Originální popis anglicky:
at - execute commands at a later time
Návod, kniha: POSIX Programmer's Manual
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
/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:
- -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>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
;
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
Shell Command Language . The
standard input shall only be used if no
-f file option is
specified.
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.
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.
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:
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 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'
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:
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:
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 ,
crontab
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
.
