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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 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(P)
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