(or "In dash, ... is not supported." when using
#!/bin/sh while IFS= read -r n do sum=$((sum+n)) done < <(program)
The easiest fix is to switch to a shell that does support process substitution, by changing the shebang to
#!/bin/bash while IFS= read -r n do sum=$((sum+n)) done < <(program)
Alternatively, process substitution can often be replaced with temporary files:
#!/bin/sh tmp="$(mktemp)" program > "$tmp" while IFS= read -r n do sum=$((sum+n)) done < "$tmp" rm "$tmp"
If streaming is important, the temporary file can be a named pipe, and the producer or consumer can be run as a background job.
If the reading command accepts input from standard input, the process substitution can be replaced with a regular pipe:
#!/bin/sh program | while IFS= read -r n do sum=$((sum+n)) done
Process substitution is a ksh and bash extension. It does not work in sh or dash scripts.
If you only intend to target shells that supports this feature, you can change the shebang to a shell that guarantees support, or ignore this warning.
You can use
# shellcheck disable=SC3000-SC4000 to ignore all such compatibility warnings.
ShellCheck is a static analysis tool for shell scripts. This page is part of its documentation.