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Quickly find a count of how many times invalid users have attempted to access your system

Disable annoying sound emanations from the PC speaker
To ensure that it will never come back, you can edit /etc/modprobe.d/blacklist Add "blacklist pcspkr" sans quotes

Find Mac address

Removing images by size
Removes all png files whose geometry is not 280x190 pixels

Know SELinux status
Usefule to check whether SELinux is in force or disabled. Though you need to be superuser while execute this command.

Explanation of system and MySQL error codes
perror should be installed if mysql-server package is installed

get rid of lines with non ascii characters
found here: https://stackoverflow.com/a/9035939

Backup your LDAP
Simple way to backup your LDAP entries: put this line on your crontab. The -n switch identifies the dbnum you want to backup (alternatively you can use -b suffix. Check man slapcat for your personal switches)

Add a DNS server on the fly
Since systemd-resolved was implemented, add a DNS server have become weirder and harder than before. With this command, you can add a DNS server on-the-fly tied to an specific interface

Which processes are listening on a specific port (e.g. port 80)
swap out "80" for your port of interest. Can use port number or named ports e.g. "http"


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