linux

os version

cat /etc/os-release

disk usage

for a direcory

$ du -hs /path/to/dir

-h human readable -s summary instead of listing all subdirectories

list the 100 biggest directories

$ du -x -h / | sort -n | tail -100

-x skip directories on different file systems

apt

• show cache locations: $ apt-config dump | grep "^Dir\( \|::Ca\)"
• show cache size: sudo du -sh /var/cache/apt
• clean only obsolete: apt-get autoclean
• see what will be removed: apt-get clean --dry-run
• remove: apt-get clean

disable password logins

• from here
• and here

edit the ssh config

$ vi /etc/ssh/sshd_config

• find PasswordAuthentication line and set to 'no'
• find ChallengeResponseAuthentication line and set to 'no'
• find UsePAM line and set to 'no'

restart the ssh service

$ service sshd restart

scp

download without password

$ scp -i "/path/to/key.pem" user@host:/path/to/file ./save/here

upload without password

$ scp -i "/path/to/key.pem" /source/file user@host:/save/here

flush dns

• osx: sudo dscacheutil -flushcache;sudo killall -HUP mDNSResponder
• linux ``

ssh

ssh copy

• ssh-copy-id

server config

vi /etc/ssh/sshd_config

add line for allowed users: AllowUsers adam

edit line to disallow root logins: PermitRootLogin no

edit line to change port: Port 222 (22 is the default)

client config

• using-the-ssh-config-file

$ touch ~/.ssh/config
$ chmod 600 ~/.ssh/config

Host <nickname>
    Hostname <ip or hostname>
    User <user>
    Port <default 22>
    IdentityFile /Users/adam/.ssh/linode_rsa

connect with

$ ssh nickname

users

list users

$ cat /etc/passwd

• create users with useradd
• create users with adduser

add a group to an existing user

$ sudo usermod -aG groupName userName

monitor logins

$ tail -fn 200 /var/log/auth.log
$ tail -fn 500 /var/log/nginx/access.log

services

show running services

$ systemctl | grep running

show the status of one of those services

$ systemctl status httpd

ssh ngrok tunnel

i haven't tried this but it looks interesting If you have a web server somewhere, you can roll your own ngrok using SSH port forwards and an nginx reverse proxy on the server.

Bash shell alias for your localhost:

tunup () {
    port=${1:-5000}
    echo "Forwarding web:5000 to local port $port"
    ssh -R 5000:127.0.0.1:$port name@webserver.tld
}

And an nginx config for e.g. tunnel.webserver.tld that does a proxy_pass to the web server's localhost:5000:

server {
    server_name tunnel.webserver.tld;
    location / {
        proxy_pass http://localhost:5000
    }
}

Then whenever you want to open the ngrok tunnel run a command like "tunup" (port 5000) or "tunup 8080" if a different local port, and access your tunnel.webserver.tld domain. When the tunnel is NOT active, your domain will return an nginx 503 Bad Gateway error.