Linux

• [[Linux day to day notes]].
• [[My usage of GNU/Linux over the years]]

Linux

The bog standard free software operating system. Runs everywhere. Good. Fast. Monolithic C kernel.

Understanding Linux performance Best Linux tips and tricks

Table of Contents

• [[related]] [[infra]]
• [[docker]] [[docker]]
  • [2018-06-15] docker system prune to cleanup
  • [2021-01-12] figuring out what takes disk space docker images | awk '{print $3}' | xargs docker inspect | less [[docker]]
  • [2021-02-12] Docker compose port mapping - Stack Overflow
  • [2021-04-14] apt-get clean and other strategies for keeping image size small? · Issue 35 · rocker-org/rocker [[docker]]
  • [2021-05-02] Attach and Detach from Docker Container | HowToProgram [[docker]] [[drill]]
• [[systemd]] [[systemd]]
  • [debugging: systemctl --failed, journalctl](#dbggngsystmctlfldjrnlctl TIDDLYLINK)
  • [profiling: systemd-analyze blame](#prflngsystmdnlyzblm TIDDLYLINK)
• [[networking]] [[network]]
  • [[who uses local port]]
  • [[testing if port is open]]
  • [2020-06-15] linux - Test if a port on a remote system is reachable (without telnet) - Super User
• [[apt]]
  • [install build dependencies: sudo apt-get build-dep package](#nstllblddpndncssdptgtblddppckg TIDDLYLINK)
• [[cron]] [[cron]]
  • [2018-02-26] if cron fails to start a job, it will send the output to local mail
  • [2018-05-10] mail subject – use bash comment operator
• [[sandboxing]] [[sandbox]]
  • [docker is excellent for quick sandboxes (just use ubuntu:latest image or something)](#dckrsxcllntfrqcksndbxsjstsbntltstmgrsmthng TIDDLYLINK) [[docker]]
  • [firejail for network sandbox](#frjlfrntwrksndbx TIDDLYLINK) [[network]] [[testing]]
• [[udev]] [[udev]]
  • [udevadm info /dev/sdc2](#dvdmnfdvsdc TIDDLYLINK)
  • [2019-03-07] when you update rules file sudo udevadm control --reload-rules
  • [2019-03-07] to trigger the target script sudo udevadm trigger
• [2017-05-08] Gateway 0.0.0.0 means that the subnet is link-local i.e. there is no gateway involved, the packets are delivered directly to the target [[network]]
• [[-----]]
• [2018-06-08] Linux ignores the setuid bit on all interpreted executables (i.e. executables starting with a #! line).
• [2020-01-29] linux - How to make htop sort by PID
• [2020-01-24] logs - View stdout/stderr of systemd service [[systemd]]
• [2020-01-29] How to read memory usage in htop? - Server Fault [[drill]]
• [2020-12-04] linux - prevent system freeze/unresponsiveness due to swapping run away memory usage - Super User
• [2018-04-27] guide to XKB
• [[-]]
• [whereis command](#whrscmmnd TIDDLYLINK) [[habit]]

related [[infra]]

docker [[docker]]

[2018-06-15] docker system prune to cleanup

[2021-01-12] figuring out what takes disk space docker images | awk '{print $3}' | xargs docker inspect | less [[docker]]

[2021-02-12] Docker compose port mapping - Stack Overflow

If you want to bind to the redis port from your nodejs container you will have to expose that port in the redis container:

version: '2'
services:
  nodejs:
    build:
      context: .
      dockerfile: DockerFile
    ports:
      - "4000:4000"
    links:
      - redis
  redis:
    build:
      context: .
      dockerfile: Dockerfile-redis
    expose:
      - "6379"
The expose tag will let you expose ports without publishing them to the host machine, but they will be exposed to the containers networks.

[2021-04-14] apt-get clean and other strategies for keeping image size small? · Issue 35 · rocker-org/rocker [[docker]]

ok, so for debian/ubuntu images don't really need autoclean – it's already run automatically

[2021-05-02] Attach and Detach from Docker Container | HowToProgram [[docker]] [[drill]]

In general, to detach from a Docker container and leave it running, we can use the CTRL-p CTRL-q key sequence.

systemd [[systemd]]

debugging: systemctl --failed, journalctl

logs – for individual jobs you can see them via status (or sdj)
presistent is not showing anythin by default?

profiling: systemd-analyze blame

networking [[network]]

who uses local port

sudo ss -tulpn | grep 5060
sudo ss -anpt  | grep 8640

testing if port is open

netcat -l 5559

one the other machine

echo "TEST" | netcat host 5559

[2020-06-15] linux - Test if a port on a remote system is reachable (without telnet) - Super User

Nice and verbose! From the man pages.
Single port:
nc -zv 127.0.0.1 80

testing if port is open

apt

install build dependencies: sudo apt-get build-dep package

cron [[cron]]

[2018-02-26] if cron fails to start a job, it will send the output to local mail

grep cron /var/log/syslog
tail -f /var/log/syslog | grep cron

[2018-05-10] mail subject – use bash comment operator

*:* TestJob; cron command

mind the space after colon!

sandboxing [[sandbox]]

docker is excellent for quick sandboxes (just use ubuntu:latest image or something) [[docker]]

firejail for network sandbox [[network]] [[testing]]

• doesn't require root!
• firejail --noprofile --net=none

udev [[udev]]

udevadm info /dev/sdc2

[2019-03-07] when you update rules file sudo udevadm control --reload-rules

[2019-03-07] to trigger the target script sudo udevadm trigger

[2017-05-08] Gateway 0.0.0.0 means that the subnet is link-local i.e. there is no gateway involved, the packets are delivered directly to the target [[network]]

-----

[2018-06-08] Linux ignores the setuid bit on all interpreted executables (i.e. executables starting with a #! line).

ok apparently setuid shell scripts are quite risky…

[2020-01-29] linux - How to make htop sort by PID

press > and select RES

[2020-01-24] logs - View stdout/stderr of systemd service [[systemd]]

Note that using the standard logging mechanism like this will not create persistent logs by default. To do that, you'll need to create /var/log/journal, and then run sudo systemctl restart systemd-journald

[2020-01-29] How to read memory usage in htop? - Server Fault [[drill]]

https://serverfault.com/questions/517483/how-to-read-memory-usage-in-htop

Hide user threads (shift + H) and close the process tree view (F5), t

[2020-12-04] linux - prevent system freeze/unresponsiveness due to swapping run away memory usage - Super User

If a process demands a lot of memory, the system moves all other process to the swap file. Including it seems, necessary processes like the X11 server or the terminal.

[2018-04-27] guide to XKB

-

whereis command [[habit]]