using bash to print index of maximum value in array

Question

I am trying to print index value of maximum value in an array. I wrote something like this:

my_array=( $(cat /etc/grub.conf | grep title | cut -d " " -f 5,7 | tr -d '()'|cut -c1-6) )
echo "${my_array[*]}" | sort -nr | head -n1
max=${my_array[0]}
for v in ${my_array[@]}; do
    if (( $v > $max )); then max=$v; fi;
done
echo $max

Output of this script is coming up like:

4.9.85 4.9.38
./grub_update.sh: line 6: ((: 4.9.85 > 0 : syntax error: invalid arithmetic operator (error token is ".9.85 > 0 ")
./grub_update.sh: line 6: ((: 4.9.38 > 0 : syntax error: invalid arithmetic operator (error token is ".9.38 > 0 ")
0

Requirement: I want to query grub.conf and read Kenrnel line followed by printing index value of latest kernel in the array

kernel /boot/vmlinuz-4.9.38-16.35.amzn1.x86_64 root=LABEL=/ console=tty1 console=ttyS0 selinux=0

Bash can't compare 4.9.85 with 0... looks like you need a little more than a simple loop. — TrebledJ
– TrebledJ, Commented Mar 22, 2019 at 18:18
sorry, I just updated the code, currently I am able to get an output but it's throwing lower value while I am looking for greater number — Todd Goodfellow
– Todd Goodfellow, Commented Mar 22, 2019 at 22:53
If you want to do a version sort, that's what the GNU sort -V argument is for. — Charles Duffy
– Charles Duffy, Commented Mar 22, 2019 at 23:47
BTW, array=( $(...) ) is an antipattern; see BashPitfalls #50. — Charles Duffy
– Charles Duffy, Commented Mar 22, 2019 at 23:48

KamilCuk · Accepted Answer · 2019-03-22 23:08:35Z

0

Comments in code:

# our array
arr=( 
     1.1.1  # first element
     2.2.2 
     4.9.85 # third element, the biggest
     4.9.38 
)

# print each array element on a separate line
printf "%s\n" "${arr[@]}" | 
# substitute the point to a space, so xargs can process it nicely
tr '.' ' ' |
# add additional zeros to handle single digit and double digit kernel versions
xargs -n3 printf "%d%02d%02d\n" |
# add line numbers as the first column
nl -w1 |
# number sort via the second column
sort -k2 -n |
# get the biggest column, the latest
tail -n1 |
# get the first field, the line/index number
cut -f1

It will output:

Live code available on tutorialspoint.

answered Mar 22, 2019 at 23:08

KamilCuk

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jhnc · Accepted Answer · 2019-03-22 23:46:50Z

0

You can do string comparisons if the digits are aligned.
printf can align the digits

data=(
    4.9.85
    4.9.38
    3.100.20.2
    4.12.2.4.5
    4.18.3
)

findmax(){
    local cur
    local best=''
    local ans

    for v in "$@"; do
        cur="$(
            # split on dots
            IFS=.
            printf '%04s%04s%04s%04s%04s' $v
        )"
        # note: sort order is locale-dependent
        if [[ $cur > $best ]]; then
            ans="$v"
            best="$cur"
        fi
    done
    echo "$ans"
}

echo "max = $(findmax "${data[@]}")"

edited Mar 22, 2019 at 23:46

answered Mar 22, 2019 at 23:41

jhnc

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