ffmpeg

Conventions used below:

in.mp4 = input file
out.mp4 = output file
Times use HH:MM:SS or seconds (e.g. 5 or 00:00:05)

1. Trimming / Cutting

Task	Command
Remove first N seconds (re-encode)	`ffmpeg -ss 5 -i in.mp4 -c:v libx264 -c:a aac out.mp4`
Remove first N seconds (no re-encode, fast, keyframe-snapped)	`ffmpeg -ss 5 -i in.mp4 -c copy out.mp4`
Cut a middle segment (start to end)	`ffmpeg -ss 00:01:00 -to 00:02:30 -i in.mp4 -c copy out.mp4`
Cut last N seconds off the end	`ffmpeg -sseof -10 -i in.mp4 -c copy out.mp4`
Cut by duration instead of end time	`ffmpeg -ss 10 -i in.mp4 -t 20 -c copy out.mp4` (start at 10s, keep 20s)

Note: -ss before -i = fast seek (jumps to nearest keyframe, may be a few frames off). -ss after -i = accurate but slower (decodes from start). -c copy = no re-encoding (instant, lossless, but cut points snap to keyframes). Drop -c copy and add -c:v libx264 -c:a aac for frame-accurate cuts at the cost of re-encoding time.

2. Format / Container Conversion

Task	Command
Convert container only (no re-encode)	`ffmpeg -i in.mkv -c copy out.mp4`
Convert with re-encode (e.g. old codec to H.264)	`ffmpeg -i in.avi -c:v libx264 -c:a aac out.mp4`
Convert to GIF	`ffmpeg -i in.mp4 -vf "fps=15,scale=480:-1" out.gif`
Extract audio only	`ffmpeg -i in.mp4 -vn -c:a copy out.aac`
Extract audio, convert to mp3	`ffmpeg -i in.mp4 -vn -c:a libmp3lame -q:a 2 out.mp3`
Strip audio, keep video only	`ffmpeg -i in.mp4 -an -c:v copy out.mp4`

3. Resizing / Scaling

Task	Command
Scale to specific width, keep aspect	`ffmpeg -i in.mp4 -vf scale=1280:-1 out.mp4`
Scale to specific height, keep aspect	`ffmpeg -i in.mp4 -vf scale=-1:720 out.mp4`
Force exact dimensions (may distort)	`ffmpeg -i in.mp4 -vf scale=1280:720 out.mp4`
Scale down only if larger (never upscale)	`ffmpeg -i in.mp4 -vf "scale='min(1280,iw)':-1" out.mp4`

4. Joining / Concatenating

Task	Command
Concat identical-codec files (fast, no re-encode)	See concat demuxer below
Concat with re-encode (mixed formats)	Use `concat` filter, not demuxer

Concat demuxer (files must share codec/resolution):

# list.txt:
file 'clip1.mp4'
file 'clip2.mp4'

ffmpeg -f concat -safe 0 -i list.txt -c copy out.mp4

Concat filter (re-encodes, handles mismatched sources):

ffmpeg -i clip1.mp4 -i clip2.mp4 -filter_complex \
  "[0:v][0:a][1:v][1:a]concat=n=2:v=1:a=1[v][a]" \
  -map "[v]" -map "[a]" out.mp4

5. Images ↔ Video

Task	Command
Images to video (fixed duration each)	`ffmpeg -framerate 1/5 -i %03d.png -c:v libx264 -pix_fmt yuv420p out.mp4`
Images to video, custom per-image duration	Use concat demuxer with `duration` lines (see your quote-slideshow project)
Video to image sequence	`ffmpeg -i in.mp4 frame_%04d.png`
Extract a single frame at a timestamp	`ffmpeg -ss 00:00:10 -i in.mp4 -frames:v 1 frame.png`
Add audio to a silent video	`ffmpeg -i video.mp4 -i audio.mp3 -c:v copy -c:a aac -shortest out.mp4`

6. Audio Adjustments

Task	Command
Change volume	`ffmpeg -i in.mp4 -filter:a "volume=1.5" out.mp4` (1.5 = 150%)
Normalize audio loudness	`ffmpeg -i in.mp4 -filter:a loudnorm out.mp4`
Fade audio in/out	`ffmpeg -i in.mp4 -af "afade=t=in:st=0:d=3,afade=t=out:st=57:d=3" out.mp4`
Merge/replace audio track	`ffmpeg -i video.mp4 -i newaudio.mp3 -map 0:v -map 1:a -c:v copy -shortest out.mp4`

7. Video Adjustments

Task	Command
Change frame rate	`ffmpeg -i in.mp4 -r 30 out.mp4`
Rotate 90° clockwise	`ffmpeg -i in.mp4 -vf "transpose=1" out.mp4`
Crop	`ffmpeg -i in.mp4 -vf "crop=w:h:x:y" out.mp4`
Add text overlay (burned-in)	`ffmpeg -i in.mp4 -vf "drawtext=text='Hello':x=10:y=10:fontsize=24:fontcolor=white" out.mp4`
Fade video in/out	`ffmpeg -i in.mp4 -vf "fade=t=in:st=0:d=2,fade=t=out:st=58:d=2" out.mp4`
Speed up / slow down	`ffmpeg -i in.mp4 -vf "setpts=0.5*PTS" -af "atempo=2.0" out.mp4` (2x speed)

8. Inspecting Files

Task	Command
Show all stream/codec info	`ffprobe -hide_banner in.mp4`
Show duration only	`ffprobe -v error -show_entries format=duration -of csv=p=0 in.mp4`
Show codec of video stream	`ffprobe -v error -select_streams v:0 -show_entries stream=codec_name -of csv=p=0 in.mp4`

9. Quality / Compression

Task	Command
Compress with target quality (CRF, lower=better, 18-28 typical)	`ffmpeg -i in.mp4 -c:v libx264 -crf 23 -c:a aac out.mp4`
Compress with target file size (2-pass)	Use `-b:v` with 2-pass encoding — ask if you need this, it's a multi-command process
Fast encode, larger file	`ffmpeg -i in.mp4 -c:v libx264 -preset ultrafast out.mp4`
Slow encode, smaller file, same quality	`ffmpeg -i in.mp4 -c:v libx264 -preset slow -crf 23 out.mp4`

Reading ffmpeg's flag order rule

Like ImageMagick, ffmpeg is order-sensitive but the rule is different: flags before -i apply to input handling (seeking, format forcing); flags after -i apply to output encoding/filtering. -ss is the classic example — its position changes both behavior and performance.

GUI Frontends (if you ever want to point-and-click instead)

These are wrappers around ffmpeg — the engine doesn't change, just the interface:

LosslessCut — AppImage, great for the exact trim task you started with (frame-accurate cut/trim without re-encoding, visual timeline)
Shutter Encoder — batch conversion, presets, GUI over most of ffmpeg's common operations
Avidemux — cutting/filtering with a visual timeline, older but stable
Curlew — simple GTK-based ffmpeg GUI, fits well with a lightweight WM setup
HandBrake — encoding/compression focused, not trimming

None of these teach you ffmpeg commands directly, but LosslessCut is worth having installed alongside your terminal workflow — it's genuinely the fastest way to do the trim-first-5-seconds job when you don't want to think about keyframes.