ffmpeg in 10 recipes: convert, trim and compress media

Tags: Linux, Developer Tips

Intro – why ffmpeg and who should use these recipes

ffmpeg is the Swiss‑army knife for audio‑video work on Linux. It can read almost any media format, apply filters, and write out a new file—all from a single command line. If you ever needed to:

change a .mov to a web‑friendly .mp4
cut the first 30 seconds off a clip
shrink a 4 GB lecture video to under 500 MB

…you’re in the right place. These 10 recipes assume you have ffmpeg installed (e.g., sudo apt install ffmpeg) and a basic command‑line comfort level. No deep codec theory, just copy‑paste commands that work out of the box.

Part 1 – Simple format conversion

#	Goal	Command (copy‑paste)	Notes
1	Convert any video to MP4 (H.264 video, AAC audio)	`bash\nffmpeg -i input.mov -c:v libx264 -c:a aac -movflags +faststart output.mp4\n`	`-movflags +faststart` moves the index to the front for progressive streaming.
2	Convert a video to WebM (VP9 + Opus)	`bash\nffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 2M -c:a libopus output.webm\n`	Adjust `-b:v` (bitrate) to control quality vs. size.
3	Extract audio as MP3	`bash\nffmpeg -i input.mp4 -q:a 2 -map a output.mp3\n`	`-q:a 2` gives near‑transparent VBR MP3; lower numbers = higher quality.
4	Extract audio as OGG Vorbis	`bash\nffmpeg -i input.mp4 -c:a libvorbis -q:a 4 output.ogg\n`	`-q:a 4` is a good balance for spoken content.

Why these flags?

-c:v / -c:a pick the encoder.
-b:v sets a target video bitrate (optional).
-q:a is a quality‑based VBR setting for audio.

Part 2 – Trimming and stitching

#	Goal	Command (copy‑paste)	Explanation
5	Cut the first 30 seconds (no re‑encoding)	`bash\nffmpeg -ss 00:00:30 -i input.mp4 -c copy -avoid_negative_ts 1 cut-start.mp4\n`	`-ss` before `-i` seeks quickly; `-c copy` avoids quality loss.
6	Cut a 10‑second segment from 01:02 to 01:12 (re‑encode)	`bash\nffmpeg -i input.mp4 -ss 00:01:02 -t 00:00:10 -c:v libx264 -c:a aac segment.mp4\n`	Placing `-ss` after `-i` gives frame‑accurate cuts at the cost of re‑encoding.
7	Concatenate two MP4 files (same codec)	`bash\n# 1️⃣ create a list file\nprintf "file 'part1.mp4'\nfile 'part2.mp4'\n" > list.txt\n# 2️⃣ run concat demuxer\nffmpeg -f concat -safe 0 -i list.txt -c copy joined.mp4\n`	All inputs must share codec, resolution, and frame rate.
8	Add a 5‑second fade‑in to the start	`bash\nffmpeg -i input.mp4 -vf \"fade=t=in:st=0:d=5\" -c:a copy faded.mp4\n`	`-vf` applies a video filter; audio is passed through unchanged.

Part 3 – Compression without sacrificing too much quality

#	Goal	Command (copy‑paste)	Tips
9	Reduce file size to ~50 % using CRF (constant‑rate‑factor)	`bash\nffmpeg -i input.mp4 -c:v libx264 -crf 23 -preset medium -c:a aac -b:a 128k compressed.mp4\n`	CRF 18–28 is typical; lower = higher quality. `-preset` trades speed for compression.
10	Target a specific output size (e.g., 700 MB)	`bash\n# 1️⃣ get source duration in seconds\nDUR=$(ffprobe -v error -show_entries format=duration -of default=noprint_wrappers=1:nokey=1 input.mp4)\n# 2️⃣ compute video bitrate (bits per second)\nTARGET=700M # desired size\nBITRATE=$(( (TARGET81024*1024) / DUR - 128000 )) # subtract 128 kbps audio\nffmpeg -i input.mp4 -c:v libx264 -b:v ${BITRATE} -c:a aac -b:a 128k sized.mp4\n`	Replace `700M` with your own target. This method uses average bitrate, not CRF.

Quick reference table for CRF values

CRF	Approx. quality (subjective)	Typical file size (vs. source)
18	Visually lossless	~90 % of source
20	High quality, minor loss	~80 %
23	Default, good for web	~60 %
26	Noticeable compression	~40 %
28	Low‑bitrate streaming	~30 %

Hands‑on example – turn a 4 GB lecture into a 500 MB web‑ready MP4

# 1️⃣ Install ffmpeg (Ubuntu/Debian)
sudo apt update && sudo apt install -y ffmpeg

# 2️⃣ Inspect the source (optional but helpful)
ffprobe -v error -show_format -show_streams lecture.mov

# 3️⃣ Convert & compress in one step
ffmpeg -i lecture.mov \
       -c:v libx264 -crf 24 -preset slow \
       -c:a aac -b:a 128k \
       -movflags +faststart \
       lecture_compressed.mp4

What the flags do

-crf 24 – a sweet spot for lecture slides (text + static graphics).
-preset slow – takes longer but squeezes out extra compression.
-movflags +faststart – makes the video start playing before the whole file downloads.

After the command finishes, check the size:

ls -lh lecture_compressed.mp4

You should see something around 500 MB, ready for upload to a learning platform.

Common mistakes & how to fix them

“ffmpeg: command not found” – Install it (sudo apt install ffmpeg or use your distro’s package manager).
Audio out of sync after cutting – Use -avoid_negative_ts 1 with copy‑only cuts, or re‑encode the audio (-c:a aac).
“Invalid argument” when using -ss – Ensure the timestamp format is hh:mm:ss or seconds (30 works too).
Output file larger than expected – Lower the CRF value or add -b:v to force a tighter bitrate.
Concat fails with “different codecs” – Re‑encode all parts to the same codec first, or use the concat filter:
```
ffmpeg -i "concat:part1.mp4|part2.mp4" -c copy joined.mp4
```

Try it yourself

Take a 2‑minute MP4 clip (sample.mp4). Trim the first 15 seconds, add a 3‑second fade‑in, and compress it to a 10 MB file.

ffmpeg -i sample.mp4 -ss 00:00:15 -t 00:01:45 \
       -vf "fade=t=in:st=0:d=3" \
       -c:v libx264 -crf 26 -preset fast \
       -c:a aac -b:a 96k \
       -movflags +faststart sample_final.mp4

Verify the result with ffprobe and ls -lh.

What’s next

Batch processing – Write a short Bash loop to apply the same recipe to every file in a folder.
Advanced filters – Explore scale, crop, and drawtext for thumbnail generation.
Streaming presets – Use -profile:v high -level 4.2 for HLS/DASH compatibility.
Hardware acceleration – Replace libx264 with h264_nvenc (NVIDIA) or h264_vaapi (Intel) for faster encoding.

Bookmark this page for a quick ffmpeg cheat sheet whenever you need to tinker with media.

Tags: Linux Developer Tips

ffmpeg in 10 recipes: convert, trim and compress media

Intro – why ffmpeg and who should use these recipes

Part 1 – Simple format conversion

Part 2 – Trimming and stitching

Part 3 – Compression without sacrificing too much quality