Ps: output modifiers vs output format control

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?

Best Answer

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.

Related Solutions

Format ‘ps’ command output without whitespace

From the man page:

-o format       user-defined format.
                format is a single argument in the form of a blank-separated or comma-separated list, which offers a
                way to specify individual output columns. The recognized keywords are described in the STANDARD FORMAT
                SPECIFIERS section below. Headers may be renamed (ps -o pid,ruser=RealUser -o comm=Command) as
                desired. If all column headers are empty (ps -o pid= -o comm=) then the header line will not be
                output. Column width will increase as needed for wide headers; this may be used to widen up columns
                such as WCHAN (ps -o pid,wchan=WIDE-WCHAN-COLUMN -o comm). Explicit width control
                (ps opid,wchan:42,cmd) is offered too. The behavior of ps -o pid=X,comm=Y varies with personality;
                output may be one column named "X,comm=Y" or two columns named "X" and "Y". Use multiple -o options
                when in doubt. Use the PS_FORMAT environment variable to specify a default as desired; DefSysV and
                DefBSD are macros that may be used to choose the default UNIX or BSD columns.

So try:

/bin/ps -o uname:1,ppid:1,pid:1

Bash – Format the output of cputime for ps

This converts the first time to seconds:

ps -p $PROCID -o cputime,etimes | awk -F'[: ]+' '/:/ {t=$3+60*($2+60*$1); print t,$NF}'

As an example, the ps command produces:

$ ps -p 5403 -o cputime,etimes
    TIME ELAPSED
01:33:38 1128931

The awk command processes that and returns:

ps -p 5403 -o cputime,etimes | awk -F'[: ]+' '/:/ {t=$3+60*($2+60*$1); print t,$NF}'
5618 1128931

Explanation

-F'[: ]+'

This tells awk to treat both colons and spaces as field separators. This way, the hours, minutes, and seconds appear as separate fields.
/:/ {t=$3+60*($2+60*$1); print t,$NF}

The initial /:/ restricts the code to working only on lines that include a colon. This removes the header lines. The number of seconds is calculated from hours, minutes, seconds via t=$3+60*($2+60*$1). The resulting value for t is then printed along side with the elapsed time.

Handling days

If ps produces days,hours,minutes,seconds, as in:

2-03:01:33

Then, use this code instead:

ps -p $PROCID -o cputime,etimes | awk -F'[-: ]+' '/:/ {t=$4+60*($3+60*($2+24*$1)); print t,$NF}'

If days may or may not be prepended to the output, then use this combination command:

ps -p $PROCID -o cputime,etimes | awk -F'[-: ]+' '/:/ && NF==5 { t=$4+60*($3+60*($2+24*$1)); print t,$NF} /:/ && NF==4 {t=$3+60*($2+60*$1); print t,$NF}'