This is what POSIX 2003 says about ps: By default, ps shall select all processes with the same effective user ID as the current user and the same controlling terminal as the invoker ps [-aA][-defl][-G grouplist][-o format]...[-p proclist][-t termlist] [-U userlist][-g grouplist][-n namelist][-u userlist] -a Write information for all processes associated with terminals. Implementations may omit session leaders from this list. -A Write information for all processes. -d Write information for all processes, except session leaders. -e Write information for all processes. (Equivalent to -A.) -f Generate a full listing. (See the STDOUT section for the con- tents of a full listing.) -g grouplist Write information for processes whose session leaders are given in grouplist. The application shall ensure that the grouplist is a single argument in the form of a <blank> or comma-separated list. -G grouplist Write information for processes whose real group ID numbers are given in grouplist. The application shall ensure that the grou- plist is a single argument in the form of a <blank> or comma- separated list. -l Generate a long listing. (See STDOUT for the contents of a long listing.) -n namelist Specify the name of an alternative system namelist file in place of the default. The name of the default file and the format of a namelist file are unspecified. -o format Write information according to the format specification given in format. Multiple -o options can be specified; the format speci- fication shall be interpreted as the <space>-separated concate- nation of all the format option-arguments. -p proclist Write information for processes whose process ID numbers are given in proclist. The application shall ensure that the pro- clist is a single argument in the form of a <blank> or comma- separated list. -t termlist Write information for processes associated with terminals given in termlist. The application shall ensure that the termlist is a single argument in the form of a <blank> or comma-separated list. Terminal identifiers shall be given in an implementation- defined format. On XSI-conformant systems, they shall be given in one of two forms: the device's filename (for example, tty04) or, if the device's filename starts with tty, just the identifier following the characters tty (for example, "04" ). -u userlist Write information for processes whose user ID numbers or login names are given in userlist. The application shall ensure that the userlist is a single argument in the form of a <blank> or comma-separated list. In the listing, the numerical user ID shall be written unless the -f option is used, in which case the login name shall be written. -U userlist Write information for processes whose real user ID numbers or login names are given in userlist. The application shall ensure that the userlist is a single argument in the form of a <blank> or comma-separated list. With the exception of -o format, all of the options shown are used to select processes. If any are specified, the default list shall be ignored and ps shall select the processes represented by the inclusive OR of all the selection-criteria options. The -o option allows the output format to be specified under user con- trol. The application shall ensure that the format specification is a list of names presented as a single argument, <blank> or comma-separated. Each variable has a default header. The default header can be overridden by appending an equals sign and the new text of the header. The rest of the characters in the argument shall be used as the header text. The fields specified shall be written in the order specified on the command line, and should be arranged in columns in the output. The field widths shall be selected by the system to be at least as wide as the header text (default or overridden value). If the header text is null, such as -o user=, the field width shall be at least as wide as the default header text. If all header text fields are null, no header line shall be written. ruser The real user ID of the process. This shall be the textual user ID, if it can be obtained and the field width permits, or a dec- imal representation otherwise. user The effective user ID of the process. This shall be the textual user ID, if it can be obtained and the field width permits, or a decimal representation otherwise. rgroup The real group ID of the process. This shall be the textual group ID, if it can be obtained and the field width permits, or a decimal representation otherwise. group The effective group ID of the process. This shall be the textual group ID, if it can be obtained and the field width permits, or a decimal representation otherwise. pid The decimal value of the process ID. ppid The decimal value of the parent process ID. pgid The decimal value of the process group ID. pcpu The ratio of CPU time used recently to CPU time available in the same period, expressed as a percentage. The meaning of "recently" in this context is unspecified. The CPU time avail- able is determined in an unspecified manner. vsz The size of the process in (virtual) memory in 1024 byte units as a decimal integer. nice The decimal value of the nice value of the process; see nice() . etime In the POSIX locale, the elapsed time since the process was started, in the form: [[dd-]hh:]mm:ss time In the POSIX locale, the cumulative CPU time of the process in the form: [dd-]hh:mm:ss tty The name of the controlling terminal of the process (if any) in the same format used by the who utility. comm The name of the command being executed ( argv[0] value) as a string. args The command with all its arguments as a string. The implementa- tion may truncate this value to the field width; it is implemen- tation-defined whether any further truncation occurs. It is unspecified whether the string represented is a version of the argument list as it was passed to the command when it started, or is a version of the arguments as they may have been modified by the application. Applications cannot depend on being able to modify their argument list and having that modification be reflected in the output of ps. Any field need not be meaningful in all implementations. In such a case a hyphen ( '-' ) should be output in place of the field value. Only comm and args shall be allowed to contain <blank>s; all others shall not. The following table specifies the default header to be used in the POSIX locale corresponding to each format specifier. Format Specifier Default Header Format Specifier Default Header args COMMAND ppid PPID comm COMMAND rgroup RGROUP etime ELAPSED ruser RUSER group GROUP time TIME nice NI tty TT pcpu %CPU user USER pgid PGID vsz VSZ pid PID There is no special quoting mechanism for header text. The header text is the rest of the argument. If multiple header changes are needed, multiple -o options can be used, such as: ps -o "user=User Name" -o pid=Process\ ID