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I need to create a script that will execute daily (cron job), calculate the uptime of the system (days only), and if that number is greater than a defined value (e.g. 30), email the system's information (uname -a) and the number of days to some address (e.g. admin@company.com).

I am getting lost in converting the value of days to something I can perform a calculation on and take action. Started by using the

uptime | grep -o "up.*day"

command but then got stuck. Using CentOS 7.8.

Thank you in advance for any help!

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    Do you have uptime -s? E.g. uptime in seconds: echo "$(( $(date +'%s') - $(date -d"$(uptime -s)" +'%s') ))". If the value is greater than 3600*24*30, then ...
    – Freddy
    May 15 '20 at 4:24
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The documentation man uptime offers this snippet

-s, --since - system up since, in yyyy-mm-dd HH:MM:SS format

So you can get an absolute date/time from which the counting commenced

uptime -s
2020-02-12 12:37:16

Subtract that from the current date/time and you'll get days. (It's a little more fiddly as date maths is usually in seconds)

# Seconds at boot
secSince=$(date --date "$(uptime -s)" +%s)

# Seconds now
secNow=$(date +%s)

# Difference
secInterval=$((secNow - secSince))

# Convert to days
daysInterval=$((secInterval / (60*60*24)))

echo "$daysInterval day(s) uptime"

If there's been a leap second added during the calculated interval the reported day may be off by one within a couple of seconds of your local midnight.

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