Substitute Scientific Notation from bytes to Megabytes [closed] - rrdtool

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I Have a .xml file that has lines which look like this:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE rrd SYSTEM "http://oss.oetiker.ch/rrdtool/rrdtool.dtd">
<!-- Round Robin Database Dump -->
<rrd>
<version>0003</version>
<step>60</step> <!-- Seconds -->
<lastupdate>1674125860</lastupdate> <!-- 2023-01-19 10:57:40 UTC -->
<ds>
<name> 1 </name>
<type> GAUGE </type>
<minimal_heartbeat>8460</minimal_heartbeat>
<min>NaN</min>
<max>NaN</max>
<!-- PDP Status -->
<last_ds>954298368</last_ds>
<value>3.8171934720e+10</value>
<unknown_sec> 0 </unknown_sec>
</ds>
<!-- Round Robin Archives -->
<rra>
<cf>AVERAGE</cf>
<pdp_per_row>1</pdp_per_row> <!-- 60 seconds -->
<params>
<xff>5.0000000000e-01</xff>
</params>
<cdp_prep>
<ds>
<primary_value>8.5981579947e+08</primary_value>
<secondary_value>0.0000000000e+00</secondary_value>
<value>NaN</value>
<unknown_datapoints>0</unknown_datapoints>
</ds>
</cdp_prep>
<database>
<!-- 2023-01-17 10:58:00 UTC / 1673953080 --> <row><v>NaN</v></row>
<!-- 2023-01-17 10:59:00 UTC / 1673953140 --> <row><v>NaN</v></row>
<!-- 2023-01-17 11:00:00 UTC / 1673953200 --> <row><v>NaN</v></row>
<!-- 2023-01-17 11:01:00 UTC / 1673953260 --> <row><v>NaN</v></row>
<!-- 2023-01-17 11:02:00 UTC / 1673953320 --> <row><v>NaN</v></row>
<!-- 2023-01-17 11:03:00 UTC / 1673953380 --> <row><v>NaN</v></row>
<!-- 2023-01-18 12:00:00 UTC / 1674043200 --> <row><v>NaN</v></row>
<!-- 2023-01-18 18:00:00 UTC / 1674064800 --> <row><v>7.9644330667e+08</v></row>
<!-- 2023-01-19 00:00:00 UTC / 1674086400 --> <row><v>7.9696554667e+08</v></row>
<!-- 2023-01-19 06:00:00 UTC / 1674108000 --> <row><v>5.8408509440e+08</v></row>
</database>
</rra>
Trying to convert the scientific notation (which is a value in bytes) and convert it to a value in megabytes and back to scientific notation in Linux bash shell or script.
So far I have this lines, but i am stuck and don't know how to put them back into the file with the calculation to divide 2x by 1024:
cat Memory_mem_used.xml | grep -Eo '[0-9]+\.[0-9]+e\+[0-9]+' | perl -ne 'printf "%d\n", $_;'
The output should look like this:
output=796443306 | output2=$(($output / 1024 / 1024)) | perl -e 'printf "%.11e\n", '$output2''
7.59000000000e+02

Answer 1

Try:
#!/bin/bash
IFS=''
while read line ; do
left=${line%%<v>*}
rest=${line#*<v>}
value=${rest%%</v>*}
right=${rest#*</v>}
if [ "$value" != "$line" ] && [ "$value" != "NaN" ] ; then # match
num_value=$(LC_ALL=C printf '%.0f' "$value")
new_value=$(LC_ALL=C printf '%.11e' $((num_value / 1048576)) )
line="$left<v>$new_value</v>$right"
fi
echo "$line"
done < input.xml