On the third side

From time to time, RedHat’s predefined kernel configuration does not suit your needs. You may want to disable or enable drivers that are not in the stock config, or you may want to build more specialized kernels. But you want to keep the build framework and all the patches. Here’s a short howto. With small changes, it works on RHEL4 as well. In Fedora, the build framework has become a bit more mature, with more options and less editing of files. You can find their corresponding document at http://fedoraproject.org/wiki/Docs/CustomKernel.

In the following you may replace “i686″ and “i386″ for “x86_64″ if you have an AMD64 or EM64T cpu on your target system. Run the following on a build system. If you are of the more clever users (you are reading this, right?), you’ll replace /usr/src with your private rpm build space, see http://users.linpro.no/ingvar/rpm.html.

rpm -ivh kernel-2.6-something.src.rpm

yum install ncurses-devel kernel-devel make gcc

cd /usr/src/redhat/SPECS
rpmbuild -bp --target=i686 kernel-2.6.spec

cd /usr/src/redhat/SOURCES
find . -name kernel\\*config
cp kernel-2.6.18-i686.config kernel-2.6.18-i686.config.orig
cp kernel-2.6.18-i686.config /usr/src/redhat/BUILD/kernel-2.6.18/linux-2.6.18.i686/.config

cd /usr/src/redhat/BUILD/kernel-2.6.18/linux-2.6.18.i686
make oldconfig
make menuconfig

vi .config

# i386

cp .config /usr/src/redhat/SOURCES/kernel-2.6.18-i686.config

cd /usr/src/redhat/SPECS
vi kernel-2.6.spec

%define buildxen 0
%define builddoc 0
%define buildkdump 0
%define buildheaders 0

%define release 8.1.15.local%{?dist}

%ifarch i686
%define buildpae 0

time rpmbuild -bb --target=i686 kernel-2.6.spec

If you do a google search for “associative arrays in bash”, you get a lot of hits that tell you that this is a feature not found in bash (though later versions may include this). Now, I find associative arrays (or hash tables if you like) a very nice feature to have available. And it is not that hard to emulate it in bash. One can use two arrays, one for the keys, and the other for the data elements, and a common counter for the indexes. A more ugly, but faster design could be direct string to variable name translation, with eval doing the dirty work.

Note that this kind of translation may be called seriously bad design, and in an interactive script, it may lead to serious security flaws. But if you know what the strings will look like, this short hand may do the work for you.

Consider the following code snippet:

#!/bin/bash
#
# Associative arrays in bash, take 1
# Using simple translation with eval

# Some test values with doublettes
values="a a a a b b c d";

# The incrementer for the keys to the array
key=-1 # Start with off-by-one, as incrementing is the first step in the loop

for val in $values; do

    # Need to check for uninitialized value, to get rid of doublettes
    # (and yes, here test(1) needs the extra character in the test)
    if eval [ 'x$'$val = "x" ]; then
        ((key++))
    fi

    # Now, fill the hash with values
    eval $val=$key;
    case $val in
        "a")
            arr[$key]="alice";
            ;;
        "b")
            arr[$key]="bob";
            ;;
        "c")
            arr[$key]="charlie";
            ;;
        "d")
            arr[$key]="diana";
            ;;
    esac
done

# Look up the value for a single key 'a'
key="a"
echo -n "Data entry for key $key is: "
eval key="$"$key
echo ${arr[$key]};

# Print the contents of the array
echo ""; echo "Whole array:"
for (( key=0; key < ${#arr[*]}; key++ ))
do
  echo ${arr[$key]}
done

## EOF

For more serious scripts, consider as mentioned, putting the keys in its own array, and search it while looking up values. This would take more time, though.

#!/bin/bash

#

# Associative arrays in bash, take 2

# Using two arrays
# Some test values with doublettes

values="a a a a b b c d";
# Search for existing keys

function getkey {

    key=$1

    for (( i=0; i <  ${#keys[*]}; i++ )) {

	if [ "${keys[$i]}" = "$key" ]; then

	    return $i

	fi

    }

    # Not found, create a new key

    keys[$i]=$key

    return $i

}
# Now, fill the hash with values

for val in $values; do
    # First, get a new key

    getkey $val; key=$?
    case $val in

	"a")

	    arr[$key]="alice";

	    ;;

	"b")

	    arr[$key]="bob";

	    ;;

	"c")

	    arr[$key]="charlie";

	    ;;

	"d")

	    arr[$key]="diana";

	    ;;

    esac

done
# Look up the value for a single key 'd'

echo -n "Data entry for key 'd' is "

getkey 'd'; key=$?

echo ${arr[$key]};
# Print the complete contents of the hash

echo ""; echo "Whole hash:"

for (( i=0; i < ${#arr[*]}; i++ ))

do

  echo -n "%arr{"

  echo -n ${keys[$i]}

  echo -n "}="

  echo ${arr[$i]}

done
# EOF