AWS : EC2 :Full Troubleshooting List

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/TroubleshootingInstancesConnecting.html#TroubleshootingInstancesServerError

Connection problems and errors

• Common causes for connection issues
• Error connecting to your instance: Connection timed out
• Error: unable to load key ... Expecting: ANY PRIVATE KEY
• Error: User key not recognized by server
• Error: Permission denied or connection closed by [instance] port 22
• Error: Unprotected private key file
• Error: Private key must begin with "-----BEGIN RSA PRIVATE KEY-----" and end with "-----END RSA PRIVATE KEY-----"
• Error: Server refused our key or No supported authentication methods available
• Cannot ping instance
• Error: Server unexpectedly closed network connection
• Error: Host key validation failed for EC2 Instance Connect

AWS : EC2: EBS : Mounting /Attaching EBS Volume : Duplicate UUID : Issue

sudo   mount -o   nouuid    /dev/xvdf2    /mnt/temp
 

https://stackoverflow.com/questions/55833917/unable-to-mount-a-volume-on-an-ec2-instance/55845557#55845557

https://stackoverflow.com/questions/55833917/unable-to-mount-a-volume-on-an-ec2-instance

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/TroubleshootingInstancesConnecting.html#TroubleshootingInstancesServerError

Linux "Stat" Comand | OCTAL Permission

 stat - -format="%a" /path1/subdir

https://www.cyberciti.biz/faq/get-octal-file-permissions-from-command-line-on-linuxunix/

https://unix.stackexchange.com/questions/188674/how-to-get-file-permission-in-octal

To just see octal file permissions on a GNU/Linux:
$ stat -c '%a' /etc/passwd

linux "Id" Command

https://www.geeksforgeeks.org/id-command-in-linux-with-examples/

Linux: List of all Users and All Groups

https://linuxize.com/post/how-to-list-users-in-linux/

• less /etc/passwd

• getent passwd
• getent passwd | cut -d: -f1

https://linuxize.com/post/how-to-list-groups-in-linux/

• less /etc/group

• getent group
• getent group | cut -d: -f1

Linux : /etc/passwd vs /usr/bin/passwd (passwd Utility) vs /etc/shadow vs /etc/group

https://stackoverflow.com/questions/50904342/etc-passwd-vs-usr-bin-passwd

The two files are different, and serve different purpose.

• /etc/passwd is user database (fun fact: contrary to its name, it doesn't store passwords - those are stored (possibly in hashed form) in /etc/shadow) - see man 5 passwd (i.e. passwd(5)), man 5 shadow (i.e. shadow(5)).

• /usr/bin/passwd is utility that is supposed to modify user records stored in /etc/passwd and /etc/shadow. See man 1 passwd (i.e. passwd(1))

Linux : File Hierarchy Standard : /bin vs /usr/bin vs usr/local/bin

https://en.wikipedia.org/wiki/Filesystem_Hierarchy_Standard

https://unix.stackexchange.com/questions/5915/difference-between-bin-and-usr-bin

https://askubuntu.com/questions/308045/differences-between-bin-sbin-usr-bin-usr-sbin-usr-local-bin-usr-local

/sbin - Binaries needed for booting, low-level system repair, or maintenance (run level 1 or S)

/bin - Binaries needed for normal/standard system functioning at any run level.

/usr/bin - Application/distribution binaries meant to be accessed by locally logged in users

/usr/sbin - Application/distribution binaries that support or configure stuff in /sbin.

/usr/share/bin - Application/distribution binaries or scripts meant to be accesed via the web, i.e. Apache web applications

*local* - Binaries not part of a distribution; locally compiled or manually installed. There's usually never a /local/bin but always a /usr/local/bin and /usr/local/share/bin.

• /bin : For binaries usable before the /usr partition is mounted. This is used for trivial binaries used in the very early boot stage or ones that you need to have available in booting single-user mode. Think of binaries like cat, ls, etc.

• /sbin : Same, but for binaries with superuser (root) privileges required.

• /usr/bin : Same as first, but for general system-wide binaries.

• /usr/sbin : Same as above, but for binaries with superuser (root) privileges required.