Grepping nonprinting characters

Here is a tip summarized from comp.unix.shell

The problem:

> For the following TAB-delimited records, I want to count number of 
> records with column-2 == -1   (should be 2) 
> ===== file.txt ====== 
> AAA    -1    2008-07-14 
> BBB    -14   2008-07-15 
> CCC    -20   2008-07-16 
> DDD    -1    2008-07-16 
> =========== 
> I tried: 
>   grep -c -- "-1\t" file.txt 
> which is not working

A solution offered for ksh93/zsh/bash shells:

grep -c -- $'-1\t' file.txt 

I will add another alternative

grep -c '-1^V^I' file.txt

Where ^V^I means type ctrl-v and ctrl-i to enter a tab character.

Related:

Advanced Bash-Scripting Guide Example 34-1
Insert ASCII Control Characters in Text

Hanging Oracle dbca

Oracle is today’s new toy for me. And by toy I mean frequent source of frustration. One issue I battled this morning was getting dbca to start. It would just hang at the command line prompt with no feedback about what it was or wasn’t doing. netca and oem would launch just fine so it didn’t seem to be a Java or X11 configuration issue.

The Oracle forums were of no help but I did find a workaround here. What to do when dbca does not start?

So, I yanked my cable a bit, then, remembering I’m on a wireless Linux laptop, shutdown the network service. Then dbca launched. Of course then dbca couldn’t connect to the databases so I had to start the network again. Good times.

This doesn’t solve the fundamental problem – I certainly couldn’t shutdown the network in a production environment and I doubt it would fly on an Oracle certification exam but at least I’m able to continue with my Oracle adventures.

perl pie in your face

I frequently use Perl’s in place file editing from the command line. What I didn’t consider until it bit me today is that the file ownership can change using this method.

Here’s the original file, owned by tomcat_6 and only readable by user and group.

$ ls -l web.xml

-rw-rw----  1 tomcat_6 tomcat 49384 Jul 10 11:38 web.xml

I belong to the tomcat group, so have write permissions to the file. The enclosing directory is also tomcat group writable. The importance of this is noted below.

Using the ‘perl pie’ one-liner to make an in place edit if the file:

$ perl -p -i -e 's;<session-timeout>\d+</session-timeout>;\
<session-timeout>1440</session-timeout>;' web.xml

Now the file is owned by me and my default group.

$ ls -l web.xml

-rw-rw----  1 crashing  daily 49384 Jul 10 21:55 web.xml

Most critically, now the file is no longer readable by the tomcat processes. This little change prevented my Tomcat server from starting. Ouch.

sed is a little nicer. It changes the owner but not the group.

$ sed -i 's;<session-timeout>.*</session-timeout>;\
<session-timeout>1445</session-timeout>;g' web.xml

$ ls -l web.xml 

-rw-rw----  1 crash tomcat 49385 Jul 10 22:44 web.xml

Neither the Perl nor the sed one-liners work if the directory is not writable because Perl and sed require unlinking the original file and replacing it with a new version.

The winner for both maintaining file ownership and working if the directory is not writable is ed.

$ ed - "web.xml" <<EOF
,s;<session-timeout>[[:digit:]]*</session-timeout>;\
<session-timeout>1440</session-timeout>;
w
EOF

ed truly does an in place edit. Nice. If only I could remember the syntax.