SC3020 – ShellCheck Wiki

In POSIX sh, `&>` is undefined.

(or "In dash, ... is not supported." when using dash)

Problematic code:

#!/bin/sh
ssh host cmd &> log

Correct code:

Use the long form instead of the shorthand:

#!/bin/sh
ssh host cmd > log 2>&1

Rationale:

&> is a bash and ksh extension for redirecting both stdout and stderr. In dash and POSIX sh, use 2>&1 explicitly.

Exceptions:

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.

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