phpMan

/* Void Main's man pages */

{ phpMan } else { main(); }

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 (con-
sult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on
Linux.

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 control-
ling terminal, except that the environment variables, current working directory, file creation mask, and other implemen-
tation-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 Guide-
lines.

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 fol-
low 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 exe-
cution (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 time-
spec, 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 international-
ization 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 stan-
dard 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 follow-
ing: 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 rel-
ative 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 func-
tionality.

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 stan-
dard 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 submit-
ting 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. Simi-
larly, 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 sys-
tems 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 vari-
able 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.