bug#7157: df should default to -P if output is not a tty

[Eric Blake]

On 10/04/2010 03:44 AM, Alain Knaff wrote:
> There is an option to prevent this behavior (-P), but apparently the
> logwatch authors were not aware of it. Understandably, I might say,
> because if their test cases happen to only have disks with short names,
> they'll never stumble upon this.

Thanks for the report.  However, I'm reluctant to make the change, on 
the grounds that df -P is documented by POSIX:

http://www.opengroup.org/onlinepubs/9699919799/utilities/df.html

Anyone that wants portable output MUST use -P, because POSIX explicitly 
implies that not using -P leads to implementation-defined formatting, 
which is therefore not machine-parseable.

Furthermore, GNU Coding Standards strongly discourage making behavior 
dependent on whether stdout is a tty.  Yes, some actions, like ls 
behavior, are mandated by POSIX, and some actions, like --color=auto 
only make sense with a tty check, but we should not be adding new 
exceptions without good cause.

One thing that might be possible, however, is to improve df output to 
adjust column alignment if $COLUMNS is not set (or is set to something 
large), so as to avoid breaking lines in the case where the output is 
usable in a long-line context.  But this still doesn't help the case 
that logwatch should have been using -P in the first place.

--
Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library http://libvirt.org