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Test Your Knowledge in Computers #503
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The flow of control (or simply control flow) is how a program processes its instructions.
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atrun(8) [freebsd man page]

ATRUN(8)						    BSD System Manager's Manual 						  ATRUN(8)

atrun -- run jobs queued for later execution SYNOPSIS
atrun [-l load_avg] [-d] DESCRIPTION
Atrun runs jobs queued by at(1). Root's crontab(5) file /etc/crontab has to contain the line */5 * * * * root /usr/libexec/atrun so that atrun gets invoked every five minutes. At every invocation, atrun will start all the jobs in the lowercase queues whose start time has elapsed. In addition, if the load average over the last minute was less than the specified limit then a maximum of one batch job (denoted by the uppercase queues) is started. Before starting a job, atrun will check the status of its owner's account with pam(3) and refuse to run the job if the account is unavail- able, e.g., locked out or expired. OPTIONS
-l load_avg Specify a limiting load factor, over which batch jobs should not be run, instead of the default of 1.5 * number of active CPUs. -d Debug; print error messages to standard error instead of using syslog(3). WARNINGS
For atrun to work, you have to start up a cron(8) daemon. FILES
/etc/pam.d/atrun pam.conf(5) configuration file for atrun /var/at/jobs Directory containing job files /var/at/spool Directory containing output spool files SEE ALSO
at(1), crontab(1), pam(3), syslog(3), crontab(5), pam.conf(5), cron(8) BUGS
The functionality of atrun should be merged into cron(8). BSD
October 30, 2012 BSD

Check Out this Related Man Page

AT(1)							      General Commands Manual							     AT(1)

at - execute commands at a later time SYNOPSIS
at [ -c ] [ -s ] [ -m ] time [ day ] [ file ] DESCRIPTION
At spools away a copy of the named file to be used as input to sh(1) or csh(1). If the -c flag (for (csh(1))) or the -s flag (for (sh(1))) is specified, then that shell will be used to execute the job; if no shell is specified, the current environment shell is used. If no file name is specified, at prompts for commands from standard input until a ^D is typed. If the -m flag is specified, mail will be sent to the user after the job has been run. If errors occur during execution of the job, then a copy of the error diagnostics will be sent to the user. If no errors occur, then a short message is sent informing the user that no errors occurred. The format of the spool file is as follows: A four line header that includes the owner of the job, the name of the job, the shell used to run the job, and whether mail will be set after the job is executed. The header is followed by a cd command to the current directory and a umask command to set the modes on any files created by the job. Then at copies all relevant environment variables to the spool file. When the script is run, it uses the user and group ID of the creator of the spool file. The time is 1 to 4 digits, with an optional following `A', `P', `N' or `M' for AM, PM, noon or midnight. One and two digit numbers are taken to be hours, three and four digits to be hours and minutes. If no letters follow the digits, a 24 hour clock time is understood. The optional day is either (1) a month name followed by a day number, or (2) a day of the week; if the word `week' follows, invocation is moved seven days further off. Names of months and days may be recognizably truncated. Examples of legitimate commands are at 8am jan 24 at -c -m 1530 fr week at -s -m 1200n week At programs are executed by periodic execution of the command /usr/libexec/atrun from cron(8). The granularity of at depends upon the how often atrun is executed. Error output is lost unless redirected or the -m flag is requested, in which case a copy of the errors is sent to the user via mail(1). FILES
/usr/spool/at spooling area /usr/spool/at/yy.ddd.hhhh.* job file /usr/spool/at/past directory where jobs are executed from /usr/spool/at/lasttimedone last time atrun was run /usr/libexec/atrun executor (run by cron(8)) SEE ALSO
atq(1), atrm(1), calendar(1), sleep(1), cron(8) DIAGNOSTICS
Complains about various syntax errors and times out of range. BUGS
Due to the granularity of the execution of /usr/libexec/atrun, there may be bugs in scheduling things almost exactly 24 hours into the future. If the system crashes, mail is not sent to the user informing them that the job was not completed. Sometimes old spool files are not removed from the directory /usr/spool/at/past. This is usually due to a system crash, and requires that they be removed by hand. 4th Berkeley Distribution October 21, 1996 AT(1)

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