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In the manpage of ps

-j Jobs format.

-f Do full-format listing.

-o format User-defined format.

$ ps -j -o ppid,sid
error: can not use output modifiers with user-defined output

$ ps -f -o ppid,sid
 PPID   SID
 3046 23122
 3046  1002
 1002  1002
 3046 13961
...

What does output modifiers mean? In the manpage of ps, -j,-f and -o are listed under OUTPUT FORMAT CONTROL, instead of OUTPUT MODIFIERS.

Generally speaking, how are output modifiers used, compared to output format control options?

Are output modifiers options or arguments?

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  • -j asks for output in "jobs format", and -o ppid,sid ask for output in "custom format". Which one do you want? Commented Feb 28, 2016 at 4:29
  • Their union? why ps -f -o ppid,sid works without error? Commented Feb 28, 2016 at 4:31
  • Yes, but -f has no impact. It should probably give an error too. Though I agree a "union" of formats could have been useful. I'm guessing you want to see -oppid,pid,pgid,sid,tty,time,cmd Commented Feb 28, 2016 at 4:42
  • Not really. I made up the examples. I want to understand: What does "output modifiers" mean? how are output modifiers used, compared to output format control options? Are output modifiers options or arguments? Commented Feb 28, 2016 at 4:48

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It seems to be a misleading error message.

If you look at the procps source, file common.h line 290:

extern unsigned        format_modifiers; /* -c -j -y -P -L... */

-j implied format_modifiers flag to be set, which cause the error if used with user defined output:

if(format_list){
  if(format_flags) return "Conflicting format options.";
  if(format_modifiers) return "Can't use output modifiers with user-defined output";
  if(thread_flags&TF_must_use) return "-L/-T with H/m/-m and -o/-O/o/O is nonsense";
  return NULL;
}

A message like Can't use output format modifiers with user-defined output would be better.

FreeBSD ps doesn't have this issue, -j option cause ps to print information about user, pid, ppid, pgid, sid, jobc, state, tt, time, and command. Adding -o makes the output aggregated:

$ ps -j -o ppid,sid
USER     PID PPID PGID  SID JOBC STAT TT     TIME COMMAND          PPID  SID
cuonglm 1196 1195 1196 1196    0 Ss    0  0:00.02 -sh (sh)         1195 1196
cuonglm 1233 1196 1233 1196    1 R+    0  0:00.00 ps -j -o ppid,si 1196 1196

Output modifiers control how information displayed, while output format controls control what information displayed.

Example the s options is an output format control, because it added process signal information to ps output:

$ ps s
  UID   PID          PENDING          BLOCKED          IGNORED           CAUGHT STAT TTY        TIME COMMAND
 1000 12831 0000000000000000 0000000000000002 0000000000384004 0000000188013003 Ss   pts/1      0:00 zsh
 1000 13067 0000000000000000 0000000000000000 0000000000000000 0000000073d3fef9 R+   pts/1      0:00 ps s

f is an output modifier, because it changed how the output displayed:

$ ps f
  PID TTY      STAT   TIME COMMAND
12831 pts/1    Ss     0:00 zsh
13238 pts/1    R+     0:00  \_ ps f

Here the output was displayed as hierarchy.

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  • Thanks. What does "output modifiers" mean? how are output modifiers used, compared to output format control options? Are output modifiers options or arguments? Commented Feb 28, 2016 at 17:19
  • Did you read the last part of my answer? What do you mean options and arguments? Commented Feb 28, 2016 at 17:37
  • Sorry, I missed your last part. Are output modifiers options or non-option arguments? Commented Feb 28, 2016 at 17:58
  • They are both options arguments Commented Feb 28, 2016 at 22:32
  • Thanks. (1) In the manpage of ps, -j means jobs format, and is listed under OUTPUT FORMAT CONTROL, instead of OUTPUT MODIFIERS. Your reply says that the source code says -j is format modifier. Is -j jobs format or format modifier? (2) -f is also listed under output format control in ps manpage. Why -f can work with -o, while -j can't? Commented Feb 29, 2016 at 22:32

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