In POSIX sh, -nt is undefined.

Problematic code:

#!/bin/sh
if [ Foo.java -nt Foo.class ]
then
  javac Foo.java
fi

Correct code:

The easiest fix is to switch to a shell that does support -nt, like bash:

#!/bin/bash
if [ Foo.java -nt Foo.class ]
then
  javac Foo.java
fi

Otherwise, find can be used:

#!/bin/sh
if [ -n "$(find Foo.java -newer Foo.class)" ]
then
  javac Foo.java
fi

Rationale:

test -nt is an extension in ksh, bash and dash, but it is not covered by POSIX.

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.

Related resources:

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