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       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
       at.allow which is located in an implementation-defined  directory.   If
       that  file  does  not  exist,  the file at.deny, which is located in an
       implementation-defined directory, shall be checked to determine whether
       the  user shall be denied access to at.  If neither file exists, only a
       process with 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
       POSIX.1‐2008, 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 timespec 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
                 format as specified by the toucht 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> characters 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 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 supported to indicate the
                           time is in  Coordinated  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 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 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 Section 1.3,  Grammar  Conventions.   This  formal  syntax
       shall  take  precedence 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
       acceptable to the shell command language described in Chapter 2,  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  at.allow  and  at.deny,  which  are  located  in   an
       implementation-defined  directory,  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  POSIX.1‐2008,  Section  8.2, Internationalization
                 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 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 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  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 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  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
       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:

               atm 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  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 processing
               at 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
       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 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
       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  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
       timezones  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 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

       The Base Definitions volume of  POSIX.1‐2008,  Chapter  8,  Environment
       Variables, Section 12.2, Utility Syntax Guidelines

COPYRIGHT

       Portions  of  this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       --  Portable  Operating  System  Interface (POSIX), The Open Group Base
       Specifications  Issue  7,  Copyright  (C)  2013  by  the  Institute  of
       Electrical and Electronics Engineers, Inc and The Open Group.  (This is
       POSIX.1-2008 with the 2013 Technical Corrigendum  1  applied.)  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.unix.org/online.html .

       Any typographical or formatting errors that appear  in  this  page  are
       most likely to have been introduced during the conversion of the source
       files   to   man   page   format.   To   report   such   errors,    see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .