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 .