Linux Commands Useful

Login using SSH

ssh user@127.127.127.127
ssh -i /path/private.key user@127.127.127.127

Remove host from ssh known hosts (when server changes, but IP stays the same)

xxxxxxxxxx
ssh-keygen -R "hostname"

Process management

htop - Activity monitor

htop for network net monitor firewall linux

sudo apt-get install jnettop

sudo jnettop

See all open ports and which programs are using

xxxxxxxxxx
sudo lsof -i -P -n

sudo lsof -i -P -n | grep LISTEN

Checking what ports are listnening

xxxxxxxxxx
sudo apt install net-tools
netstat -nlt

Test specific port

xxxxxxxxxx
telnet ADDRESS PORT

Debug incoming requests HTTP netcat

xxxxxxxxxx
nc -l PORT

Speedtest

xxxxxxxxxx
sudo apt install speedtest-cli

speedtest-cli

crontab - automation

Edit tasks

xxxxxxxxxx
crontab -e 

Format

xxxxxxxxxx
@reboot /media/hdd/automation_cron/startup_automations_evalds_1.sh
*/5 * * * * /media/hdd/automation_cron/run_automations_evalds_1.sh

Global crontab (with user names admin )

xxxxxxxxxx
sudo vim /etc/crontab

Restart

xxxxxxxxxx
sudo service cron restart

Logs

xxxxxxxxxx
sudo grep CRON /var/log/syslog 

Chmod

xxxxxxxxxx
0: No permission
1: Execute
2: Write
3: Write and execute
4: Read
5: Read and execute
6: Read and write
7: Read, write, and execute

xxxxxxxxxx
The owner can read, write, and execute the file.
The group can read, write, and execute the file.
Anyone else can read, write, and execute the file.

xxxxxxxxxx
sudo chmod 660 /shome/file

Make folder write/read for all users

xxxxxxxxxx
sudo chmod -R a+wr /some/folder

Make SH file executable

xxxxxxxxxx
sudo chmod +x /some/folder

List files in directory

xxxxxxxxxx
ls -ltr /path/dir
ls -ltr /path/dir | grep "filter-text"

https://www.tecmint.com/sort-ls-output-by-last-modified-date-and-time/

List current directory

xxxxxxxxxx
pwd

Find file

xxxxxxxxxx
find /dir -name "*Something*

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

Find text in files

xxxxxxxxxx
grep -i -A 5 -B 5 "string" FILE_PATTERN

https://www.thegeekstuff.com/2009/03/15-practical-unix-grep-command-examples/

Kill process

xxxxxxxxxx
kill -9 PROCES_ID
kill -9 $(ps aux | grep 'SOME TEXT IN PROCESS' | awk '{print $2}')

Zip a folder

xxxxxxxxxx
zip -r <output_file> <folder_1>

Find location of some app in global path

xxxxxxxxxx
whereis app_name

Symlinks

Create

xxxxxxxxxx
ln -s /path/source/folder_or_file /path/destination

Will result in /path/destination/folder_or_file

Find source

xxxxxxxxxx
readlink -f /path/to/syslink
readlink -f which command

Linux Screen

create with name screen -S newname

List screens: screen -ls

Attach to existing screen -rd newname

Detach from screen ctrl+a => d

Kill screen ctrl+a => k

Rename screen

Scroll mode

Screen shortcuts

Send commands to screen:

xxxxxxxxxx
screen -dmS server
screen -r server -X stuff "/media/hdd/Document/server.sh\n"

Alternative to Linux screens - tmux https://superuser.com/questions/236158/tmux-vs-screen