Identify the error's cause

Tell users exactly what went wrong. Be specific—vague error messages frustrate users.

Not recommended

Bad directory.

Recommended

The specified directory exists but is not writable. To add files to this directory, the directory must be writable. [Explanation of how to make this directory writable.]

Not recommended

Invalid field 'picture'.

Recommended

The 'picture' field can only appear once on the command line; this command line contains the 'picture' field <N> times.
Note: Prior to version 2.1, you could specify the 'picture' field more than once, but more recent versions no longer support this.

Next unit: Identify the user's invalid inputs