AT(1P) POSIX Programmer's Manual AT(1P)PROLOG
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.
NAMEat - execute commands at a later time
SYNOPSISat [-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
/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.
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:
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.
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 time‐
spec arguments, the results are unspecified.
-r Remove the jobs with the specified at_job_id operands that were
previously scheduled by the at utility.
Submit the job to be run at the time specified by the time
option-argument, which the application shall ensure has the for‐
mat as specified by the touch -t time utility.
The following operands shall be supported:
The name reported by a previous invocation of the at utility at
the time the job was scheduled.
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 cate‐
gories in the POSIX locale shall be recognized in a case-insensitive
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 sup‐
ported to indicate the time is in Coordinated Universal Time. In
the POSIX locale, the time field can also be one of the follow‐
Indicates the time 12:00 am (00:00).
Indicates the time 12:00 pm.
Indicates the current day and time. Invoking at <now>
shall submit an at-job for potentially immediate execu‐
tion (that is, subject only to unspecified scheduling
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:
Indicates the current day.
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.
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 com‐
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 prece‐
dence 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.
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].
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].
A minute is a one or two-digit number whose value can be [0,9]
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.
A year_number is a four-digit number representing the year A.D., in
which the at_job is to be run.
The inc_number is the number of times the succeeding increment
period is to be added to the specified date and time.
The name of an optional timezone suffix to the time field, in an
One of the values from the mon or abmon keywords in the LC_TIME
One of the values from the day or abday keywords in the LC_TIME
One of the values from the am_pm keyword in the LC_TIME locale
timespec : time
| time date
| time increment
| time date increment
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
date : month_name day_number
| month_name day_number "," year_number
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 accept‐
able to the shell command language described in Shell Command Language
. The standard input shall only be used if no -f file option is speci‐
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 utili‐
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 Vari‐
ables 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.
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).
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.
Determine the location of message catalogs for the processing of
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 implemen‐
tation shall do one of the following: use that shell; use sh;
use the login shell from the user database; or any of the pre‐
ceding accompanied by a warning diagnostic about which was cho‐
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 time‐
zone shall be used.
When standard input is a terminal, prompts of unspecified format for
each line of the user input described in the STDIN section may be writ‐
ten 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 com‐
mand interpreter, shall be considered a diagnostic that changes the
Diagnostic messages, if any, shall be written to standard error.
The following exit values shall be returned:
0 The at utility successfully submitted, removed, or listed a job
>0 An error occurred.
CONSEQUENCES OF ERRORS
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 dif‐
ferent 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 descrip‐
tors, traps, and priority inherited from the invoking environment are
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
job ... at ...
1. This sequence can be used at a terminal:
at-m 0730 tomorrow
sort < file >outfile
2. This sequence, which demonstrates redirecting standard error to a
pipe, is useful in a command procedure (the sequence of output re‐
direction 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
# 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
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 interna‐
tionalized 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 histor‐
ical, 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 suit‐
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 appropri‐
ate privileges to access the work of other users. Since this is primar‐
ily 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-
at 1200 friday
mean the same thing if there is a file named friday in the current
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 time‐
zones 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 exam‐
TZ= at noon ...
gives the desired BSD result.
While the yacc-like grammar specified in the OPERANDS section is lexi‐
cally 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.
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(1P)