storagealert.sh

#!/bin/bash

# Setup:
# 0. Have a mail server set up, or something that provides `sendmail` with proper sender permissions.
# 1. Adjust the $drive, $buckets, $from and $to variables below to suit your needs.
# 2. Create /var/local/storagealert/$drive.txt and write 0 to it.
# 3. Put this script at /etc/cron.hourly/storagealert with chmod 0755.

set -e;

drive='PLACEHOLDER';
buckets=('95' '90' '80');
from='server@example.com';
to='me@example.com';

pct="$(df --output=pcent "/dev/$drive" | egrep -v '^Use%$')";
if [ "$(wc -l <<<"$pct")" -ne '1' ]; then
    echo '`df` returned multiple lines of output:' >&2;
    echo "$pct" >&2;
    exit 1;
fi;

if ! grep -Pq '^\s*\d+%$' <<<"$pct"; then
    echo '`df` returned bad percentage:' >&2;
    echo "$pct" >&2;
    exit 1;
fi;

pct="$(sed -E 's#^[[:space:]]*([[:digit:]]+)%$#\1#' <<<"$pct")";
old="$(cat "/var/local/storagealert/$drive.txt")";
echo "$pct" >"/var/local/storagealert/$drive.txt";

do_mail=false;
crossed=0;
for b in "${buckets[@]}"; do
    if [ "$pct" -ge "$b" ] && [ "$old" -lt "$b" ]; then
        do_mail=true;
        crossed="$b";
        break;
    fi;
done;

if "$do_mail"; then
    sendmail -t <<EOF
From: $from
To: $to
Subject: Storage more than $crossed% full
Content-Type: text/plain; charset="US-ASCII"

Drive $drive is $pct% full.

EOF
fi;