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Ubuntu commands cheatsheet

Purpose Command More info
List all users cut -d: -f1 /etc/passwd
List all groups cat /etc/group
Check groups of user groups <username>
Create group addgroup [-r or --system] [-g or --gid GID] <groupname> GID is Group ID, an unique positive number. -r or --system create system group
Create user adduser <username>
Create a new user or update default new user information useradd <username>
Add user into group adduser <username or uid> <groupname or gid> or usermod -a -G <groupname> <username>
Delete user from group deluser <username> <groupname>
Add user into sudo group adduser <username> sudo or usermod -aG sudo <username>
Change user's password passwd <username>
Change user's login name usermod -l <current_username> <new_username>
Compress with tar tar -czvf [--exclude=regex-path] <filename> <directory-path> -c: create, -z: using gzip, -v: verbose, -f: specify filename, --exclude: exclude path. Should use extension .tar.gz or .tgz
Uncompress with tar tar -xzvf <filename> [-C <directory-path>] -x: extract
Check port netstat [options] grep <port> -n: no foreign IP check, -p: show protocol, -t: tcp, -u: udp, -l: list
View crontab cat /etc/crontab System crontab
Make symbolic link ln -s <src> <dst> Windows: mklink /D /H /J <src> <dst>
Check IP tables cat /etc/sysconfig/iptables OR iptables -L
Restore IP table sudo iptables-restore < /etc/iptables/rules.v4
Save IP table sudo iptables-save > /etc/iptables/rules.v4
Generate private key and cerificate RSA, PEM format openssl req -newkey rsa:2048 -x509 -days 3650 -keyout ved_pri.pem -out cert.cer -new -nodes
Generate private key and request cerificate RSA, PEM format openssl req -newkey rsa:2048 -x509 -days 3650 -keyout ved_pri.pem -out cert.cer
Check package available versions apt-cache policy <package_name>
SSH key print openSSH public key to PEM format ssh-keygen -f path_to_pub_file -e -m pem
Change hostname Edit file /etc/hostname and Edit 127.0.0.1 <hostname> in /etc/hosts Can do either reload network or reboot machine
Check version of SSL/TLS of a server `openssl ciphers -v awk '{print $2}'

Make process still runs even close terminal

nohup is defined by POSIX while disown is not. This means that while many shells (e.g. bash, zsh, ksh) have it, others (for example tcsh, csh, dash and sh) won't have it.

nohup will already disown the process.

disown can be used after a command has been launched while nohup must be used before.

Disown then background

  1. Press Ctrl + Z, take a look at the number of the process
  2. disown -h %<number>
  3. bg 1

For example

$ sudo tailf /var/log/my_log1.log
# Ctrl + Z
[1]+  Stopped                 sudo tailf /var/log/my_log1.log
$ disown -h %1
$ bg 1
$ sudo tailf /var/log/my_log2.log
# Ctrl + Z
[2]+  Stopped                 sudo tailf /var/log/my_log2.log
$ disown -h %2
$ bg 2

Nohup

nohup <command> &

Optimized server

Open files limit

Most OS can change the open-files limit. Example:

$ ulimit -n 65536

To permanently set the soft and hard values for all users of the system to allow for up to 65536 open files. Edit /etc/security/limits.conf and append the following two lines:

*       soft    nofile  65535
*       hard    nofile  65535

Save file. Start a new session so that the limits take effect (Logout -> Login).

More ports for testing

sudo sysctl -w net.ipv4.ip_local_port_range="1025 65535"

Increase the maximum number of possible open file descriptors:

echo 300000 | sudo tee /proc/sys/fs/nr_open
echo 300000 | sudo tee /proc/sys/fs/file-max

Kernel and Network Tuning

Edit /etc/sysctl.conf

net.ipv4.tcp_max_syn_backlog = 40000
net.core.somaxconn = 40000
net.core.wmem_default = 8388608
net.core.rmem_default = 8388608
net.ipv4.tcp_sack = 1
net.ipv4.tcp_window_scaling = 1
net.ipv4.tcp_fin_timeout = 15
net.ipv4.tcp_keepalive_intvl = 30
net.ipv4.tcp_tw_reuse = 1
net.ipv4.tcp_moderate_rcvbuf = 1
net.core.rmem_max = 134217728
net.core.wmem_max = 134217728
net.ipv4.tcp_mem  = 134217728 134217728 134217728
net.ipv4.tcp_rmem = 4096 277750 134217728
net.ipv4.tcp_wmem = 4096 277750 134217728
net.core.netdev_max_backlog = 300000

Add user to sudo

adduser <username> sudo

or

usermod -aG sudo <username>

or

visudo

# Add this line
<username> ALL=(ALL) [NOPASSWD:]ALL

Changing shell

chsh -s /bin/bash <username>

If not success, use the following command

usermod -s /bin/bash <username>

Timezones

Ubuntu 16

View current timezone timedatectl

View list timezones timedatectl list-timezones

Set timezone timedatectl set-timezone Asia/Ho_Chi_Minh