nutshell/4.linux.Rmd at master · dongzhuoer/nutshell · GitHub

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# (PART) OS {-}


# Master Linux


## miscellaneous

- when you always see a system alert, `sudo rm /var/crash/*`
- If you find something wrong abount SSH, ensure that the user's login shell is `/bin/bash`.
- `unar` (`lsar`) 可自动解决 `.zip` 乱码问题
- [understanding `~/.bashrc`](https://dongzhuoer.com/post/caprice/2018/bashrc/)


## inspect and monitor

- list package files

```bash
dpkg -c *.deb
dpkg -L package       # installed
apt-file list package # apt repository
```

- system version

```bash
uname -r                       # kernel version
cat /etc/debian_version        # Debian version
cat /etc/lsb-release           # especially useful for Docker image
```

- cpu

```bash
grep 'physical id' /proc/cpuinfo | sort -u          # processors
grep 'core id'     /proc/cpuinfo | sort -u | wc -l  # cores per processors
grep 'processor'   /proc/cpuinfo | sort -u | wc -l  # threads
```

- port and traffic

```bash
sudo lsof -i -P -n | grep LISTEN    # IPv4/6, ssh -> 22, localhost -> 127.0.0.1
netstat -tulpn      # tcp, udp, listen, show process, ssh -> 22
sudo iftop -P
sudo tcptrack -i wlp2s0  # port 1080
```

- file system

```bash
du . -d 1 -h
df -h
sudo blkid
lsblk # -f
```

- process

```bash
ps -aux  # 观察系统所有的程序数据
ps -axjf # 连同部分程序树状态
pstree
```


## account

- add account

Don't use `-p password`, it's wrong and **unsafe**. Because it saves the password directly instead of hashed code into `/etc/shadow` ^[You can use `sudo grep username /etc/shadow` to verify, note that `password` is shown unencrypted.].

```bash
sudo useradd -m -s /bin/bash username
echo 'username:password' | sudo chpasswd
```

- group

Add user to `sudo` group (`wheel` for CentOS) can assign root privilege.

```bash
sudo usermod -g group username                       # change initial group
sudo usermod [-a] -G group1, group2 username      # append or change
sudo gpasswd --delete username group
```

- others
  + In `man` page, `LOGIN` (`-l` option) means username
  + `id`, `who`


## systemd

- [阮一峰中文教程](http://www.ruanyifeng.com/blog/2016/03/systemd-tutorial-commands.html)
- [Red Hat documentation](https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/System_Administrators_Guide/chap-Managing_Services_with_systemd.html)
- `man systemd.service`
- [override some settings in `.service` without modifying original file](https://dongzhuoer.com/post/caprice/2018/systemd-crash-teach-me-overwriting-service)