How can I check if a reboot is required on Arch Linux?

#!/usr/bin/bash

s1=$(pacman -Q linux | sed 's/linux //')
s2=$(uname -r | sed 's/-ARCH//')

if [ "$s1" == "$s2" ]; then
  echo OK
else
  echo REBOOT
fi

Seems to work. Source: bbs.archlinux.org/viewtopic.php?id=173508

I use this script to check if the boot kernel matches the current kernel and if a process is using any old libraries.

#!/bin/bash

get_boot_kernel() {
    local get_version=0
    for field in $(file /boot/vmlinuz*); do
        if [[ $get_version -eq 1 ]]; then
            echo $field
            return
        elif [[ $field == version ]]; then
            # the next field contains the version
            get_version=1
        fi
    done
}

rc=1

libs=$(lsof -n +c 0 2> /dev/null | grep 'DEL.*lib' | awk '1 { print $1 ": " $NF }' | sort -u)
if [[ -n $libs ]]; then
    cat <<< $libs
    echo "# LIBS: reboot required"
    rc=0
fi

active_kernel=$(uname -r)
current_kernel=$(get_boot_kernel)
if [[ $active_kernel != $current_kernel ]]; then
    echo "$active_kernel < $current_kernel"
    echo "# KERNEL: reboot required"
    rc=0
fi
exit $rc

Sample output:

Xorg: /usr/lib/libedit.so.0.0.63
Xorg: /usr/lib/libgssapi_krb5.so.2.2
Xorg: /usr/lib/libk5crypto.so.3.1
Xorg: /usr/lib/libkrb5.so.3.3
Xorg: /usr/lib/libkrb5support.so.0.1
Xorg: /usr/lib/libzstd.so.1.4.5
# LIBS: reboot required
5.10.8-arch1-1 < 5.10.10-arch1-1
# KERNEL: reboot required

If you only have processes using old libraries you can restart the processes instead of rebooting.