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AIX function example with "shift" command

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# 1  
Old 05-04-2014
AIX function example with "shift" command
Hello,

I am reading one of the AIX manuals about shell scripting and (AIX 5) and I found this example when introducing to functions:

Code:
function usage
{
  prog="$1"; shift
  print -u2 "$prog: usage: $prog $@"
  exit 1
}

This example is meant to be easy but I don't understand what it is for or what is trying to demonstrate.

The shift command is meant to pass to next argument.. but I don't see what is the purpose here (?)

Then it prints into the STDERR because it prints into file descriptor 2.. but what for?

So.. I am confused. This is probably a very simple thing but I am stuck since yesterday and I still don't figure it out.

Any hint will be appreciated.

Thanks.
# 2  
Old 05-04-2014
By using shift, $1 becomes $2, $2 becomes $3, etc.. in the following line:
Code:
  print -u2 "$prog: usage: $prog $@"

there is "$@" which stands for all parameters which originally would have been all parameters to the function, but because of the shift it has become parameters $2, $3, etc of the original parameters to the function.

Programmatically I do not really see what the purpose is in this case, since they could also have used:

Code:
function usage
{
  print -u2 "$1: usage: $@"
  exit 1
}

But I would have to say the original is easier to read and understand, so that may be the reason..

The other thing - it prints to stderr - is logical since the usage is often printed when a user uses a wrong syntax.
Last edited by Scrutinizer; 05-04-2014 at 10:50 AM..
This User Gave Thanks to Scrutinizer For This Post:
Kibou
# 3  
Old 05-04-2014
I've just realised I missed these lines.. Smilie

Code:
# Invoke function usage with arguments: script
# followed by arglist. Note the exit status!

But well.. now I understand better..

So any time I invoke the script, I'll have an output like this:

Code:
myscript: usage: myscript start stop refresh

And I can see this usage message which goes to stderr.

Well, I think the purpose is making something like this:

Code:
function usage
{
  prog="$1"; shift
  print -u2 "$prog: usage: $prog $@"
  exit 1
}

if [[ "$@" == "" ]];then
  usage script start stop refresh
fi

So only when I invoke the script without arguments I will be prompted with the usage message, which is also an error message.
# 4  
Old 05-04-2014
Quote:
Originally Posted by Scrutinizer
Programmatically I do not really see what the purpose is in this case, since they could also have used:

Code:
function usage
{
  print -u2 "$1: usage: $@"
  exit 1
}

As it is: no, they couldn't have done that. The original function is meant as a usage message and suppose it would have been called this way:

Code:
usage myscript -x -y -z

then the output in the original case would be:

Code:
myscript: usage: -x -y -z

whereas your function would have produced:

Code:
myscript: usage: myscript -x -y -z

You need to use the "shift" somehow to avoid the duplication of the first parameter (once in "$1", another as first part of "$@").

I hope this helps.

bakunin
# 5  
Old 05-05-2014
Hi Bakunin. No, actually . With your input both functions will produce:
Code:
myscript: usage: myscript -x -y -z


Code:
function usage
{
  prog="$1"; shift
  print -u2 "$prog: usage: $prog $@"
  exit 1
}

usage myscript -x -y -z

Output:
Code:
myscript: usage: myscript -x -y -z

# 6  
Old 05-05-2014
Quote:
Originally Posted by Scrutinizer
Hi Bakunin. No, actually . With your input both functions will produce:
Code:
myscript: usage: myscript -x -y -z

Oops. I have to admit i overlooked the second "$prog" completely. Struck of blindness, i presume. Sorry.

bakunin
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