Ubuntu Manpage: at - execute commands at a later time

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 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>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 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 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
;

date : month_name day_number
| month_name day_number "," year_number
| day_of_week
| "today"
| "tomorrow"
;

increment : "+" inc_number inc_period
| "next" inc_period
;

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 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 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 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.

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)