In my Unix system, /bin/sh is symlinked to /bin/bash.

/bin/sh -> bash*

As this is the case, I expect that if an executable has a shebang of:

#!/bin/sh

it will be equivalent to:

#!/bin/bash

However, this is not true.
For example, below code uses process substitution which is supported by bash only:

#!/bin/bash
echo ${BASH_VERSION}
diff <(echo -e "1n2") <(echo -e "2n2")

Result:

4.2.46(2)-release
1c1
< 1
---
> 2

If I change the shebang to /bin/sh, as follows:

#!/bin/sh
echo ${BASH_VERSION}
diff <(echo -e "1n2") <(echo -e "2n2")

Result is:

4.2.46(2)-release
test.sh: line 3: syntax error near unexpected token `('
test.sh: line 3: `diff <(echo -e "1n2") <(echo -e "2n2")'

Moreover, even if remove the shebang, and run the script explicitly:

/bin/sh test.sh

I get the same error.

Weirdly (or maybe not), running the following will work:

`realpath /bin/sh` test.sh

I’m looking for an explanation for this behavior – as this is unexpected in my understanding, since /bin/sh -> bash. Also, the value of BASH_VERSION is consistent.

Thank you.