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PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
AT(1P) POSIX Programmer's Manual AT(1P)
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.
at — execute commands at a later time
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...]
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.
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 touch −t time utility.
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"
;
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.
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.
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.
Default.
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).
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.
None.
None.
The following exit values shall be returned:
0 The at utility successfully submitted, removed, or listed a job
or jobs.
>0 An error occurred.
The job shall not be scheduled, removed, or listed.
The following sections are informative.
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 ...
$
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'
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.
None.
batch(1p), crontab(1p)
The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment
Variables, Section 12.2, Utility Syntax Guidelines
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 .
IEEE/The Open Group 2013 AT(1P)
Pages that refer to this page: batch(1p), crontab(1p)