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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 |
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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.1‐2017, 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_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.1‐2017, 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.
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.1‐2017, Chapter 8,
Environment Variables, Section 12.2, Utility Syntax Guidelines
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, 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(1P)
Pages that refer to this page: batch(1p), crontab(1p)
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HTML rendering created 2023-12-22 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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