UNIX for Beginners Questions & Answers

Welcome to the world of scripting and its infinite possibilities. Let us go step by step, because you do several things wrongly here (as does every beginner - don't worry).

1) DO NOT use backticks - never! They are a leftover from a very old shell named "Bourne shell", to which bash is downwards compatible, hence the name "Bourne-again-shell". Bourne shell was the standard shell for UNIX systems in the early eighties, but these times are gone for some time now. Since you use bash you should use the modern way of "process substitution", which reads:

Code:

ORACLE_HOME=$(cat /var/opt/oracle/oratab|grep -v "^#"|cut -d ':' -f2)

2) You made the error of "useless use of cat". grep can read a file by itself and you don't need to dump the file contents into a data stream using cat. You can simply write:

Code:

ORACLE_HOME=$(grep -v "^#" /var/opt/oracle/oratab|cut -d ':' -f2)

This may look like a small point, but in fact it needs one process less to start. Starting a process is taxing on the resources of a system and while a single process more or less won't hurt at all if you use such a construct in an often-run loop the effects are considerable. It is never too early to start good habits and if you start doing it right in little scripts like this you will do it right once you start writing big ones either.

3) The grep command might not do what you think it does: grep filters lines. In your case you filter all lines starting with the character "#" - in fact you reversed this filter, so all lines NOT starting with "#" show up. But i.e empty lines or every other line not starting with "#" will show up too. I don't know your oratab file, but my suspicion is that your problem comes from there. Whenever you create regular expressions work on the following checklist:

- does the regex match the correct lines/text parts? (Obvious)
- are there misses i want matched? (false negatives)
- are there matches i did not want to match? (false positives)

Even the most seasoned creators of regular expressions will not always get it right on the first try. The trick is to be persistent and try to imagine the cases mentioned in the checklist above and then modify the regexp accordingly until satisfaction is reached.

In your case i suppose you are not interested in "any non-comment lines", but in a specific line - the one starting with "SGCD", as rbatte1 already suggested. So your line should probably read:

Code:

ORACLE_HOME=$(grep '^SGCD:' /var/opt/oracle/oratab|cut -d ':' -f2)

4) in your scripts try to be as robust as possible. That is: try to anticipate possible problems and deal with them. Suppose, for instance, the oratab file does not contain a line starting with "SGCD:". Then the ORACLE_HOME would be empty and your PATH statement (supposing the rest works correct) would look like:

Code:

PATH=/path/one:/path/two::

You could avoid this by testing if a line you search for is even present at all and stop processing if it isn't:

Code:

if grep -q '^SGCD:' /var/opt/oracle/oratab ; then
           ORACLE_HOME=$(grep '^SGCD:' /var/opt/oracle/oratab|cut -d ':' -f2)
else
      echo "no SGCD-line found in oratab"
fi

and, finally, a last point: whenever you use grep and then cut you could use sed instead which can do alone what these two do combined:

Code:

ORACLE_HOME=$(sed '/^SGCD:/ s/^[^:]*:\([^:]*\):.*$/\1/' /var/opt/oracle/oratab)

I hope this helps.

bakunin

And again, sed by default reports the non-matching lines.
Quick fix:

Code:

ORACLE_HOME=$(sed -n '/^DGCS:/ s/^[^:]*:\([^:]*\):.*$/\1/p' /var/opt/oracle/oratab)

-n suppress default print
/p print if a match was found

Yes, indeed. This was what i had in mind, but didn't write. Thank you for correcting it.

bakunin