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



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

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.1-2017, 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___i_ds
                 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 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> 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.1-2017, 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:


               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 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.1-2017, 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 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       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 .



IEEE/The Open Group                  2017                                AT(1)
