I BASHed the terminal and sed 's/ex...'

Like MySQL, BASH scripting is something I've always done frequently, but very light as far as complexity and depth.

Batching commands is easy, but now I'm coming into a new phase of my career where scripting is crucial to the day-to-day operations.  I'm finding help in the form of co-workers and tldp.org.  The following is a sanitized version of the check and insert I was asked to create that will turn on fasttcp on CentOS/RHEL, but not if the kernel is "2.6.18-238.9.1.el5.fsl_2.0.2.1"

So we need to:
a) Check kernel to exclude a certain build.
      i. (if yes) Insert 'kernel*' to the exclude on yum.conf
     ii. Report if the yum.conf was modified or not.
b) download and install
c) chkconfig on resources

I'm a total noob at scripting, so I made sure I didn't break anything on the system; I created the script as a non-root user and copied the yum.conf to that user's home folder with a unique name.

cp /etc/yum.conf /home/dave/dave-practice-yum.conf

First, the beginning of any (good) shell script calls the shell you're working in.  I'm from BSD most-recently, so !/bin/sh is what I would generally use, but now that I'm a CentOS guy at my new job, !/bin/bash is the shell for me.

#!/bin/bash

Defining variables: I need to find out if the kernel is the mystical kernel that isn't supposed to get updated, so I'll establish the KRNL variable.  I also am going to insert 'kernel*' into the 'exclude=' line of the yum.conf if it is a match, so I will also define the EXCL variable to make sure I'm not over-writing an already-established 'exclude='.

KRNL=`uname -r`
EXCL=`grep "exclude=" dave-practice-yum.conf|cut -d= -f2`

Now we write in the actual check with an IF statement:

if [ $KRNL = 2.6.18-238.9.1.el5.fsl_2.0.2.1 ]
then
        if [ -z $EXCL ]
        then
                sed -i 's/exclude=/exclude=kernel*/' dave-practice-yum.conf
                echo "yum.conf modified."
        else
                sed -i '/exclude=/s/$/,kernel*/g' dave-practice-yum.conf
        fi
else
        echo "No modification to yum.conf"
fi

The else in the nested condition is pretty ugly.  I didn't want to over-write if there were already any excludes, so I appended the 'kernel*' to the 'exclude=' if nothing were present; if there is something present on the exclude line when the script runs, it will simply put a ',kernel*'.  This works, but if 'kernel*' is there, it will be repeated.

Then we download the source package from the internal repository, and chkconfig the services on (in this case FASTTCP).

/usr/sbin/wget -qO - http://server/files/fasttcp/ftcp_rc.tar|tar xf - -C /
/sbin/chkconfig --add fasttcp
/sbin/chkconfig --levels 2345 fasttcp on
/etc/init.d/fasttcp start

Remarkably, this script works; thanks in large part to my co-workers, and James in particular.  Below, find the script in its entirety (without blogspot formatting).

#!/bin/bash

KRNL=`uname -r`

EXCL=`grep "exclude=" dave-practice-yum.conf|cut -d= -f2`

if [ $KRNL = 2.6.18-238.9.1.el5.fsl_2.0.2.1 ]

then

        if [ -z $EXCL ]

        then

                sed -i 's/exclude=/exclude=kernel*/' dave-practice-yum.conf

                echo "yum.conf modified."

        else

                sed -i '/exclude=/s/$/,kernel*/g' dave-practice-yum.conf

        fi

else echo "No modification to yum.conf"

fi

/usr/sbin/wget -qO - http://server/files/fasttcp/ftcp_rc.tar|tar xf - -C /

/sbin/chkconfig --add fasttcp

/sbin/chkconfig --levels 2345 fasttcp on

/etc/init.d/fasttcp start