Error messages from compilers should look like this:
Error messages from other noninteractive programs should look like this:
when there is an appropriate source file, or like this:
when there is no relevant source file.
In an interactive program (one that is reading commands from a terminal), it is better not to include the program name in an error message. The place to indicate which program is running is in the prompt or with the screen layout. (When the same program runs with input from a source other than a terminal, it is not interactive and would do best to print error messages using the noninteractive style.)
The string message should not begin with a capital letter when it follows a program name and/or filename. Also, it should not end with a period.
Error messages from interactive programs, and other messages such as usage messages, should start with a capital letter. But they should not end with a period.