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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. Detemine the largest files on the filesystem:
   du -ah / --exclude="/proc" | sort -h -r | head -n 5 2>/dev/null
   
4. Check the output for something that can be deleted.
   
5. 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.
   
6. 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
   
7. Check the inode usage with this: df -i
   
8. 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

Use the Linux Standard Base command as follows:
lsb__release -a

The du command is very useful when you are looking at disk space usage. Here are some of the most commonly used parameters:

-h human-readable Displays sizes in human-readable format, using units such as KB, MB, GB, etc
-a, –all Lists all files with their sizes (combine with h to get the best result)
-d, –max-depth=N
–time Lists all files with their sizes and the timestamp of its last modification
-X, –exclude=<pattern> Lists all files except those that match <pattern>

To find the disk usage of all files in the directory except the tarred files:

du -ah --exclude=".tgz" --exclude=".tar.gz"

To find the 10 largest files in the current directory ordered by file size:
(This gets both directories and files by size)
for i in G M K; do du -ah | grep [0-9]$i | sort -nr -k 1; done | head -n 11
or
du -ah . | sort -n -r | head -n 10

(This gets only files ordered by size)
du -hsx * | sort -rh | head -10
or
du -ah --max-depth=1 | sort -h

#Run iostat to get the average cpu usage every 10 seconds for 10 times:
iostat -x 10 10 >> /home/<user>/iostat.out.$(date +"%Y-%m-%d")

#Run dstat to get cpu stats for cpus 1, 3 and the total:
dstat -C 0,3,total

top and htop are free CLI process viewers. They list the top resource using processes being run.The information includes:

• PID — Process Id The task’s unique process ID
• PPID — Parent Process PID (Process ID) of a task’s parent
• RUSER — Real User NameThe real user name of the task’s owner 
• UID — User IdThe effective user ID of the task’s owner
• USER — User NameThe effective user name of the task’s owner
• GROUP — Group NameThe effective group name of the task’s owner.
• NI — Nice value The nice value of the task. A negative nice value means higher priority, whereas a positive nice value means lower priority. Zero in this field simply means priority will not be adjusted.

top options:
–a : Sort by memory usage This switch makes top to sort the processes by allocated memory
–d : Delay time update interval as: -d ss.tt (seconds.tenths)
–p : Monitor PIDs as: -pN1 -pN2 … or -pN1, N2 [,…]
–u : <somebody> Monitor only processes with an effective UID or user name matching <somebody>.
htop options:
-u –user=USERNAME Show only the processes of a given user
-d –delay=DELAY Delay between updates, in tenths of seconds

Examples:

To run top in batch mode (updating every 3 seconds) and write it to a file in my directory
top -b -d 10 -n 3 >> /home/<user>/top-file

To run htop to get only processes for my user:
htop -u <user>

Watch disk usage updating every 3 seconds:
watch -d -n 3 'df -h'

Sed strings make VIM very powerful. You can use them to do many things. The changes will not be written to the file until you save the file, so you are safe to experiment a bit.

To delete lines 4-12:

:4,12 d

To delete the current line:
:d

To replace the word foo with the word bar starting at the line where the cursor is through the end of the file:

:.,$s/foo/bar/

To add the word foo at the end of lines 24 – 34:

:24,34 s/$/foo/

To remove the spaces in the middle of a pip list output line and replace it with == so it can be used to install (if you have to recreate your pyenv):

1.) remove the header lines:
:1,2 d
2.) replace the spaces with double equals:
:%s/ +/==/g

To remove == and everything after from a pip list so that pip will install the newest version:

:%s/==.*//

To remove all lines that contain the strings error, warn, or fail (remove the /d to show the lines that the command will delete):

:g/error\|warn\|fail/d

To remove all lines that don’t contain the strings error, warn, or fail (remove the /d to show the lines that the command will delete):

:g!/error\|warn\|fail/d

v can replace the g! if you prefer:

:v/error\|warn\|fail/d

To reformat a paragraph in vim:

1. Use <CTRL-J> to join all lines in the paragraph
2. :gq

To remove all commented and blank lines from a file (remove the /d to show the lines that the command will delete):

:g/\v^(#|$)/d

To change the last character in a line to a semi-colon:

:%s/.{1}$/\;/

To replace all single quotes in a file with double quotes:

:%s/'/\"/g

To remove all extra spaces in all lines of a file:

:%s/ +//g

Use g to execute a cmd on regex to delete blank lines:

:g/^$/d

Substitute starting string to eol with newstring followed by comma:

:% s/<string>.*/<newstring>,/

Substitute invalid non-printable characters (U+00A0) with spaces:

%s/\%u00a0/ /g

Remove everything on line from the comma to eol:
:%s/,.*//

Remove all lines not beginning with IPv4 IP:
:%v/^\(\d\+\.\)\{3}\d\+/d