sed 's/replace all occurences of this/with this/g'

awk '/before this line/{print "insert this line"}{print $0}'

awk '{print $0}/after this line/{print "insert this line"}'

sed

kernel=yes
webmin=yes
samba=no

if kernel=yes then ...

# I've always got these around...  but they're usually much more elaborate...

warn () {
        echo "Warning: $*"
}

fatal () {
        echo "Fatal! $*"
        exit 2
}

d='/'   # In case / is just too painful

replaceStringInFile () {
        # I use this order to make this easier to extend
        from="$1"
        to="$2"
        file="$3"

        [ $# -eq 3 ] || { warn "bad arg count" ; return 1; }
        [ -z "$from" ] || { warn "No from pattern" ; return 1; }
        # Note that an empty to pattern makes perfect sense...
        [ -z "$file" ] || { warn "No filename given" ; return 1; }
        [ -f "$file" ] || { warn "No such file: $file" ; return 1; }
        [ -r "$file" ] || { warn "Cannot read file: $file" ; return 1; }
        [ -w "$file" ] || { warn "Cannot write file: $file" ; return 1; }
        # You should also check that the directory is writable, etc., ...
        tmpFile="$file.tmp.$$"
        cp -p "$file" "$tmpFile" || {
                warn "Cannot create temporary file: $tmpFile" ; return 1; }
        sed -e "$d$from$ds$d$d$to$dg" <"$file" >"$tmpFile" || {
                warn "Cannot edit file: $file" ; return 1; }
        mv "$file" "$file~" || {
                warn "Cannot rename file: $file" ; return 1; }
        mv "$tmpFile" "$file" || {
                warn "Cannot replace file: $file" ; return 1; }
        return 0
}

if [ "$variableNameHere"  = "yes" ] ; then
    doYesStuff
else
    doNoStuff
fi

case "$var" in
[Yy]|[Yy][Ee][Ss])
    doYesStuff
    ;;
[Nn]|[Nn][Oo])
    doNoStuff
    ;;
*)
    doWTHStuff
    ;;
esac