a bit counter-intuitive, but: which reason would the shell have to change the value of "$1" as long as it isn't being changed?

After this explanation the solution how to clear the variable "$1":

Either provide an empty value to make it empty:

Code:

# . ./testme xxx
1=xxx
# echo $1
xxx
# . ./testme ""
1=
# . ./testme
1=

or use "shift" to rotate out $1:

Code:

# . ./testme xxx
1=xxx
# echo $1
xxx
# shift
# echo $1

# . ./testme
1=

One word about the implementation of bash: *this* in fact is a bug and the failure of the Linux pdksh to do the same as the AIX ksh should be regarded as sloppiness of implementation, not as feature.

bakunin

Sorry but I disagree. Where is the variable "1" being set? The line "echo 1=$1" does not define or assign any variable. It tells the shell to expand $1 (in this case, XXX) and output "1=XXX"

Code:

$ echo ${.sh.version}
Version M 93s+ 2008-01-31
$ cat testme
echo 1=$1
$ . ./testme XXX
1=XXX
$ echo $1

$ . ./testme
1=
$

You are right and wrong, fpmurphy: The variable 1 is not set in the echo-statement. It is merely displayed there. It is set when the script is started and as the script is started in the current environment the variable is set in the current environment.

When you execute "script 1", you start a new shell environment, then set the variable 1 in this environment to "1" and then the script starts to run, yes? If you issue ". script 1" you do not start a new environment but the rest of the procedure occurs quite the same.

bakunin

Good, Bakunin, we agree that the variable "1" is not set by the "echo 1=$1" statement.

You claim that $1 (first positional paramater) should be available in your current environment upon sourcing a file such as in the example provided

Sorry, again, I must disagree. Positional parameters are initially assigned when you invoke a shell, temporarily replaced when a shell function is invoked, and can only be reassigned with the set special built-in command.

$1 is available within the testme script but not outside it. To illustrate this point, try the
following:

Code:

#!/usr/bin/ksh
echo "Positional Parameter 1=$1"
. ./testme XXX
echo "Positional Parameter 1=$1"

which gives the following output

Code:

$ ./demo FPM
Positional Parameter 1=FPM
1=XXX
Positional Parameter 1=FPM
$

Have you a counter example to disprove my assertion?