Unix Tips and Tricks

Inline Search and Replacement One Liners

2008-05-05T16:49:00.002-04:00

Here are two ways to do inline file replacement of patterns using perl and sed. Both examples will do a global replacement of the string orig with the string replacement.

First, we have a perl one liner:
perl -pi -e 's/orig/replacement/g' file

And here we do the same with sed:
sed -i -e 's/orig/replacement/g' file

This example finds and replaces a simple string in a file. You can also use a regex (regular expression) to do more powerful substitutions.

To replace all words that begin abb with ABB in /usr/share/dict/words:
sed -i -e 's/^abb/ABB/g' /usr/share/dict/words

$ egrep '^abd' /usr/share/dict/words
abdomen
abdomens
abdominal
abduct
abducted
abduction
abductions
abductor
abductors
abducts
$ sed -i -e 's/^abb/ABB/g' /usr/share/dict/words
$ egrep '^abb' /usr/share/dict/words
$ egrep '^ABB' /usr/share/dict/words
ABBe
ABBey
ABBeys
ABBot
ABBots
ABBreviate
ABBreviated
ABBreviates
ABBreviating
ABBreviation
ABBreviations
$

Of course I used a copy of the words file and didn't actually clobber the original.

Patch Your Linux Kernel and Apply Code Without Rebooting

2008-04-25T09:57:00.004-04:00

KSplice from MIT allows you to apply security patches to your running Linux kernel without rebooting. This is great news for organizations that can not reboot their systems. KSplice would primarily be used to apply security updates as it is limited to making code changes that do not introduce semantic changes to data structures. Therefore, KSplice is best used to apply security updates that fix "off by one" and similar style exploits.

Check out the project at http://web.mit.edu/ksplice

David
http://tributetogrr.blogspot.com

Multiple Ways to Delete Blank Lines From a Text File

2008-04-16T12:41:00.002-04:00

You have a text file that has a bunch of blank lines in it. You want to do some operation on that text file but first want to delete the blank lines. You could open the file in vi and manually delete them all but that isn't the Unix way. Here are a number of easy ways to delete blank lines.

If you already have the text file open in vi/vim, the following will delete all blank lines while in command mode:
:g/^$/d

Here are a few different options from the shell using standard GNU tools:

GNU awk

awk '/./' filename

awk 'NF > 0' filename

Warning: This will delete all blank lines AND lines containing only whitespace

GNU sed

sed -i '/^$/d' filename

This makes use of the regular expression ^$ representing blank lines. The -i flag tells sed to edit the file in place. If you want to preserve the original file, simple remove the -i and redirect the output to a new file:

sed '/^$/d' filename > newfilename

GNU grep

grep -v '^$' filename > newfilename

This is the same as sed with the use of the regex ^$

Of course there are many other ways to accomplish this task. That is the beauty of Unix.

Display the Contents of an RPM or Tar Archive

2008-04-11T21:51:00.002-04:00

An RPM is just an archive of files. I believe that all of the various archiving programs (tar, cpio, etc.) provide you with a way to list the contents of their archives. An RPM is no different. You need to figure out if foo.rpm provides libfoo.so. Making use of the rpm2cpio, cpio, and grep programs, we can do just this:

unix% rpm2cpio foo.rpm | cpio -t | grep "libfoo.so"
/usr/lib/libfoo.so.2
unix%

Conversely, you can display the contents of a tar archive in one step:
unix% tar -tvf foo.tar
foo/
foo/bar.txt
foo/bills
foo/private/
foo/private/tax-return.pdf
unix%

Neither of these steps will actually extract files from the archive.

Being a diehard Debian user, it feels weird to write my first post about an RPM tip. By day, I admin a large number of Suse servers for a financial company. For me, there is no escaping the dreaded RPM.

Unix questions are always welcome.
Send questions to unix@tributetogrr.blogspot.com