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#!/usr/bin/env bash
# Emulate genhostid(1) available on RHEL/CENTOS, for use on distros
# which do not provide that utility.
#
# Usage:
# zgenhostid
# zgenhostid <value>
#
# If /etc/hostid already exists and is size > 0, the script exits immediately
# and changes nothing. Unlike genhostid, this generates an error message.
#
# The first form generates a random hostid and stores it in /etc/hostid.
# The second form checks that the provided value is between 0x1 and 0xFFFFFFFF
# and if so, stores it in /etc/hostid. This form is not supported by
# genhostid(1).
hostid_file=/etc/hostid
function usage {
echo "$0 [value]"
echo "If $hostid_file is not present, store a hostid in it." >&2
echo "The optional value must be an 8-digit hex number between" >&2
echo "1 and 2^32-1. If no value is provided, a random one will" >&2
echo "be generated. The value must be unique among your systems." >&2
}
# hostid(1) ignores contents of /etc/hostid if size < 4 bytes. It would
# be better if this checked size >= 4 bytes but it the method must be
# widely portable.
if [ -s $hostid_file ]; then
echo "$hostid_file already exists. No change made." >&2
exit 1
fi
if [ -n "$1" ]; then
host_id=$1
else
# $RANDOM goes from 0..32k-1
number=$((((RANDOM % 4) * 32768 + RANDOM) * 32768 + RANDOM))
host_id=$(printf "%08x" $number)
fi
if egrep -o '^0{8}$' <<< $host_id >/dev/null 2>&1; then
usage
exit 2
fi
if ! egrep -o '^[a-fA-F0-9]{8}$' <<< $host_id >/dev/null 2>&1; then
usage
exit 3
fi
a=${host_id:6:2}
b=${host_id:4:2}
c=${host_id:2:2}
d=${host_id:0:2}
echo -ne \\x$a\\x$b\\x$c\\x$d > $hostid_file
exit 0
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