Kudithipudi.Org

I was looking for a quick way to search for credit card numbers in a file and ran across this excellent post by Adrian Rollett. I tweaked his suggestion a bit to show some additional data.

Original suggestion

 grep '\(^\|[^0-9]\)\{1\}\([345]\{1\}[0-9]\{3\}\|6011\)\{1\}[-]\?[0-9]\{4\}[-]\?\[0-9]\{2\}[-]\?[0-9]\{2\}-\?[0-9]\{1,4\}\($\|[^0-9]\)\{1\}' FILE_TO_SEARCH 

My modification

 grep '\([345]\{1\}[0-9]\{3\}\|6011\)\{1\}[ -]\?[0-9]\{4\}[ -]\?[0-9]\{2\}[-]\?[0-9]\{2\}[ -]\?[0-9]\{1,4\}' --color -H -n FILE_TO_SEARCH 

The modified command will show the name of the file the number was found and at which line. You can tweak it further using additional options for grep. A good reference guide can be found here.

Quick way to figure out, which ports a particular program is using in linux

 netstat -plan | grep -i PROGRAM_NAME 

Example : Check which ports SSH is listening on

samurai@samurai:~$ sudo /bin/netstat -plan | grep sshd
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      5257/sshd
tcp        0     52 123.123.123.123:22      124.124.124.124:32846     ESTABLISHED 3551/sshd: samurai
tcp6       0      0 :::22                   :::*                    LISTEN      5257/sshd
unix  3      [ ]         STREAM     CONNECTED     5893     3551/sshd: samurai
unix  2      [ ]         DGRAM                    5849     3551/sshd: samurai

Most Redhat/Fedora users are used to chkconfig and service for controlling the services/programs that startup at boot time. Here is how you do it in Ubuntu

• Check status of a particular service

 sudo SERVICE_NAME status 

Example : Check the status of Apache Web Service

samurai@samurai:~$ sudo service apache2 status
Apache is running (pid 3496).

• Add a service to start on bootup

 update-rc.d SERVICE_NAME add 

Example : Configure squid to start on bootup

 update-rc.d squid add 

• Stop a service from starting on bootup

 update-rc.d SERVICE_NAME remove 

Example : Configure squid to NOT start on bootup

 update-rc.d squid remove 

NOTE : You need to have a startup script in /etc/init.d for the service to ensure update-rc.d works fine.

For my notes.. if you ever want to check the IO capability of a disk (local or network) on a linux machine, use the following command

 dd if=/dev/zero of=test.file bs=4M count=1000 

The above command make a copy of the output from /dev/zero to a file called test.file (you can locate the file on the disk you want to measure) with a block size of 4M for a total file size of 4000Mb.

There are several ways to run commands repetively in Linux.. (cron, while loop etc). Here is a simple trick. Say you want to check the size of a directory using “du” every 5 seconds, you can use the following command

watch -n 5 du -ch

Check out all the options available in watch by running “man watch” .