If you're going to use bash-only syntax like $(<...), your script must be run with bash, not sh.

Thus, either run bash yourscript or add a #!/bin/bash (or similar) shebang, flag the file executable, and invoke it as a command, for example ./yourscript

As an alternative that's both efficient and compatible with POSIX sh:

read SERVER_IP_PORT <"$IPAddressFile" 

If you're going to use bash-only syntax like $(<...), your script must be run with bash, not sh.

Thus, either run bash yourscript or add a #!/bin/bash (or similar) shebang, flag the file executable, and invoke it as a command, for example ./yourscript

As an alternative that's both efficient and compatible with POSIX sh:

IFS= read -r SERVER_IP_PORT <"$IPAddressFile"