The ps(1) display's list of columns is now first built without taking
into account the '-O' options. In a second step, all columns passed via
'-O' are finally inserted after the built-so-far display's first PID
column (if it exists, else at start), in their order of appearance as
arguments to the '-O' options.
This new two-step procedure suppresses the following undesirable
behaviors:
- '-O' used to insert the columns of the default display even if the user also passed other options to request specific columns.
- Each occurence of '-O' beyond the first would just insert the passed keywords after those requested by the previous options, as if by '-o', inconsistently with the behavior for the first occurence.
These behaviors had more annoying consequences:
- Where the columns of some '-O' occurence appear in the display used to depend on the actual position of '-O' with respect to other options, despite the natural expectation that they should go near a single PID column considered as an anchor regardless of other options adding columns.
- Columns specified with multiple '-O' options would not be grouped together.
- It used to be impossible to specify custom headers but for the last column for columns that are next to each other (i.e., specified by a single '-O' occurence).
which are now all lifted.
With these changes, '-O' can still be used alone to amend the default
display, but can now be used also in conjunction with any specific
display, and in particular "canned" ones invoked by '-j', '-l', '-u' or
'-v'.
This part discusses other ps(1) implementations' behaviors and compares
them to the one established by this change.
NetBSD seems to be the only other BSD having refined the meaning of
ps(1)'s '-O' option. While the behavior there is similar to the new one
here on the surface, it differs on the following points:
- If no options requesting a specific display are present before the first '-O' option, the appearance of '-O' triggers the insertion of the default display, regardless of whether such specific display options appear *after* it.
- If options requesting a specific display appear before the first '-O' and none specify a PID column, columns listed in the first '-O' are appended to them (as '-o' would do), but columns passed by further '-O' options are then inserted next to the columns of the first '-O' options.
Behavior of point 1 seems to have only one advantage: To allow to
customize the default display by first using '-O' and then other options
appending to it, but as the default display finishes with the COMMAND
column, it is unlikely that one wants to use '-o' or other specific
display options after '-O' (NetBSD's ps(1) does not suppress duplicate
columns). A much simpler and easy-to-understand way to reach that
effect in FreeBSD, if it really proves useful, would be to introduce
a new explicit option that inserts the default display.
The column-appending behavior of the first '-O' option in point 2 can be
also achieved by using '-o' instead. As '-O' is used to insert columns
after the PID one, which is located near the left in the default and all
"canned" displays, we found it more consistent and practical to push its
columns completely to the left on the absence of a PID column. The
effect of multiple '-O' options in NetBSD when no PID column has been
requested beforehand is also cumbersome and inconsistent with the
documentation (it is likely a bug).
Both behaviors have the disadvantage that the position of '-O' options
with respect to other ones is meaningful in ways that are not obvious
and that are arguably not desirable as '-O' is meant to append columns
after an anchor (the PID column).
Linux's procps-ng's ps(1) is very limited in its handling of '-O', and
more generally when mixing options tweaking the display. '-O' causes
insertion of the default display (like NetBSD does). If '-o' options
are specified, '-O' must come before them. '-O' is not usable with
canned display options. Additionally, only one '-O' option may appear
on the command line, limiting header customization.
The only case in which these implementations and ours behave in the same
way with respect to '-O' is if only a single '-O' option and no other
options changing the display are specified.