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@tauon @dalias @orman I could consider giving the errno (specially once I add locales) but to give an example of what I do here:
cat: error: Failed opening file '/home/haelwenn/foobar': No such file or directory

• name of the tool (useful for logs)
• error/warning/… clearly labelled (and reusing the error: pattern from compilers)
• quotes around user input (here the path), although I'd consider putting it at the end so no they wouldn't be needed anymore but sometimes there's two arbitrary parts (like when copying)