Identify the error's cause

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

Not recommended

Bad directory.

Recommended

The [Name of directory] 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