Linux tips

Things I forget and search everytime.

Disk

File system

$ df -h
Filesystem        Size  Used Avail Use% Mounted on
devtmpfs          1.9G     0  1.9G   0% /dev
tmpfs             1.9G     0  1.9G   0% /dev/shm
tmpfs             1.9G  432K  1.9G   1% /run
tmpfs             1.9G     0  1.9G   0% /sys/fs/cgroup
/dev/nvme0n1p1    8.0G  5.3G  2.8G  66% /
/dev/nvme0n1p128   10M  3.8M  6.3M  38% /boot/efi
tmpfs             386M     0  386M   0% /run/user/1000

Directories

$ du -BM -d1
2M      ./.git
169M    ./packages
408M    ./node_modules
579M    .

Swap

$ sudo mkdir /swaps
$ sudo dd if=/dev/zero of=/swaps/1 bs=1M count=2048
$ sudo mkswap /swaps/1
$ sudo chmod 0600 /swaps/1
$ sudo swapon /swaps/1
$ cat /proc/swaps
Filename                                Type            Size            Used            Priority
/swaps/1                                file            2097148         0               -2

For persistance, add one line setting:

$ sudo vi /etc/fstab

/swaps/1          swap                    swap    defaults        0 0

Errors

$ sudo swapon /swaps/1
swapon: /swaps/1: swapon failed: Invalid argument

Check your file system. The "overlay" file system (most likely Docker) does not support swap.

$ df -Th /swaps
Filesystem     Type     Size  Used Avail Use% Mounted on
overlay        overlay   32G   16G   15G  52% /

Prepare for make

make: g++: Command not found

Run either:

$ sudo apt install build-essential

$ sudo yum groupinstall -y 'Development Tools'

VS Code to watch files

VS Code might show some error popup.

• https://code.visualstudio.com/docs/setup/linux#_visual-studio-code-is-unable-to-watch-for-file-changes-in-this-large-workspace-error-enospc

Or you might see error in console:

Warning: ENOSPC: System limit for number of file watchers reached, watch '/path/to/files/'

$ cat /proc/sys/fs/inotify/max_user_watches
$ sudo vi /etc/sysctl.conf

fs.inotify.max_user_watches=524288

$ sudo sysctl -p

Monitoring

$ sudo yum install htop -y
$ htop

Hit q to quit.

HTTPS

• Tutorial: Configure SSL/TLS on Amazon Linux 2 - Amazon Elastic Compute Cloud

1. Open HTTP (80) port
2. Install certbot
3. Obtain the keys

$ cd ~
$ sudo wget -r --no-parent -A 'epel-release-*.rpm' https://dl.fedoraproject.org/pub/epel/7/x86_64/Packages/e/
$ sudo rpm -Uvh dl.fedoraproject.org/pub/epel/7/x86_64/Packages/e/epel-release-*.rpm
$ sudo yum-config-manager --enable epel*
$ sudo yum repolist all
$ sudo yum install -y certbot
$ sudo certbot certonly --register-unsafely-without-email -- standalone -d xxx.example.com

certbot option --register-unsafely-without-email is for development, not for production.

Files:

$ sudo ls -l /etc/letsencrypt/live/xxx.example.com/
total 4
lrwxrwxrwx 1 root root  38 Dec  4 21:29 cert.pem -> ../../archive/xxx.example.com/cert1.pem
lrwxrwxrwx 1 root root  39 Dec  4 21:29 chain.pem -> ../../archive/xxx.example.com/chain1.pem
lrwxrwxrwx 1 root root  43 Dec  4 21:29 fullchain.pem -> ../../archive/xxx.example.com/fullchain1.pem
lrwxrwxrwx 1 root root  41 Dec  4 21:29 privkey.pem -> ../../archive/xxx.example.com/privkey1.pem
-rw-r--r-- 1 root root 692 Dec  4 21:29 README

Example:

const express = require('express')
const https = require('https')
const fs = require('fs')

const PORT = 8080;

const app = express();
app.get('/', (req, res) => res.send('OK'));

// you have to copy these 2 files and set permission
const httpsOptions = {
  cert: fs.readFileSync('keys/cert.pem'),
  key: fs.readFileSync('keys/privkey.pem'),
};
const server = https.createServer(httpsOptions, app);
server.listen(PORT, () => console.log(`Port: ${PORT}`));