at - execute commands at a later time
at [ -c ] [ -s ] [ -m ] time [ day ] [ file ]
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
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 under-
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).
/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))
atq(1), atrm(1), calendar(1), sleep(1), cron(8)
Complains about various syntax errors and times out of range.
Due to the granularity of the execution of /usr/libexec/atrun, there may be bugs in sched-
uling 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
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)