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

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

open shell in a running container

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

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

default:

ps2pdf input.pdf output.pdf

ghostscript options -dPDFSETTINGS

ps2pdf -dPDFSETTINGS=/ebook input.pdf output.pdf

possible options:

1. dPDFSETTINGS=/screen (72 dpi)
2. dPDFSETTINGS=/ebook (150 dpi)
3. dPDFSETTINGS=/prepress (300 dpi)
4. dPDFSETTINGS=/printer (300 dpi)
5. dPDFSETTINGS=/default Selects the output which is useful for multiple purposes. Can cause large PDFS.

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