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To reconfigure you Display manager:
1. Determine which Display Manager is being used:
cat /etc/X11/default-display-manager
2. Reconfigure Display Manager as needed:
dpkg-reconfigure <display-manager>

To break up a large .csv file into multiple smaller files with 1000 lines:
sed -n '1,1001p' large_table.csv > output_1.csv
sed -n '1002,2001p' large_table.csv > output_2.csv

To see memory and CPU usage for all containers:
docker stats

b. To see disk usage for all containers:
docker ps -s

c. To see memory usage for all containers:
docker stats --no-stream --format "{{.Name}}: {{.MemUsage}}"

d. To see which container has been hit by the OOM killer:
for each in $(docker ps --format "{{.Names}}"); do if docker inspect --format '{{.State.OOMKilled}}' $each | grep -q 'true'; then echo "$each is OOMKilled"; fi; done

Use ssh to execute a command on a remote system:
ssh user@user@server.com <command>

Create a log of an ssh session:
ssh user@server.com | tee -a ~/server.com.ssh.log

Copy a file from your local directory to a directory on a remote system:
scp [filename] [user]@[server.com]:/remote/directory

Remove the ABCD– prefix from a filename:
$ s='ABCD--11212239.pdf'
$ mv "$s" "${s#ABCD--}"

Remove the prefix from all files in current directory recursively:
while IFS= read -r -d '' filename; do
mv "$filename" "${filename#ABCD--}"
done < <(find . -type f -name 'ABCD--*' -print0)

Remove the .pdf suffix from a filename:
$ s='ABCD--11212239.pdf'
$ mv "$s" "${s%.pdf}"

Remove the “r_” prefix from all the files in the current directory:
for each in ls r_*; do mv $filename ${filename#r_}; done

Remove the “pdf” suffix from all the files in the current directory recursively:
while IFS= read -r -d '' filename; do
mv "$filename" "${filename%.pdf}"
done < <(find . -type f -name '*.pdf' -print0)

#Check size of Volume Group
df -h -T | grep vg
#Display volume group info. Check for free space
sudo vgdisplay
#Display logical volume info
sudo lvdisplay
#Test run
sudo lvextend -t -v -l +100%FREE /dev/ubuntu-vg/ubuntu-lv
#Actual run
sudo lvextend -l +100%FREE /dev/ubuntu-vg/ubuntu-lv
#Resize logical volume
sudo resize2fs /dev/ubuntu-vg/ubuntu-lv
#Verify
df -h -T | grep vg

To boot into single user mode from the Grub menu:

In Ubuntu, the Grub Menu should appear if you press and hold Shift while Grub is loading, if you boot using BIOS. If your Ubuntu system boots using UEFI, press Esc instead. You might have to press the key multiple times.

When you get to the Grub Menu, edit the line that starts with linux by adding any one of these to the end of the line:
s (lower case s)
S (upper case s)
1 (number 1)
If the system is using systemd, add one of these to the end of the line that starts with linux and use the key sequence that is listed at the bottom to boot the new configuration (usually ,<ctrl>X or F10):
systemd.target=rescue.target
rescue

Since single user mode gives you root access to the system, it is a good idea to require the root password when booting into it. To do this,

If the system is using systemd, edit the ExecStart line in both /lib/systemd/system/rescue.service and /lib/systemd/system/emergency.service by changing sushell to sulogin. After saving these changes, when you edit the Grub menu and boot into single user mode, you will be required to enter the root password for access.

1. Log into system: ssh <system>
   
2. Check for full filesystem: df -h
   
3. Determine the largest directories on the filesystem:
   sudo du -ah / --exclude="/proc" | sort -h -r | head -n 5 2>/dev/null
   
4. Determine the 5 largest files on the filesystem:
   sudo find / -type f -exec ls -la {} + 2>/dev/null | sort -k5 -rn | head -5
   
5. Check the output for something that can be deleted.
   
6. If the du command doesn’t show anything, consider whether something that was running might have created a temporary file that exceeded the remaining space on the filesystem.
   
7. If you find temporary files created at run-time that exceed the remaining drive space, you have 3 choices:
   A. add more drive space
   B. spread out the run times of jobs so that concurrent jobs don’t exceed the remaining drive space
   C. consider adding compression to the temporary files before writing them to the filesystem
   
8. Check the inode usage with this: df -i
   
9. If you find the issue is inode usage, you have 2 choices:
   A. delete file s of directories that use a large number of inodes:
   1. identify which files and directories are using the most inodes by running this:
for i in /*; do echo $i; find $i | wc -l; done
   2. examine the output for toplevel directories that are using a lot of inodes
   3. re-run the command in step 1. replacing /* with the toplevel directory to check
   4. Once you determine where the issue is: delete expendable files or directories
   B. increase the number of available inodes:
   1. unmount the full filesystem: umount <filesystem mountpoint>
   2. update the number of inodes: mkfs.ext3 /dev/<device> -N <desired num of inodes>
   3. re-mount the filesystem: mount -a
   

Calculate MySQL Memory Usage – Quick Stored Procedure

To see what systems on the 192.168.11.0 network are listening on port 22:
nmap -sV -p 22 192.168.11.0/24

To do an nmap ping scan on 192.168.11.0 network:
nmap -sp 192.168.11.0/24

To scan the top five ports on 192.168.11.7:
nmap --top-ports 5 192.168.11.7