This section assumes ffmpeg -i input.mkv and output.mkv or mp4, etc.

-c:v hevc -c:a copy

-acodec aac (most common one)
-acodec libopus (prefer opus at 32kbps for talking only and 96kbps for movies and music)

-c:v hevc -c:a libopus -b:a 32k
 -c:v hevc -c:a aac -b:a 64k

resolution 3840:2160, 1920:1080, 1280:720, 854 or 720:480, 640:360, 426:240 (16:9 aspect ratios) to preserve aspect ratio while downscaling resolution or upscaling resolution use -2 value
-vf scale=-2:640 will downscale 1060x720 to 640p and maintain its aspect ratio

To concat files together make a text file with your videos putting file before them. Put # in front of them if not used. Makes modifying list easy.

# videos.txt file '01.mp4' file '02.mp4' file '03.mp4' file '04.mp4' file '05.mp4' #file '06.mp4' #file '07.mp4'

or use this command to generate the text file

for f in *.mp4; do echo "file '$f'" >> videos.txt; done

concat without re-encoding (make sure all have same aspect ratio, resolution, video and audio codec

ffmpeg -f concat -i videos.txt -c copy output.mp4

To re-encode video and audio

ffmpeg -f concat -i videos.txt -c:v hevc -c:a libopus -b:a 64k output.mp4

To replace original audio track with a new one

ffmpeg -i video.mp4 -i audio.wav -map 0:v -map 1:a -c:v hevc -vf scale="-2:720" -c:a copy -ss 1:17 -to 4:16 output.mp4

sets the audio track to start at 1:17 and end at 4:16.

ffmpeg -i input.mp4 -i newaudio.opus -map 0:v -map 1:a -c:v copy -c:a copy -shortest output.mp4

-shortest option sets video and audio track to the same duration determined by the shortest one.

To add an additional audio track (-map 0 chooses all streams rather than -map 0:v)

ffmpeg -i video.mkv -i audio.mp3 -map 0 -map 1:a -c:v copy -c:a copy -shortest output.mkv

To merge two audio tracks into one

ffmpeg -i video.mkv -i audio.m4a -filter_complex "[0:a][1:a]amerge=inputs=2[a]" -map 0:v -map "[a]" -c:v copy -ac 2 -shortest output.mp4

Add Title and Author metadata to an m4b audiobook.

Title is Linux Mint and Author is Evil Penguin

ffmpeg -i LinuxMint.m4b -c copy -metadata title="Linux Mint" -metadata artist="Evil Penguin" LinuxMint1.m4b

adding a cover to an m4b file isn't so easy if you don't use m4b-tool so best way in linux is to use TagEditor

Max OS Tips

dark mode in Safari with SafariDarkMode.css

Dark mode in Safari make a CSS text file using the CSS code below copy/paste into TextEdit. In TextEdit choose View | Make Plain Text then save it calling it like SafariDarkMode.css and save it to your documents folder.

color (text is silver), background-color and background are night black. Notice that background doesn't have !important tag otherwise it'll block out many elements.  a:link is purple,  a:visited is red a:hover is pink. Change the hex colors as you wish.

* { color: #BCC6CC !important; background-color: #0C090A !important; background: #0C090A; }

a:link { color: #7A5DC7 !important; }

a:visited { color: #CA226B !important; }

a:hover { color: #C48793 !important; }

img { filter: grayscale(40%); }

CMD + , (comma) to quickly bring up advance settings in Safari to toggle dark mode

Reduce files of PDFs by a huge margin while maintaining acceptable image quality

Create your own filters using Color Sync Utility

open PDF - File | Export

select your custom filter

Mac OS
Screen Capturing

cmd + shift + 3 = captures whole screen
cmd + shift + 4 = captures selected region
cmd + shift + 4 + spacebar = captures window with camera icon

Use Preview as an image and PDF viewer. Basic image editing functions are Tools - Crop, Adjust Size to make the dimensions smaller, Export as jpg to reduce file size.

56Kbps = 7KB/s (dial up 1995, DSL 1997) 64Kbps = 8KB/s (ISDN) 128Kbps = 16KB/s (ISDN) 512Kbps = 64KB/s 1024Kbps = 128KB/s 3G (1,000Kbps) = 125KB/s 1.5Mbps = 188KB/s (some DSL & T1) 6Mbps = 800KB/s (cable and DSL) 10Mbps = 1.3MB/s (cable) 25Mbps = 3MB/s (cable) 50Mbps = 6MB/s (cable) 100Mbps = 12MB/s (fiber, 10/100Mbps ethernet) 1000Mbps = 120MB/s (if all switches, routers are gigabit ethernet)

USB 2.0 = 57MB/s WiFi 802.11b = 1.4MB/s WiFi 802.11g = 6.8MB/s WiFi 802.11n = 31MB/s

Regex (regular expressions)

([\d]+) remove all digits (numbers) 3453 peace 897 -----> peace

([^\d]+) remove everything except digits 5343 life -----> 5343