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====== Linux Cheat Sheet ======

===== snap =====

Clean unused/disabled snap packages
  snap list --all | while read snapname ver rev trk pub notes; do if [[ $notes = *disabled* ]]; then snap remove "$snapname" --revision="$rev"; fi; done


===== iptables/netfilter =====

Restrict to single host, e.g. for prometheus node exporter
  iptables -N prometheus  
  iptables -A prometheus --src <server-ip> -j ACCEPT
  iptables -A prometheus -j DROP
  iptables -I INPUT -m tcp -p tcp --dport 9100 -j prometheus
  
===== docker =====

open shell in a running container

  docker exec -ti <container-name> /bin/bash


===== email =====

==== sender guidelines ====

  - https://support.google.com/a/answer/14229414
  - https://senders.yahooinc.com/best-practices/
  - https://senders.yahooinc.com/faqs/
  - https://www.bsi.bund.de/dok/TR-03182 


===== Video =====

compress video for email sending

  ffmpeg -i input.mp4 output_compressed.mp4

adjust volume 
  ffmpeg -i input.avi -af "volumedetect" -vn -sn -dn -f null /dev/null
  
gives:
  [Parsed_volumedetect_0 @ 0x55f12ff28780] n_samples: 1839104
  [Parsed_volumedetect_0 @ 0x55f12ff28780] mean_volume: -43.4 dB
  [Parsed_volumedetect_0 @ 0x55f12ff28780] max_volume: -31.5 dB
  [Parsed_volumedetect_0 @ 0x55f12ff28780] histogram_31db: 64
  [Parsed_volumedetect_0 @ 0x55f12ff28780] histogram_32db: 455
  [Parsed_volumedetect_0 @ 0x55f12ff28780] histogram_33db: 1884


increase max volume:
  ffmpeg -i input.avi -af "volume=30dB" -c:v copy -c:a libmp3lame -q:a 2 output.mp4
  
  
flip image
  ffmpeg -i input.mp4 -vf "transpose=2" output.mp4
  
transpose options:
  0 = 90° counter-clockwise and vertical flip (default)
  1 = 90° clockwise
  2 = 90° counter-clockwise
  3 = 90° clockwise and vertical flip