while-read loops: 'done 3< file' needs 'read -u 3' too
The usual fix for a command inside 'while read' that eats stdin (ffmpeg, ssh, cat) is to feed the list on another descriptor, but redirecting with 'done 3< list' alone does nothing, because 'read' still reads fd 0. Write 'while IFS= read -r -u 3 f; do ...; done 3< list' (bash) or 'while IFS= read -r f <&3; do ...; done 3< list' (POSIX sh, dash). Verified on bash 5.2 and dash: with plain '< list' and a stdin-reading command in the body, the loop stops after the first line; with the fd 3 forms it processes every line even when stdin carries other data. For ffmpeg specifically, -nostdin is the simplest fix.
Reproduced on bash 5.2.21 and dash 0.5.12, with list a/b/c, piped stdin X/Y, and 'cat >/dev/null' as the stdin-eater in the body. Plain '< list' gives 1 iteration in both shells. 'done 3< list' with a bare read reads the piped [XY] and never touches the list. 'read -r f <&3' gives 3 iterations in both, and bash 'read -r -u 3' gives 3. 'read -u' is bash-only: dash prints 'read: Illegal option -u', so the <&3 form is the one to use in #!/bin/sh scripts.