Linux troubleshooting spells

I sincerely wish the reader not to use these spells ever. They are only used when you are lost in a unknown server where you need to troubleshoot it to find out what is wrong.

Space disk checking

$ df -kh /opt/java11/

Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/Volgroup01-lx_root
                       19G   15G  2.9G  84% /

$ du -hx --max-depth=1n /opt/java11/ | sort -hr

297M        /opt/java11/
220M        /opt/java11/lib
77M /opt/java11/jmods
544K        /opt/java11/legal
512K        /opt/java11/bin
228K        /opt/java11/include
140K        /opt/java11/conf

ls -lrth /opt/java11/
total 28K
-rw-r--r--.  1  668  668 1.2K Aug 23  2018 release
drwxr-xr-x.  2 root root 4.0K Apr 22  2021 bin
drwxr-xr-x.  4 root root 4.0K Apr 22  2021 conf
drwxr-xr-x.  3 root root 4.0K Apr 22  2021 include
drwxr-xr-x.  2 root root 4.0K Apr 22  2021 jmods
drwxr-xr-x. 72 root root 4.0K Apr 22  2021 legal
drwxr-xr-x.  6 root root 4.0K Apr 22  2021 lib