Source: https://unix.stackexchange.com/a/194058/223965

Two days back:

journalctl --vacuum-time=2d

Retain only the past 500 MB:

journalctl --vacuum-size=500M

To get the checksum of a string, use this.

echo -n "yourstring" |md5sum
echo -n "yourstring" |sha1sum
echo -n "yourstring" |sha256sum

source Stackoverflow

If you want to have the original Oracle JDK instead of OpenJDK, follow this:

1. Download the *.tar.gz file (e.g. jdk-10.0.2_linux-x64_bin.tar.gz) at https://www.oracle.com/technetwork/java/javase/downloads/jdk10-downloads-4416644.html
2. untar it to /opt

tar xvzf jdk-8u191-linux-x64.tar.gz --directory=/opt

3. Update the alternatives so the system knows about your Java version (the pathes have to be changed)

update-alternatives --install \
    /usr/bin/java java /opt/jdk1.8.0_191/bin/java 20000 \
    --slave /usr/bin/keytool keytool /opt/jdk1.8.0_191/bin/keytool \
    --slave /usr/bin/orbd orbd /opt/jdk1.8.0_191/bin/orbd \
    --slave /usr/bin/pack200 pack200 /opt/jdk1.8.0_191/bin/pack200 \
    --slave /usr/bin/policytool policytool /opt/jdk1.8.0_191/bin/policytool \
    --slave /usr/bin/rmid rmid /opt/jdk1.8.0_191/bin/rmid \
    --slave /usr/bin/rmiregistry rmiregistry /opt/jdk1.8.0_191/bin/rmiregistry \
    --slave /usr/bin/tnameserv tnameserv /opt/jdk1.8.0_191/bin/tnameserv \
    --slave /usr/bin/unpack200 unpack200 /opt/jdk1.8.0_191/bin/unpack200

4. change the defualt java:

update-alternatives --config java

5. check the version

 java -version

remove old java

update-alternatives --remove "java" "/opt/jdk1.8.0_151/bin/java"
rm -r /opt/jdk1.8.0_151/bin/java

If you have a big home drive and want to search only in the hidden folders, use this command:

find . -path '*/\.*' -name <filename>

Sign vboxdrv so Secure Boot has no problem with it. (Source AskUbuntu)
Short version:

sudo /usr/src/linux-headers-$(uname -r)/scripts/sign-file sha256 ~/.ssh/MOK.priv .ssh/MOK.der $(modinfo -n vboxdrv)

To create the keys MOK.priv and MOK.der, this steps are required:

# create key pair
openssl req -new -x509 -newkey rsa:2048 -keyout MOK.priv -outform DER -out MOK.der -nodes -days 36500 -subj "/CN=Signing Key for Secure Boot/"

# use mokutil to test, if the key is enrolled (should not, if you just created it)
mokutil --test-key .ssh/MOK.der

# import key
sudo mokutil --import .ssh/MOK.der

# sign vboxdrv
sudo /usr/src/linux-headers-$(uname -r)/scripts/sign-file sha256 .ssh/MOK.priv .ssh/MOK.der $(modinfo -n vboxdrv)

# check vboxdrv and activate
sudo modinfo -n vboxdrv
sudo modprobe vboxdrv

If you have a lot of open windows, then the task list could run out of space. The grouping of the windows is a good feature to don't loose the overview.
This is what I mean with "grouping":

There is a good Applet called Icing Task Manager. Install steps:

1. Open Applets via System Settings
   
2. Go to Download and search and install Icing Task Manager
3. At tab Manage activate Icing Task Manager and open its settings
4. In tab Panel change Title display to "Title" (or "App"). Otherwise only the icon is displayed and you can't tell them apart from the panel launchers.
   
5. Now you have to remove the old Window list.
   1. Right click on the taskbar, chose "Panel edit mode"
   2. Right click on the old Window list and choose "remove Window list"
6. have fun

If you e.g. can't unmount because a file is busy, then you could find it with this command:

lsof | grep <path>

Replace <path> with your mount point, so you see which program uses your mount point.

lsof has this columns:

COMMAND     PID   TID             USER   FD      TYPE             DEVICE  SIZE/OFF       NODE NAME

Good questions to ask if you want to hire a Linux Sysadmin.
Or good questions to ask yourself before before applying to a new job.

Ha a look at: trimstray/test-your-sysadmin-skills

Simply run

 df -h | grep -v tmpfs

-v inverts the grep