mirror of https://github.com/mkerrisk/man-pages
707 lines
21 KiB
Groff
707 lines
21 KiB
Groff
.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
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.TH "AT" 1P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
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.\" at
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.SH NAME
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at \- execute commands at a later time
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.SH SYNOPSIS
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.LP
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\fBat\fP \fB[\fP\fB-m\fP\fB][\fP\fB-f\fP \fIfile\fP\fB][\fP\fB-q\fP
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\fIqueuename\fP\fB]\fP \fB-t\fP \fItime_arg\fP\fB
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.br
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.sp
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at\fP \fB[\fP\fB-m\fP\fB][\fP\fB-f\fP \fIfile\fP\fB][\fP\fB-q\fP \fIqueuename\fP\fB]\fP
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\fItimespec\fP
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\fB\&...
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.br
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.sp
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at -r\fP \fIat_job_id\fP \fB...
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.br
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.sp
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at -l -q\fP \fIqueuename\fP\fB
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.br
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.sp
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at -l\fP \fB[\fP\fIat_job_id\fP \fB...\fP\fB]\fP\fB\fP
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\fB
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.br
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\fP
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.SH DESCRIPTION
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.LP
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The \fIat\fP utility shall read commands from standard input and group
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them together as an \fIat-job\fP, to be executed at a
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later time.
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.LP
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The at-job shall be executed in a separate invocation of the shell,
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running in a separate process group with no controlling
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terminal, except that the environment variables, current working directory,
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file creation mask, and other implementation-defined
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execution-time attributes in effect when the \fIat\fP utility is executed
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shall be retained and used when the at-job is
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executed.
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.LP
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When the at-job is submitted, the \fIat_job_id\fP and scheduled time
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shall be written to standard error. The \fIat_job_id\fP
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is an identifier that shall be a string consisting solely of alphanumeric
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characters and the period character. The \fIat_job_id\fP
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shall be assigned by the system when the job is scheduled such that
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it uniquely identifies a particular job.
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.LP
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User notification and the processing of the job's standard output
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and standard error are described under the \fB-m\fP
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option.
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.LP
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Users shall be permitted to use \fIat\fP if their name appears in
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the file \fB/usr/lib/cron/at.allow\fP. If that file does not
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exist, the file \fB/usr/lib/cron/at.deny\fP shall be checked to determine
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whether the user shall be denied access to \fIat\fP. If
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neither file exists, only a process with the appropriate privileges
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shall be allowed to submit a job. If only \fBat.deny\fP exists
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and is empty, global usage shall be permitted. The \fBat.allow\fP
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and \fBat.deny\fP files shall consist of one user name per
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line.
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.SH OPTIONS
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.LP
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The \fIat\fP utility shall conform to the Base Definitions volume
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of IEEE\ Std\ 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
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.LP
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The following options shall be supported:
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.TP 7
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\fB-f\ \fP \fIfile\fP
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Specify the pathname of a file to be used as the source of the at-job,
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instead of standard input.
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.TP 7
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\fB-l\fP
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(The letter ell.) Report all jobs scheduled for the invoking user
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if no \fIat_job_id\fP operands are specified. If
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\fIat_job_id\fPs are specified, report only information for these
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jobs. The output shall be written to standard output.
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.TP 7
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\fB-m\fP
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Send mail to the invoking user after the at-job has run, announcing
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its completion. Standard output and standard error produced
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by the at-job shall be mailed to the user as well, unless redirected
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elsewhere. Mail shall be sent even if the job produces no
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output.
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.LP
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If \fB-m\fP is not used, the job's standard output and standard error
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shall be provided to the user by means of mail, unless
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they are redirected elsewhere; if there is no such output to provide,
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the implementation need not notify the user of the job's
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completion.
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.TP 7
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\fB-q\ \fP \fIqueuename\fP
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.sp
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Specify in which queue to schedule a job for submission. When used
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with the \fB-l\fP option, limit the search to that particular
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queue. By default, at-jobs shall be scheduled in queue \fIa\fP. In
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contrast, queue \fIb\fP shall be reserved for batch jobs; see
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\fIbatch\fP. The meanings of all other \fIqueuename\fPs are implementation-defined.
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If
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\fB-q\fP is specified along with either of the \fB-t\fP \fItime_arg\fP
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or \fItimespec\fP arguments, the results are
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unspecified.
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.TP 7
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\fB-r\fP
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Remove the jobs with the specified \fIat_job_id\fP operands that were
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previously scheduled by the \fIat\fP utility.
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.TP 7
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\fB-t\ \fP \fItime_arg\fP
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Submit the job to be run at the time specified by the \fItime\fP option-argument,
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which the application shall ensure has the
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format as specified by the \fItouch\fP \fB-t\fP \fItime\fP utility.
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.sp
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.SH OPERANDS
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.LP
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The following operands shall be supported:
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.TP 7
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\fIat_job_id\fP
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The name reported by a previous invocation of the \fIat\fP utility
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at the time the job was scheduled.
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.TP 7
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\fItimespec\fP
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Submit the job to be run at the date and time specified. All of the
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\fItimespec\fP operands are interpreted as if they were
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separated by <space>s and concatenated, and shall be parsed as described
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in the grammar at the end of this section. The date
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and time shall be interpreted as being in the timezone of the user
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(as determined by the \fITZ\fP variable), unless a timezone
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name appears as part of \fItime\fP, below.
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.LP
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In the POSIX locale, the following describes the three parts of the
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time specification string. All of the values from the
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\fILC_TIME\fP categories in the POSIX locale shall be recognized in
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a case-insensitive manner.
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.TP 7
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\fItime\fP
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.RS
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The time can be specified as one, two, or four digits. One-digit and
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two-digit numbers shall be taken to be hours; four-digit
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numbers to be hours and minutes. The time can alternatively be specified
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as two numbers separated by a colon, meaning
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\fIhour\fP:\fIminute\fP. An AM/PM indication (one of the values from
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the \fBam_pm\fP keywords in the \fILC_TIME\fP locale
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category) can follow the time; otherwise, a 24-hour clock time shall
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be understood. A timezone name can also follow to further
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qualify the time. The acceptable timezone names are implementation-defined,
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except that they shall be case-insensitive and the
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string \fButc\fP is supported to indicate the time is in Coordinated
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Universal Time. In the POSIX locale, the \fItime\fP field
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can also be one of the following tokens:
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.TP 7
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\fBmidnight\fP
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.RS
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Indicates the time 12:00 am (00:00).
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.RE
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.TP 7
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\fBnoon\fP
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.RS
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Indicates the time 12:00 pm.
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.RE
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.TP 7
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\fBnow\fP
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.RS
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Indicates the current day and time. Invoking \fIat\fP <\fBnow\fP>
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shall submit an at-job for potentially immediate
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execution (that is, subject only to unspecified scheduling delays).
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.RE
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.sp
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.RE
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.TP 7
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\fIdate\fP
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.RS
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An optional \fIdate\fP can be specified as either a month name (one
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of the values from the \fBmon\fP or \fBabmon\fP keywords
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in the \fILC_TIME\fP locale category) followed by a day number (and
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possibly year number preceded by a comma), or a day of the
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week (one of the values from the \fBday\fP or \fBabday\fP keywords
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in the \fILC_TIME\fP locale category). In the POSIX locale,
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two special days shall be recognized:
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.TP 7
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\fBtoday\fP
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.RS
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Indicates the current day.
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.RE
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.TP 7
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\fBtomorrow\fP
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.RS
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Indicates the day following the current day.
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.RE
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.sp
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.LP
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If no \fIdate\fP is given, \fBtoday\fP shall be assumed if the given
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time is greater than the current time, and
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\fBtomorrow\fP shall be assumed if it is less. If the given month
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is less than the current month (and no year is given), next year
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shall be assumed.
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.RE
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.TP 7
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\fIincrement\fP
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.RS
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The optional \fIincrement\fP shall be a number preceded by a plus
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sign ( \fB'+'\fP ) and suffixed by one of the following:
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\fBminutes\fP, \fBhours\fP, \fBdays\fP, \fBweeks\fP, \fBmonths\fP,
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or \fByears\fP. (The singular forms shall also be
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accepted.) The keyword \fBnext\fP shall be equivalent to an increment
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number of +1. For example, the following are equivalent
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commands:
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.sp
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.RS
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.nf
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\fBat 2pm + 1 week
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at 2pm next week
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\fP
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.fi
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.RE
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.RE
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.sp
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.sp
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.LP
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The following grammar describes the precise format of \fItimespec\fP
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in the POSIX locale. The general conventions for this
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style of grammar are described in \fIGrammar Conventions\fP . This
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formal syntax shall
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take precedence over the preceding text syntax description. The longest
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possible token or delimiter shall be recognized at a given
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point. When used in a \fItimespec\fP, white space shall also delimit
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tokens.
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.sp
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.RS
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.nf
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\fB%token hr24clock_hr_min
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%token hr24clock_hour
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/*
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An hr24clock_hr_min is a one, two, or four-digit number. A one-digit
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or two-digit number constitutes an hr24clock_hour. An hr24clock_hour
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may be any of the single digits [0,9], or may be double digits, ranging
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from [00,23]. If an hr24clock_hr_min is a four-digit number, the
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first two digits shall be a valid hr24clock_hour, while the last two
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represent the number of minutes, from [00,59].
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*/
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.sp
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%token wallclock_hr_min
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%token wallclock_hour
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/*
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A wallclock_hr_min is a one, two-digit, or four-digit number.
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A one-digit or two-digit number constitutes a wallclock_hour.
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A wallclock_hour may be any of the single digits [1,9], or may
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be double digits, ranging from [01,12]. If a wallclock_hr_min
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is a four-digit number, the first two digits shall be a valid
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wallclock_hour, while the last two represent the number of
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minutes, from [00,59].
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*/
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.sp
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%token minute
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/*
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A minute is a one or two-digit number whose value can be [0,9]
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or [00,59].
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*/
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.sp
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%token day_number
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/*
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A day_number is a number in the range appropriate for the particular
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month and year specified by month_name and year_number, respectively.
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If no year_number is given, the current year is assumed if the given
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date and time are later this year. If no year_number is given and
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the date and time have already occurred this year and the month is
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not the current month, next year is the assumed year.
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*/
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.sp
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%token year_number
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/*
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A year_number is a four-digit number representing the year A.D., in
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which the at_job is to be run.
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*/
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.sp
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%token inc_number
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/*
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The inc_number is the number of times the succeeding increment
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period is to be added to the specified date and time.
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*/
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.sp
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%token timezone_name
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/*
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The name of an optional timezone suffix to the time field, in an
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implementation-defined format.
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*/
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.sp
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%token month_name
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/*
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One of the values from the mon or abmon keywords in the LC_TIME
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locale category.
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*/
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.sp
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%token day_of_week
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/*
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One of the values from the day or abday keywords in the LC_TIME
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locale category.
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*/
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.sp
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%token am_pm
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/*
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One of the values from the am_pm keyword in the LC_TIME locale
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category.
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*/
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.sp
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%start timespec
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%%
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timespec : time
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| time date
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| time increment
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| time date increment
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| nowspec
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;
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.sp
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nowspec : "now"
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| "now" increment
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;
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.sp
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time : hr24clock_hr_min
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| hr24clock_hr_min timezone_name
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| hr24clock_hour ":" minute
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| hr24clock_hour ":" minute timezone_name
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| wallclock_hr_min am_pm
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| wallclock_hr_min am_pm timezone_name
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| wallclock_hour ":" minute am_pm
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| wallclock_hour ":" minute am_pm timezone_name
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| "noon"
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| "midnight"
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;
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.sp
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date : month_name day_number
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| month_name day_number "," year_number
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| day_of_week
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| "today"
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| "tomorrow"
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;
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.sp
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increment : "+" inc_number inc_period
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| "next" inc_period
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;
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.sp
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inc_period : "minute" | "minutes"
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| "hour" | "hours"
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| "day" | "days"
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| "week" | "weeks"
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| "month" | "months"
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| "year" | "years"
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;
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\fP
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.fi
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.RE
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.SH STDIN
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.LP
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The standard input shall be a text file consisting of commands acceptable
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to the shell command language described in \fIShell Command Language\fP
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\&. The standard input shall only be used if no \fB-f\fP \fIfile\fP
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option is specified.
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.SH INPUT FILES
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.LP
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See the STDIN section.
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.LP
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The text files \fB/usr/lib/cron/at.allow\fP and \fB/usr/lib/cron/at.deny\fP
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shall contain zero or more user names, one per line,
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of users who are, respectively, authorized or denied access to the
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\fIat\fP and \fIbatch\fP
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utilities.
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.SH ENVIRONMENT VARIABLES
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.LP
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The following environment variables shall affect the execution of
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\fIat\fP:
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.TP 7
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\fILANG\fP
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Provide a default value for the internationalization variables that
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are unset or null. (See the Base Definitions volume of
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IEEE\ Std\ 1003.1-2001, Section 8.2, Internationalization Variables
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for
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the precedence of internationalization variables used to determine
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the values of locale categories.)
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.TP 7
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\fILC_ALL\fP
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If set to a non-empty string value, override the values of all the
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other internationalization variables.
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.TP 7
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\fILC_CTYPE\fP
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Determine the locale for the interpretation of sequences of bytes
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of text data as characters (for example, single-byte as
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opposed to multi-byte characters in arguments and input files).
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.TP 7
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\fILC_MESSAGES\fP
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Determine the locale that should be used to affect the format and
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contents of diagnostic messages written to standard error and
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informative messages written to standard output.
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.TP 7
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\fINLSPATH\fP
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Determine the location of message catalogs for the processing of \fILC_MESSAGES
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\&.\fP
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.TP 7
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\fILC_TIME\fP
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Determine the format and contents for date and time strings written
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and accepted by \fIat\fP.
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.TP 7
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\fISHELL\fP
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Determine a name of a command interpreter to be used to invoke the
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at-job. If the variable is unset or null, \fIsh\fP shall be used.
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If it is set to a value other than a name for \fIsh\fP, the implementation
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shall do one of the following: use that shell; use \fIsh\fP; use the
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login shell from the user database; or any of the preceding accompanied
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by a warning
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diagnostic about which was chosen.
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.TP 7
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\fITZ\fP
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Determine the timezone. The job shall be submitted for execution at
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the time specified by \fItimespec\fP or \fB-t\fP
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\fItime\fP relative to the timezone specified by the \fITZ\fP variable.
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If \fItimespec\fP specifies a timezone, it shall
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override \fITZ .\fP If \fItimespec\fP does not specify a timezone
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and \fITZ\fP is unset or null, an unspecified default timezone
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shall be used.
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.sp
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.SH ASYNCHRONOUS EVENTS
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.LP
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Default.
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.SH STDOUT
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.LP
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When standard input is a terminal, prompts of unspecified format for
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each line of the user input described in the STDIN section
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may be written to standard output.
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.LP
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|
In the POSIX locale, the following shall be written to the standard
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output for each job when jobs are listed in response to the
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\fB-l\fP option:
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.sp
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.RS
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.nf
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\fB"%s\\t%s\\n",\fP \fIat_job_id\fP\fB, <\fP\fIdate\fP\fB>
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\fP
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.fi
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.RE
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.LP
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where \fIdate\fP shall be equivalent in format to the output of:
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.sp
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.RS
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.nf
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\fBdate +"%a %b %e %T %Y"
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\fP
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.fi
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.RE
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.LP
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The date and time written shall be adjusted so that they appear in
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the timezone of the user (as determined by the \fITZ\fP
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variable).
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.SH STDERR
|
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.LP
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In the POSIX locale, the following shall be written to standard error
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when a job has been successfully submitted:
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.sp
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.RS
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.nf
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\fB"job %s at %s\\n",\fP \fIat_job_id\fP\fB, <\fP\fIdate\fP\fB>
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\fP
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.fi
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.RE
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.LP
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where \fIdate\fP has the same format as that described in the STDOUT
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section. Neither this, nor warning messages concerning the
|
|
selection of the command interpreter, shall be considered a diagnostic
|
|
that changes the exit status.
|
|
.LP
|
|
Diagnostic messages, if any, shall be written to standard error.
|
|
.SH OUTPUT FILES
|
|
.LP
|
|
None.
|
|
.SH EXTENDED DESCRIPTION
|
|
.LP
|
|
None.
|
|
.SH EXIT STATUS
|
|
.LP
|
|
The following exit values shall be returned:
|
|
.TP 7
|
|
\ 0
|
|
The \fIat\fP utility successfully submitted, removed, or listed a
|
|
job or jobs.
|
|
.TP 7
|
|
>0
|
|
An error occurred.
|
|
.sp
|
|
.SH CONSEQUENCES OF ERRORS
|
|
.LP
|
|
The job shall not be scheduled, removed, or listed.
|
|
.LP
|
|
\fIThe following sections are informative.\fP
|
|
.SH APPLICATION USAGE
|
|
.LP
|
|
The format of the \fIat\fP 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.
|
|
.LP
|
|
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.
|
|
.LP
|
|
Some implementations do not allow substitution of different shells
|
|
using \fISHELL .\fP System V systems, for example, have used
|
|
the login shell value for the user in \fB/etc/passwd\fP. To select
|
|
reliably another command interpreter, the user must include it
|
|
as part of the script, such as:
|
|
.sp
|
|
.RS
|
|
.nf
|
|
|
|
\fB$\fP \fBat 1800
|
|
myshell myscript
|
|
EOT
|
|
\fP\fBjob ... at ...
|
|
$\fP
|
|
.fi
|
|
.RE
|
|
.SH EXAMPLES
|
|
.IP " 1." 4
|
|
This sequence can be used at a terminal:
|
|
.sp
|
|
.RS
|
|
.nf
|
|
|
|
\fBat -m 0730 tomorrow
|
|
sort < file >outfile
|
|
EOT
|
|
\fP
|
|
.fi
|
|
.RE
|
|
.LP
|
|
.IP " 2." 4
|
|
This sequence, which demonstrates redirecting standard error to a
|
|
pipe, is useful in a command procedure (the sequence of output
|
|
redirection specifications is significant):
|
|
.sp
|
|
.RS
|
|
.nf
|
|
|
|
\fBat now + 1 hour <<!
|
|
diff file1 file2 2>&1 >outfile | mailx mygroup
|
|
!
|
|
\fP
|
|
.fi
|
|
.RE
|
|
.LP
|
|
.IP " 3." 4
|
|
To have a job reschedule itself, \fIat\fP can be invoked from within
|
|
the at-job. For example, this daily processing script
|
|
named \fBmy.daily\fP runs every day (although \fIcrontab\fP is a more
|
|
appropriate vehicle
|
|
for such work):
|
|
.sp
|
|
.RS
|
|
.nf
|
|
|
|
\fB# my.daily runs every day
|
|
\fP\fIdaily processing\fP\fBat now tomorrow < my.daily
|
|
\fP
|
|
.fi
|
|
.RE
|
|
.LP
|
|
.IP " 4." 4
|
|
The spacing of the three portions of the POSIX locale \fItimespec\fP
|
|
is quite flexible as long as there are no ambiguities.
|
|
Examples of various times and operand presentation include:
|
|
.sp
|
|
.RS
|
|
.nf
|
|
|
|
\fBat 0815am Jan 24
|
|
at 8 :15amjan24
|
|
at now "+ 1day"
|
|
at 5 pm FRIday
|
|
at '17
|
|
utc+
|
|
30minutes'
|
|
\fP
|
|
.fi
|
|
.RE
|
|
.LP
|
|
.SH RATIONALE
|
|
.LP
|
|
The \fIat\fP 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.
|
|
.LP
|
|
The \fB-t\fP \fItime\fP option was added as a new capability to support
|
|
an internationalized way of specifying a time for
|
|
execution of the submitted job.
|
|
.LP
|
|
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 \fIat_job_id\fP 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 \fIat\fP functionality is useful in simple
|
|
environments, but in large or complex situations, the functionality
|
|
provided by the Batch Services option is more suitable.
|
|
.LP
|
|
The \fB-q\fP option historically has been an undocumented option,
|
|
used mainly by the \fIbatch\fP utility.
|
|
.LP
|
|
The System V \fB-m\fP 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.
|
|
.LP
|
|
The behavior of \fIat\fP <\fBnow\fP> was changed in an early proposal
|
|
from being unspecified to submitting a job for
|
|
potentially immediate execution. Historical BSD \fIat\fP 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.
|
|
.LP
|
|
On BSD-based systems, a \fB-u\fP \fIuser\fP 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.
|
|
.LP
|
|
The \fB-f\fP \fIfile\fP 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:
|
|
.sp
|
|
.RS
|
|
.nf
|
|
|
|
\fBat 1200 friday
|
|
\fP
|
|
.fi
|
|
.RE
|
|
.LP
|
|
mean the same thing if there is a file named \fBfriday\fP in the current
|
|
directory?
|
|
.LP
|
|
The \fIat_job_id\fP 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.
|
|
.LP
|
|
The \fIat\fP utility varies between System V and BSD systems in the
|
|
way timezones are used. On System V systems, the \fITZ\fP
|
|
variable affects the at-job submission times and the times displayed
|
|
for the user. On BSD systems, \fITZ\fP 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 \fIat\fP command
|
|
immediately following an unsetting or null assignment to \fITZ
|
|
\&.\fP For example:
|
|
.sp
|
|
.RS
|
|
.nf
|
|
|
|
\fBTZ= at noon ...
|
|
\fP
|
|
.fi
|
|
.RE
|
|
.LP
|
|
gives the desired BSD result.
|
|
.LP
|
|
While the \fIyacc\fP-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 \fIday_number\fP, \fIyear_number\fP, and so
|
|
on, based on the context.
|
|
.SH FUTURE DIRECTIONS
|
|
.LP
|
|
None.
|
|
.SH SEE ALSO
|
|
.LP
|
|
\fIbatch\fP , \fIcrontab\fP
|
|
.SH COPYRIGHT
|
|
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 .
|