Shell Programming and Scripting

Ahem, this won't work in AIX either. The only place where you can skip the "$" and have the variable expanded anyway is inside double-brackets:

Code:

var=1
(( var = var + 1 ))
print - $var

This is standard ksh and it will definitely work in Linux-ksh as well as in AIX-ksh.


The rule is: the comparison will work, regardless if the variable is quoted or not. The problem one avoids with quoting variables is this: if the variable contains whitespace, the expanded variable will lead to a syntax error once it is expanded, because the shell treats the resulting expanded value as two (or more words):

Code:

typeset var="abc"
if [ $var == abc ] ; then         # this is first expanded to
if [ abc == abc ]  ; then         # which is a legal statement

typeset var="abc def"
if [ $var == abc ] ; then                 # this is first expanded to
if [ abc def == abc ]  ; then             # which is NOT a legal statement, but
if [ "$var" == "abc" ] ; then             # this is first expanded to
if [ "abc def" == "abc" ]  ; then         # which is legal again

You might want to do that because otherwise it would be misleading, but it is not required at all. The OS only cares for the shebang and the executable flag and whcih extension the file has doesn't matter at all.

In AIX "/bin" is a link to "/usr/bin". This is one of my 7.1.3-testsystems:

Code:

# ls -l /bin
lrwxrwxrwx    1 bin      bin               8 Feb  5 09:27 /bin -> /usr/bin

According to the POSIX documents this ("-n <number>") is the modern and suggested form and should be used anyway. If at all the old syntax is only supported for backwards compatibility purposes.

I hope this helps.

bakunin

I got hit with my ksh scripts just going from RedHat 5 to RedHat 6. Yes, they changed the behavior of the builtin echo command. But they added a "-e" flag to get the old behavior. This means that a script can easily test whih echo is in operation. Just do a "echo -e" and capture the output. If you get "-e" you have the old echo. If you get no characters, you have the new echo.

If I have the old echo, I'm good. My scripts will work the way I want. But if I have the new echo, I'm in trouble. I need to arrange to add the "-e" flag to all of my echo commands. It sounds like a big deal when I explain it, but I really just add a few lines of code to each script near the top (and certainly before any echo statements)...

Code:

function myecho
{
        ''echo -e ${1:+"$@"}
        return $?
}
echo=$(echo -e)
((${#echo})) || alias echo=myecho
unset echo

Bakunin, it has just been tested: variables without a preceding $ work in AIX. It works with simple [ ].

We were really surprised about this because we never saw it before. But I am talking about hundreds of scripts written like this for a bank.

At the beginning we thouhgt those scripts didn't work, but after realising that there were hundreds we started to suspect that they actually worked that way.

This is part of the intricacies of the test command, and its peculiar ways.

I know that the only important thing is the shebang. Although it is only a visual matter, to me it looks like something unproperlly finished.. Anyone that sees those scripts without opening them will think that is ksh.
This is my point of view and I explain it like this: if I tell you that I have a friend and my friend's name is Pamela, what would you think? Is it a woman or a man? With the information you have, the name, you suppose that is a girl. I am sure you can "investigate" more and maybe is a boy, but someone with that name looks at first, a girl.

This case is the same, the name is information that could be used to make our life easier, to avoid the need to open the file if the name is self-explicative.

I understand your point though, but I would change the name as well.

I know it is going to be more difficult for the rest of the programs when they have to call the scripts...

The problem here is that in the scripts from AIX they used something like

Code:

/usr/bin/fuser

so I had to change it into

Code:

/bin/fuser

In the latest version of RHEL it doesn't work without the -n option. I tested it myself.

Regards.

I always preferred the equally builtin "print" over "echo". It has a lot of advantages (the "-u" switch which "echo" lacks, the "-p", ...) and no disadvantages that i know of. Original versions of the Korn shell didn't have the "echo" built in but use the external "/usr/bin/echo", which is why i started the habit of using "print" instead.

Could you please post such a script part? I administrate AIX systems for the last 25 years now (started with 3.2.3 on a RS/6000 model 32H) and i never came across such a behavior. To be honest i can't believe that.

You are right and i would change that too, no matter what. It still is not a requirement, though, and i wanted to exmphasize that difference.

bakunin

The only way is testing it on an AIX machine. Do you have access yourself to an AIX machine so you can test this simple thing?

Code:

cont=1
if [ cont -eq 1 ]
then
  echo "It works"
fi

I have access to the 300+ AIX systems i am in charge of, ranging from the two 5.3-systems we need to keep at that level to the latest 7.1.3 SP3 and anything in between. Today is my day off, but i will report here what i find out tomorrow.

bakunin

Today is bank holiday in my country so I am not at work either...

300 AIX is a lot ^^ I just need one!

I haven't got as much experience with AIX as you have, but there's a workmate working with me that has been working with AIX for many years as well and he was shocked after testing this code.

I'll wait, no problem. Enjoy your day off!