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AT(P)				    POSIX Programmer's Manual				    AT(P)

NAME
       at - execute commands at a later time

SYNOPSIS
       at [-m][-f file][-q queuename] -t time_arg

       at [-m][-f file][-q queuename] timespec ...

       at -r at_job_id ...

       at -l -q queuename

       at -l [at_job_id ...]

DESCRIPTION
       The  at	utility shall read commands from standard input and group them together as an at-
       job, to be executed at a later time.

       The at-job shall be executed in a separate invocation of the shell, running in a  separate
       process group with no controlling terminal, except that the environment variables, current
       working directory, file creation mask,  and  other  implementation-defined  execution-time
       attributes  in  effect when the at utility is executed shall be retained and used when the
       at-job is executed.

       When the at-job is submitted, the at_job_id and scheduled time shall be written	to  stan-
       dard  error.  The  at_job_id  is an identifier that shall be a string consisting solely of
       alphanumeric characters and the period character. The at_job_id shall be assigned  by  the
       system when the job is scheduled such that it uniquely identifies a particular job.

       User  notification  and the processing of the job's standard output and standard error are
       described under the -m option.

       Users  shall  be  permitted  to	 use   at   if	 their	 name	appears   in   the   file
       /usr/lib/cron/at.allow.	If that file does not exist, the file /usr/lib/cron/at.deny shall
       be checked to determine whether the user shall be denied access to  at.	If  neither  file
       exists,	only  a process with the appropriate privileges shall be allowed to submit a job.
       If only at.deny exists and is empty, global usage shall be  permitted.  The  at.allow  and
       at.deny files shall consist of one user name per line.

OPTIONS
       The  at utility shall conform to the Base Definitions volume of IEEE Std 1003.1-2001, Sec-
       tion 12.2, Utility Syntax Guidelines.

       The following options shall be supported:

       -f  file
	      Specify the pathname of a file to be used as the source of the at-job,  instead  of
	      standard input.

       -l     (The  letter  ell.) Report all jobs scheduled for the invoking user if no at_job_id
	      operands are specified. If at_job_ids are specified, report  only  information  for
	      these jobs. The output shall be written to standard output.

       -m     Send mail to the invoking user after the at-job has run, announcing its completion.
	      Standard output and standard error produced by the at-job shall be  mailed  to  the
	      user  as well, unless redirected elsewhere. Mail shall be sent even if the job pro-
	      duces no output.

       If -m is not used, the job's standard output and standard error shall be provided  to  the
       user by means of mail, unless they are redirected elsewhere; if there is no such output to
       provide, the implementation need not notify the user of the job's completion.

       -q  queuename

	      Specify in which queue to schedule a job for submission.	When  used  with  the  -l
	      option,  limit  the  search  to that particular queue. By default, at-jobs shall be
	      scheduled in queue a. In contrast, queue b shall be reserved for	batch  jobs;  see
	      batch.  The  meanings of all other queuenames are implementation-defined.  If -q is
	      specified along with either of the -t time_arg or timespec arguments,  the  results
	      are unspecified.

       -r     Remove  the  jobs with the specified at_job_id operands that were previously sched-
	      uled by the at utility.

       -t  time_arg
	      Submit the job to be run at the time specified by the time  option-argument,  which
	      the application shall ensure has the format as specified by the touch -t time util-
	      ity.

OPERANDS
       The following operands shall be supported:

       at_job_id
	      The name reported by a previous invocation of the at utility at the  time  the  job
	      was scheduled.

       timespec
	      Submit  the job to be run at the date and time specified. All of the timespec oper-
	      ands are interpreted as if they were separated by <space>s  and  concatenated,  and
	      shall  be  parsed  as described in the grammar at the end of this section. The date
	      and time shall be interpreted as being in the timezone of the user  (as  determined
	      by the TZ variable), unless a timezone name appears as part of time, below.

       In  the	POSIX  locale,	the following describes the three parts of the time specification
       string. All of the values from the LC_TIME categories in the POSIX locale shall be  recog-
       nized in a case-insensitive manner.

       time
	      The time can be specified as one, two, or four digits. One-digit and two-digit num-
	      bers shall be taken to be hours; four-digit numbers to be hours  and  minutes.  The
	      time  can  alternatively	be specified as two numbers separated by a colon, meaning
	      hour:minute. An AM/PM indication (one of the values from the am_pm keywords in  the
	      LC_TIME locale category) can follow the time; otherwise, a 24-hour clock time shall
	      be understood. A timezone name can also follow to further  qualify  the  time.  The
	      acceptable  timezone  names  are	implementation-defined, except that they shall be
	      case-insensitive and the string utc is supported to indicate the time is in Coordi-
	      nated  Universal	Time.  In the POSIX locale, the time field can also be one of the
	      following tokens:

	      midnight
		     Indicates the time 12:00 am (00:00).

	      noon
		     Indicates the time 12:00 pm.

	      now
		     Indicates the current day and time. Invoking at <now> shall submit an at-job
		     for  potentially  immediate  execution (that is, subject only to unspecified
		     scheduling delays).

       date
	      An optional date can be specified as either a month name (one of	the  values  from
	      the  mon or abmon keywords in the LC_TIME locale category) followed by a day number
	      (and possibly year number preceded by a comma), or a day of the week  (one  of  the
	      values from the day or abday keywords in the LC_TIME locale category). In the POSIX
	      locale, two special days shall be recognized:

	      today
		     Indicates the current day.

	      tomorrow
		     Indicates the day following the current day.

	      If no date is given, today shall be assumed if the given time is greater	than  the
	      current  time,  and  tomorrow shall be assumed if it is less. If the given month is
	      less than the current month (and no year is given), next year shall be assumed.

       increment
	      The optional increment shall be a number preceded by a plus sign ( '+' )	and  suf-
	      fixed  by one of the following: minutes, hours, days, weeks, months, or years. (The
	      singular forms shall also be accepted.) The keyword next shall be equivalent to  an
	      increment number of +1. For example, the following are equivalent commands:

		     at 2pm + 1 week
		     at 2pm next week

       The  following  grammar	describes the precise format of timespec in the POSIX locale. The
       general conventions for this style of grammar are described in Grammar Conventions .  This
       formal  syntax shall take precedence over the preceding text syntax description. The long-
       est possible token or delimiter shall be recognized at a given point. When used in a time-
       spec, white space shall also delimit tokens.

	      %token hr24clock_hr_min
	      %token hr24clock_hour
	      /*
		An hr24clock_hr_min is a one, two, or four-digit number. A one-digit
		or two-digit number constitutes an hr24clock_hour. An hr24clock_hour
		may be any of the single digits [0,9], or may be double digits, ranging
		from [00,23]. If an hr24clock_hr_min is a four-digit number, the
		first two digits shall be a valid hr24clock_hour, while the last two
		represent the number of minutes, from [00,59].
	      */

	      %token wallclock_hr_min
	      %token wallclock_hour
	      /*
		A wallclock_hr_min is a one, two-digit, or four-digit number.
		A one-digit or two-digit number constitutes a wallclock_hour.
		A wallclock_hour may be any of the single digits [1,9], or may
		be double digits, ranging from [01,12]. If a wallclock_hr_min
		is a four-digit number, the first two digits shall be a valid
		wallclock_hour, while the last two represent the number of
		minutes, from [00,59].
	      */

	      %token minute
	      /*
		A minute is a one or two-digit number whose value can be [0,9]
		or [00,59].
	      */

	      %token day_number
	      /*
		A day_number is a number in the range appropriate for the particular
		month and year specified by month_name and year_number, respectively.
		If no year_number is given, the current year is assumed if the given
		date and time are later this year. If no year_number is given and
		the date and time have already occurred this year and the month is
		not the current month, next year is the assumed year.
	      */

	      %token year_number
	      /*
		A year_number is a four-digit number representing the year A.D., in
		which the at_job is to be run.
	      */

	      %token inc_number
	      /*
		The inc_number is the number of times the succeeding increment
		period is to be added to the specified date and time.
	      */

	      %token timezone_name
	      /*
		The name of an optional timezone suffix to the time field, in an
		implementation-defined format.
	      */

	      %token month_name
	      /*
		One of the values from the mon or abmon keywords in the LC_TIME
		locale category.
	      */

	      %token day_of_week
	      /*
		One of the values from the day or abday keywords in the LC_TIME
		locale category.
	      */

	      %token am_pm
	      /*
		One of the values from the am_pm keyword in the LC_TIME locale
		category.
	      */

	      %start timespec
	      %%
	      timespec	  : time
			  | time date
			  | time increment
			  | time date increment
			  | nowspec
			  ;

	      nowspec	  : "now"
			  | "now" increment
			  ;

	      time	  : hr24clock_hr_min
			  | hr24clock_hr_min timezone_name
			  | hr24clock_hour ":" minute
			  | hr24clock_hour ":" minute timezone_name
			  | wallclock_hr_min am_pm
			  | wallclock_hr_min am_pm timezone_name
			  | wallclock_hour ":" minute am_pm
			  | wallclock_hour ":" minute am_pm timezone_name
			  | "noon"
			  | "midnight"
			  ;

	      date	  : month_name day_number
			  | month_name day_number "," year_number
			  | day_of_week
			  | "today"
			  | "tomorrow"
			  ;

	      increment   : "+" inc_number inc_period
			  | "next" inc_period
			  ;

	      inc_period  : "minute" | "minutes"
			  | "hour" | "hours"
			  | "day" | "days"
			  | "week" | "weeks"
			  | "month" | "months"
			  | "year" | "years"
			  ;

STDIN
       The  standard  input  shall  be a text file consisting of commands acceptable to the shell
       command language described in Shell Command Language . The standard input  shall  only  be
       used if no -f file option is specified.

INPUT FILES
       See the STDIN section.

       The text files /usr/lib/cron/at.allow and /usr/lib/cron/at.deny shall contain zero or more
       user names, one per line, of users who are, respectively, authorized or denied  access  to
       the at and batch utilities.

ENVIRONMENT VARIABLES
       The following environment variables shall affect the execution of at:

       LANG   Provide  a  default  value for the internationalization variables that are unset or
	      null. (See the Base Definitions volume of IEEE Std 1003.1-2001, Section 8.2, Inter-
	      nationalization Variables for the precedence of internationalization variables used
	      to determine the values of locale categories.)

       LC_ALL If set to a non-empty string value, override the values of all the  other  interna-
	      tionalization variables.

       LC_CTYPE
	      Determine  the  locale for the interpretation of sequences of bytes of text data as
	      characters (for example, single-byte as opposed to multi-byte characters	in  argu-
	      ments and input files).

       LC_MESSAGES
	      Determine the locale that should be used to affect the format and contents of diag-
	      nostic messages written to standard error and informative messages written to stan-
	      dard output.

       NLSPATH
	      Determine the location of message catalogs for the processing of LC_MESSAGES .

       LC_TIME
	      Determine the format and contents for date and time strings written and accepted by
	      at.

       SHELL  Determine a name of a command interpreter to be used to invoke the at-job.  If  the
	      variable	is unset or null, sh shall be used.  If it is set to a value other than a
	      name for sh, the implementation shall do one of the following: use that shell;  use
	      sh; use the login shell from the user database; or any of the preceding accompanied
	      by a warning diagnostic about which was chosen.

       TZ     Determine the timezone. The job shall be submitted for execution at the time speci-
	      fied  by timespec or -t time relative to the timezone specified by the TZ variable.
	      If timespec specifies a timezone, it shall override TZ . If timespec does not spec-
	      ify  a  timezone	and TZ is unset or null, an unspecified default timezone shall be
	      used.

ASYNCHRONOUS EVENTS
       Default.

STDOUT
       When standard input is a terminal, prompts of unspecified format for each line of the user
       input described in the STDIN section may be written to standard output.

       In  the	POSIX  locale, the following shall be written to the standard output for each job
       when jobs are listed in response to the -l option:

	      "%s\t%s\n", at_job_id, <date>

       where date shall be equivalent in format to the output of:

	      date +"%a %b %e %T %Y"

       The date and time written shall be adjusted so that they appear in  the	timezone  of  the
       user (as determined by the TZ variable).

STDERR
       In  the POSIX locale, the following shall be written to standard error when a job has been
       successfully submitted:

	      "job %s at %s\n", at_job_id, <date>

       where date has the same format as that described in the STDOUT section. Neither this,  nor
       warning	messages concerning the selection of the command interpreter, shall be considered
       a diagnostic that changes the exit status.

       Diagnostic messages, if any, shall be written to standard error.

OUTPUT FILES
       None.

EXTENDED DESCRIPTION
       None.

EXIT STATUS
       The following exit values shall be returned:

	0     The at utility successfully submitted, removed, or listed a job or jobs.

       >0     An error occurred.

CONSEQUENCES OF ERRORS
       The job shall not be scheduled, removed, or listed.

       The following sections are informative.

APPLICATION USAGE
       The format of the at command line shown here is guaranteed  only  for  the  POSIX  locale.
       Other  cultures	may be supported with substantially different interfaces, although imple-
       mentations are encouraged to provide comparable levels of functionality.

       Since the commands run in a separate shell invocation, running in a separate process group
       with  no  controlling  terminal, open file descriptors, traps, and priority inherited from
       the invoking environment are lost.

       Some implementations do not allow substitution of different shells using SHELL . System	V
       systems,  for  example,	have  used  the login shell value for the user in /etc/passwd. To
       select reliably another command interpreter, the user must  include  it	as  part  of  the
       script, such as:

	      $ at 1800
	      myshell myscript
	      EOT
	      job ... at ...
	      $

EXAMPLES
	1. This sequence can be used at a terminal:

	   at -m 0730 tomorrow
	   sort < file >outfile
	   EOT

	2. This sequence, which demonstrates redirecting standard error to a pipe, is useful in a
	   command procedure (the sequence of output redirection specifications is significant):

	   at now + 1 hour <<!
	   diff file1 file2 2>&1 >outfile | mailx mygroup
	   !

	3. To have a job reschedule itself, at can be invoked from within the at-job.  For  exam-
	   ple, this daily processing script named my.daily runs every day (although crontab is a
	   more appropriate vehicle for such work):

	   # my.daily runs every day
	   daily processingat now tomorrow < my.daily

	4. The spacing of the three portions of the POSIX locale timespec is  quite  flexible  as
	   long  as there are no ambiguities.  Examples of various times and operand presentation
	   include:

	   at 0815am Jan 24
	   at 8 :15amjan24
	   at now "+ 1day"
	   at 5 pm FRIday
	   at '17
	       utc+
	       30minutes'

RATIONALE
       The at utility reads from standard input the commands to be executed at a later	time.  It
       may  be	useful	to  redirect standard output and standard error within the specified com-
       mands.

       The -t time option was added as a new capability to support an  internationalized  way  of
       specifying a time for execution of the submitted job.

       Early proposals added a "jobname" concept as a way of giving submitted jobs names that are
       meaningful to the user submitting them. The historical, system-specified  at_job_id  gives
       no indication of what the job is. Upon further reflection, it was decided that the benefit
       of this was not worth the change in historical interface. The at functionality  is  useful
       in  simple environments, but in large or complex situations, the functionality provided by
       the Batch Services option is more suitable.

       The -q option historically has been an undocumented option, used mainly by the batch util-
       ity.

       The  System  V  -m option was added to provide a method for informing users that an at-job
       had completed. Otherwise, users are only informed when output to standard error	or  stan-
       dard output are not redirected.

       The  behavior  of at <now> was changed in an early proposal from being unspecified to sub-
       mitting a job for potentially immediate execution. Historical BSD at implementations  sup-
       port this. Historical System V implementations give an error in that case, but a change to
       the System V versions should have no backwards-compatibility ramifications.

       On BSD-based systems, a -u user option has allowed those with  appropriate  privileges  to
       access  the  work  of other users. Since this is primarily a system administration feature
       and is not universally implemented, it has been omitted. Similarly,  a  specification  for
       the output format for a user with appropriate privileges viewing the queues of other users
       has been omitted.

       The -f file option from System V is used instead of the BSD method of using the last oper-
       and as the pathname. The BSD method is ambiguous-does:

	      at 1200 friday

       mean the same thing if there is a file named friday in the current directory?

       The  at_job_id  is  composed  of a limited character set in historical practice, and it is
       mandated here to invalidate systems that might try using  characters  that  require  shell
       quoting or that could not be easily parsed by shell scripts.

       The  at	utility varies between System V and BSD systems in the way timezones are used. On
       System V systems, the TZ variable affects the at-job submission times and the  times  dis-
       played  for  the  user.	On BSD systems, TZ is not taken into account. The BSD behavior is
       easily achieved with the current specification.	If the user wishes to have  the  timezone
       default	to  that of the system, they merely need to issue the at command immediately fol-
       lowing an unsetting or null assignment to TZ . For example:

	      TZ= at noon ...

       gives the desired BSD result.

       While the yacc-like grammar specified in the OPERANDS  section  is  lexically  unambiguous
       with  respect  to  the digit strings, a lexical analyzer would probably be written to look
       for and return digit strings in those cases. The parser could then check whether the digit
       string returned is a valid day_number, year_number, and so on, based on the context.

FUTURE DIRECTIONS
       None.

SEE ALSO
       batch , crontab

COPYRIGHT
       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable	Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
       the Institute of Electrical and Electronics Engineers, Inc and  The  Open  Group.  In  the
       event  of  any  discrepancy  between this version and the original IEEE and The Open Group
       Standard, the original IEEE and The Open Group Standard is the referee document. The orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					    AT(P)
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