# Miscellaneous

## Check version

```console
$ cat /etc/debian_version
bookworm/sid
$ cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=22.04
DISTRIB_CODENAME=jammy
DISTRIB_DESCRIPTION="Ubuntu 22.04.5 LTS"
```

## User management

Add a new user:

```console
$ sudo adduser xxxx
```

Add the new user to the `sudo` group:

```console
$ sudo usermod -aG sudo xxxx
```

Add a new user with other specified attributes:

```console
$ sudo adduser --disabled-password --disabled-login --no-create-home --gecos "XXXX" --uid 1001 xxxx
```

Delete a user:

```console
$ sudo deluser xxxx
Removing user `xxxx' ...
Warning: group `xxxx' has no more members.
Done.
$ sudo rm -rf /home/xxxx
```

## hostname

Show hostname:

```console
$ hostname
```

Temporarily set hostname:

```console
$ sudo hostname xxxx
```

Edit `/etc/hostname` to set it permanently (effective after reboot), and make it effective immediately after editing the file:

```console
$ sudo hostname -F /etc/hostname
```

You may need to edit `/etc/hosts` to map the hostname accordingly.

## systemctl editor

The editor to do `systemctl edit` can be changed to `vi` by:

```sh
export SYSTEMD_EDITOR=vi
```

Often we run the command with `sudo`, but the envs are cleared for security reasons. So we need to config `sudo` to keep the necessary envs:

```console
$ sudo visudo
```

:::{literalinclude} /_files/ubuntu/etc/sudoers
:diff: /_files/ubuntu/etc/sudoers.orig
:::

:::{note}
Make sure the owner of `/etc/sudoers` is `root:root` and its mode is `0440`, or `sudo` will refuse to run. Also, the rule is for `sudo` group, so the current user must be in this group.
:::

## Clear iptables

Delete all rules in all chains:

```console
$ sudo iptables -F
```

Delete all chains (except the default ones):

```console
$ sudo iptables -X
```

List rules:

```console
$ sudo iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination 
```

## List port listening

```console
$ sudo lsof -i :3306
COMMAND     PID  USER   FD   TYPE  DEVICE SIZE/OFF NODE NAME
mariadbd 200092 mysql   20u  IPv4 1866849      0t0  TCP localhost:mysql (LISTEN)
```