This is VERY disturbing. The command:

Code:

cat abc.txt | sed -e 's/$a/$b/g'  >  abc.txt

will NOT do what you said it will do on any Linux system, on any UNIX system (including AIX), nor on any BSD system.

You haven't bothered to tell us what shell you're using, but with any of the common shells available on any of the systems listed above that I am aware of, the above command will do the same thing on every one of those systems: truncate abc.txt so that it will be an empty file.

If you weren't redirecting output to your input file (thereby destroying your input file before cat and sed ever see it), you are still using single quotes that cause $a and $b to be treated as literal strings instead of having the shell expand them.

If you want to run a script on an AIX system and on a Linux system and have it behave the same way on both of those systems, you cannot use options that are only available on one of those systems.

If you expect us to help you, you need to show us the actual results that you are getting on both systems. I.e., show us:and tell us: