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AT(1P)			   POSIX Programmer's Manual			AT(1P)

PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the	 corresponding
       Linux  manual page for details of Linux behavior), or the interface may
       not be implemented on Linux.

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 standard 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, Section 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
	      produces 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 time‐
	      spec arguments, the results are unspecified.

       -r     Remove  the jobs with the specified at_job_id operands that were
	      previously scheduled 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 for‐
	      mat as specified by the touch -t time utility.

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 operands 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	 cate‐
       gories  in  the	POSIX locale shall be recognized in a case-insensitive
       manner.

       time
	      The time can be specified as one, two, or four digits. One-digit
	      and  two-digit  numbers  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 sup‐
	      ported to indicate the time is in Coordinated Universal Time. In
	      the  POSIX locale, the time field can also be one of the follow‐
	      ing 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 execu‐
		     tion (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 suffixed 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 com‐
	      mands:

		     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 prece‐
       dence  over the preceding text syntax description. The longest possible
       token or delimiter shall be recognized at a given point. When used in a
       timespec, 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 accept‐
       able to the shell command language described in Shell Command  Language
       .  The standard input shall only be used if no -f file option is speci‐
       fied.

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	utili‐
       ties.

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,  Internationalization Vari‐
	      ables 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 internationalization 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 arguments and input files).

       LC_MESSAGES
	      Determine the locale that should be used to  affect  the	format
	      and  contents  of	 diagnostic messages written to standard error
	      and informative messages written to standard 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  implemen‐
	      tation  shall  do	 one of the following: use that shell; use sh;
	      use the login shell from the user database; or any of  the  pre‐
	      ceding  accompanied by a warning diagnostic about which was cho‐
	      sen.

       TZ     Determine the timezone. The job shall be submitted for execution
	      at  the  time  specified	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 specify a
	      timezone and TZ is unset or null, an unspecified	default	 time‐
	      zone 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 writ‐
       ten 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 com‐
       mand 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 dif‐
       ferent interfaces, although implementations 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 descrip‐
       tors,  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  re‐
	   direction 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 example, 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 commands.

       The -t time option was added as a new capability to support an interna‐
       tionalized 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 histor‐
       ical,  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 suit‐
       able.

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

       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 standard output are not redirected.

       The  behavior  of  at <now> was changed in an early proposal from being
       unspecified to submitting a job for  potentially	 immediate  execution.
       Historical  BSD	at  implementations  support 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 appropri‐
       ate privileges to access the work of other users. Since this is primar‐
       ily 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	 operand 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 time‐
       zones are used. On System V systems, the TZ variable affects the at-job
       submission times and the times displayed 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 following an unsetting or null assignment to TZ . For exam‐
       ple:

	      TZ= at noon ...

       gives the desired BSD result.

       While  the yacc-like grammar specified in the OPERANDS section is lexi‐
       cally 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 original Standard can be  obtained	online
       at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group		     2003				AT(1P)
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