View on GitHub

General Notes

Compilations of General Notes on topics like git, aws, linux, ubuntu, scp, vscode, windows, github.

Linux/Ubuntu

Visitors

Commands & Notes

Login Command for EC2 machine using SSH

ssh -i "your-ec2-pem-file.pem" ubuntu@ip.address

View System Logs

Command: cat /var/log/syslog You can also use tail and grep for filter search

System Services Location & Commands

Command: cd /etc/systemd/system/ will open the folder containing all system services

Command: sudo systemctl start application.service as the command parameter suggests will start the service

Command: sudo systemctl stop application.service as the command parameter suggests will stop the service

Command: sudo systemctl remove application.service as the command parameter suggests will remove the service

Command: sudo systemctl restart application.service as the command parameter suggests will restart the service

Command: sudo systemctl reload application.service will reload the configuration of the service

Command: sudo systemctl status application.service will show the status of the service

Command: sudo systemctl enable application.service will enable the service to autostart on boot

Command: sudo systemctl disable application.service will disable the service to not to start on boot

Command: sudo systemctl is-active application.service will check if the service is active

Command: sudo systemctl is-enabled application.service will check if the service is enabled to start on system boot

Docker Commands

Command: docker ps will list all running containers. You can use docker ps -a or docker ps --all to list all containers

netstat Command

Command Description
sudo netstat -lntu Display all services listening for TCP and UDP, all free open ports on the local machine
sudo netstat -nlp Use this command to check which port is occupied by which process pid
Parameter Description
-a Displays all active connections and the TCP and UDP ports on which the computer is listening
-n Displays active TCP connections, however, addresses and port numbers are expressed numerically and no attempt is made to determine names
-p protocol Shows connections for the protocol specified by protocol. In this case, protocol can be tcp, udp, tcpv6, or udpv6. If this parameter is used with -s to display statistics by protocol, protocol can be tcp, udp, icmp, ip, tcpv6, udpv6, icmpv6, or ipv6
-p Show which processes are using which sockets (similar to -b under Windows) (you must be root to do this)
-t Display only TCP connections
-u Display only UDP connections

ss (socket statistics) Command

The ss (socket statistics) tool is a CLI command used to show network statistics.

The ss command is a simpler and faster version of the now obsolete netstat command

Command Description
sudo ss List all connections
sudo ss -t To view only tcp or udp or unix connections use the t, u or x option
sudo ss -ua List all udp connections
sudo ss -lptn Use this command to check which port is occupied by which process pid
sudo ss -ltn Show only listening sockets

lsof (list open files) Command

lsof is a command meaning “list open files”, which is used in many Unix-like systems to report a list of all open files and the processes that opened them.

Command Description
sudo lsof -n -i :{port} Use this command to check pid for given port

.bashrc

.bashrc is a Bash shell script that Bash runs whenever it is started interactively. It initializes an interactive shell session. You can put any command in that file that you could type at the command prompt.

You put commands here to set up the shell for use in your particular environment, or to customize things to your preferences. A common thing to put in .bashrc are aliases that you want to always be available.

Adding Alias To .bashrc

Just to be safe I always take a backup of the file before editing it. So I advice to make a copy of .bashrc before proceeding ahead. To view the file use below command

cat $HOME/.bashrc

Now lets create a alias for command that takes us to our project folder, which for me is cd /usr/local/test so lets edit the .bashrc file using below command

vi $HOME/.bashrc

When the editor opens, press i to enable editing mode, and go to the last line of the code. Now the syntax for creating an alias is as follows: alias shortcommandname="long custom command that you want to use" So here since we are creating a alias for cd /usr/local/test so our alias code will look like given below:

# Custom Alias
alias letswork="cd /usr/local/test"

So basically cd /usr/local/test has an alias letswork. Now save the file and exit .bashrc using command :wq!. Now we have our alias created, but to make is active/usable you will have to either restart your ec2 terminal or simply use source command. Sample code to load the .bashrc is given below:

source ~/.bashrc

Now you can hit your alias and check if it executes the same command as you specified. Which in my case will take me to my test folder when i press letswork command on console.

Environment Variable on EC2

/etc/environment vs ~/.profile

/etc/environment ~/.profile
/etc/environment is a system-wide configuration file, which means it is used by all users. It is owned by root though, so you need to be an admin user and use sudo to modify it. ~/.profile is one of your own user’s personal shell initialization scripts. Every user has one and can edit their file without affecting others.
This file is specifically meant for system-wide environment variable settings. It is not a script file, but rather consists of assignment expressions, one per line.
FOO=bar
Note: Variable expansion does not work in /etc/environment.		 Files with the .sh extension in the /etc/profile.d directory get executed whenever a bash login shell is entered (e.g. when logging in from the console or over ssh), as well as by the DisplayManager when the desktop session loads.
Editing /etc/environment is not recommended /etc/profile and /etc/profile.d/*.sh are the global initialization scripts that are equivalent to ~/.profile for each user. The global scripts get executed before the user-specific scripts though; and the main /etc/profile executes all the *.sh scripts in /etc/profile.d/ just before it exits.

You can for instance create the file /etc/profile.d/myenvvars.sh and set variables like this:

export JAVA_HOME=/usr/lib/jvm/jdk1.7.0
export PATH=$PATH:$JAVA_HOME/bin
# With White Spaces
VARWITHWHITESPACE="`id -un`"
export VARWITHWHITESPACE

If you want to add variable into /etc/environment use below format. Always take backup before you do any changes

cat /etc/environment
vi /etc/environment

SAMPLEVAR="value"

Environment Variable In systemd Service

cd /etc/systemd/system Open system services folder using the given command

Now before proceeding, You will have to stop the service using command systemctl stop myservice and then take a backup of your service file before editing. Now to view the service file use this command: cat myservice.service. The given file is editable but its not recommended to edit it directly. Instead the better way is to use command: systemctl edit myservice which will create an overide file which we can edit. /etc/systemd/system/myservice.service.d inside this there is a file with extension .conf i.e override.conf. Here you can add your environment variable.

To add environment variable use below similar syntax and just replace customvar=value and you are done. Now start the service

[Service]
Environment="customvar=value"

Note: If the directory (myservice.service.d) exists and is empty then your service will be disabled, so if you don’t intend to put something in the directory then delete it.

kill Command

The syntax of the kill command takes the following form: kill [OPTIONS] [PID]... The kill command sends a signal to specified processes or process groups, causing them to act according to the signal.

Commands Description
kill [PID] Kill a process by Process ID
killall [PNAME] Kill a process by Process name
kill -l Get a list of all the signals that can be sent to the kill command
Signal Name Single Value Effect
SIGHUP 1 Hangup
SIGINT 2 Interrupt from keyboard
SIGKILL 9 Kill signal
SIGTERM 15 Termination signal
SIGSTOP 17, 19, 23 Stop the process 
kill -1 PID
kill -SIGHUP PID

To terminate or kill a process with the kill command, first you need to find the process ID number (PID). You can do this using different commands such as top, ps , pidof and pgrep

Issue: What is a defunct process and why doesn’t it get killed?

defunct means the process has either completed its task or has been corrupted or killed, but its child processes are still running or these parent process is monitoring its child process. To kill this kind of process, kill -9 PID doesn’t work. You can try to kill them with this command but it will show this again and again.

Solution: sudo kill -9 pid ppid where pid is process id and ppid is parent process id

Use ps -ef | grep defunct to get ppid

ps Command

Use sudo as required

Command Description
ps -efj Command to view the process group IDs (GIDs)
ps -ef | grep defunct Command to view the defunct process

Reference

Visitors