How do you convert an entire directory with ffmpeg?

And on Windows:

FOR /F "tokens=*" %G IN ('dir /b *.flac') DO ffmpeg -i "%G" -acodec mp3 "%~nG.mp3"

Previous answer will only create 1 output file called out.mov. To make a separate output file for each old movie, try this.

for i in *.avi;
  do name=`echo "$i" | cut -d'.' -f1`
  echo "$name"
  ffmpeg -i "$i" "${name}.mov"
done

For Windows:

Here I'm Converting all the (.mp4) files to (.mp3) files.
Just open cmd, goto the desired folder and type the command.

Shortcut: (optional)
1. Goto the folder where your (.mp4) files are present
2. Press Shift and Left click and Choose "Open PowerShell Window Here"
or "Open Command Prompt Window Here"
3. Type "cmd" [NOTE: Skip this step if it directly opens cmd instead of PowerShell]
4. Run the command

for %i in (*.mp4) do ffmpeg -i "%i" "%~ni.mp3"

If you want to put this into a batch file on Windows 10, you need to use %%i.

For Linux and macOS this can be done in one line, using parameter expansion to change the filename extension of the output file:

for i in *.avi; do ffmpeg -i "$i" "${i%.*}.mp4"; done