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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 dif-
       fer (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 iden-
       tifies 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 con-
       sist 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 Syn-
       tax 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  speci-
              fied.  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 unspeci-
              fied.

       -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 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 val-
       ues 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  key-
              words  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 fol-
              lowing: 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 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 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, Internationalization Variables for the prece-
              dence 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 exam-
              ple, 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  writ-
              ten 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  submit-
       ted:


              "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 con-
       cerning the selection of the command interpreter, shall be considered a diagnostic that changes the  exit  sta-
       tus.

       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  compara-
       ble 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 example, this daily process-
           ing 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 redi-
       rect 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. Other-
       wise, 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 implemen-
       tations give an error in that case, but a change to the System V versions should have no  backwards-compatibil-
       ity 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 invali-
       date 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

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard  for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifica-
       tions 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 Stan-
       dard, 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)