Run Commands at Specified Times
Usage: cron [-asLwW!h-] [-o<opt>] [-d <dir>]
[ crontab1 crontab2 ... ] |
-install [-N <name>] [-D <dispname>]
[-u <user>] [-p <password>]
[-H <directory] [-MI] <arguments> |
-start [-N <name>] [-D <dispname>] <arguments> |
-stop [-N <name>] [-D <dispname>] |
-pause [-N <name>] [-D <dispname>] |
-continue [-N <name>] [-D <dispname>] |
-delete [-N <name>] [-D <dispname>] |
-status [-N <name>] [-D <dispname>]
cron executes commands at times according to the crontab
(.crt) files specifed. If any path specified is a directory,
cron will run any crontab files in that directory or any
subdirectory. If no files are specified, cron will read
the crontab from stdin.
Each file is expected to contain a list of activities to be
run at certain times, specified one per line. There are six
fields per line, separated by white space. The first five
specify:
minute (0-59)
hour (0-23)
day of the month (1-31)
month of the year (1-12)
day of the week (0-6, 0 is Sunday)
Each of these fields may be an asterisk (meaning all legal
values) or a comma-separated list of elements, where each
element is either a single number or a range, given as two
numbers separated by a minus. If either end of a range is
omitted, it defaults to the highest/lowest legal value, as
appropriate. If the days are specified in both day of the
month and day of the week fields, the union of those fields
is used; to specify the days by only one field, the other
should be an asterisk. For example, 0 0 1 * 1 means run the
command at midnight on the first of every month and also
on every Monday.
The sixth field of each line is a command that is to be
executed by the shell at the specified times. Any percent
characters in this field (unless escaped by the escape
character) are translated to carriage return/newline
combinations. Only the first line (up to the first % or
end of line) is passed as a command; the other lines are
written to the shell as stdin. If no input lines are
given, the shell will inherit stdin opened to the nul
device.
Blank lines and any lines beginning with # will be considered
as comments.
If the crontab was read from a file, the shell will inherit
stdout and stderr handles for writing (appending, if the file
already exists) to a file created by replacing the .crt
extension on the crontab file with .nnn, where nnn is the
linenumber in the .crt file. (If the crontab was read from
stdin, then the shell will inherit the stdout and stderr
that cron inherited when it started.)
If a syntax or other error is encountered in a crontab, that
line, but not the rest of the file will be ignored. The
rationale is that cron should continue running even if it
encounters problems; it should not be necessary to restart
it just because one entry in one crontab contained an error.
By default, if the crontab is read from a file, commands
will be run asynchronously, meaning that all entries that
are ready to run at any given time will be quickly spawned,
one right after another, without waiting for each to complete
before the next is spawned. If the crontab is read from
stdin, the default will be synchronous execution, meaning
each must complete before the next starts; that's so any
output from one won't be jumbled with that of another. These
defaults can be overridden with command-line options.
Once it's started, cron will make an initial scan of the
crontab files or directories. Once every minute after that
it will look for any changes by comparing timestamps on the
files.
Running Cron as a Service:
Under Windows NT, there are two ways to run cron. The
simplest is as an ordinary process. But cron can also be
run as a service, which means it can run even under a
different ID than the person currently logged in or even
when no one is logged in. When run as a service, cron can
be set up to start up automatically when Windows NT is booted
or for manual startup using the Services applet in the
Control Panel. Finally, as service, cron can be either
interactive, meaning it and its children get access to the
console, or non-interactive, meaning it and its children
will run invisibly, with no display output and no means for
input through the keyboard or mouse. Any error messages
will be appended to a cron.log file in the home directory
since messages written to the screen would be lost.
The choice of running cron as either an interactive or non-
interactive service is this: if you run it interactively,
then there may be a security hole since windows will appear
and your users may be able to interact with the applications
cron invokes in ways you didn't intend. Also, the user
might be annoyed to see windows popping up and going
away for all those activities scheduled with cron.
On the other hand, if you run cron non-interactively, then
you may not be able to use it to schedule everything you
need. For example, if you start notepad from a non-
interactive copy of cron, notepad will hang because it
simply can't run properly without access to the console.
Also, you won't be able to kill the notepad process without
rebooting due to the way the security mechanism works.
An alternative to deciding between these differing ways of
running cron is to run several copies instead. Each can be
set up with its own startup and interactive/non-interactive
characteristics. But each copy must have a unique service
and display name.
Service Options:
-install Install cron as a service. The following
options may be specified with -install:
-u <user> Run cron under the specified user
name. The user name should be
in the form DomainName\Username or,
if the account is in the current
domain, as .\Username. The default
is to run under the LocalSystem ID.
To be able to start a service under
a user ID, that user must have the
right to log on as a service. To
grant that right, use the User
Manager. Pull down Policies, select
User Rights and in the popup, check
Show Advanced User Rights. You can
then select "Log on as a service"
and grant that right to selected
groups or users.
-p <password> Login password for the specified
user name.
-H <directory> Home directory that the cron service
should start up in. Default is the
current directory at the time the
install was done.
-M Manual startup. Default is
to startup automatically when
Windows NT is booted.
-I Interactive. Default is non-
interactive. A service can not be
installed as interactive if the
-u option is specified.
<arguments> Any run-time arguments to be passed
to cron when it starts. Note that
any options being set up for the
service MUST follow any -install
options. The first option that's
not an -install option is assumed to
be a run-time option. No syntax or
other checks are made during
installation that the run-time
arguments are sensible. If no
arguments are given with either
-install or -start, cron will look
for crontab (.crt) files in the
home directory.
-start <arguments>
Start the previously installed cron service,
passing it whatever arguments may be desired.
If the -install operation also specified an
argument list, this argument list will override
it.
-stop Stop the cron service.
-pause Pause the cron service.
-continue Allow cron to continue execution after having
been paused.
-delete Delete (uninstall) the cron service.
-status Report the status of the cron service.
All the service options above accept the following additional
options for specifying the name of the service:
-N <name> Service name for naming the entry
in the Registry.
-D <dispname> Display name for use in the Services
applet.
Under NT, each service has two names: one is the service
name, which is the name under which all the information
about that service is stored in the registry. The second
is the display name, which is the more human-readable name
shown in the Services applet in the Control Panel. By
default, the service name for cron is "Hamilton_Cron"
and the display name is "Hamilton Cron Service".
When installing a service, both the service name and the
display name must be unique; that is, there can't be any
other services already installed with that service name
or with that display name. Once installed, either name
may be used to refer to the service.
General Options:
-a Asynchronous execution. Don't wait for one
command to complete before spawning the next,
even if the crontab was read from stdin.
-s Synchronous execution. Always wait for one
command to complete before spawning the next,
even if the crontab was read from a file.
-L Logging. As each command is spawned, timestamp
and log it to stdout and to the output file if
that's not the same as stdout.
-w Wait until the next whole minute. If cron wasn't
started near a whole minute (i.e., zero to three
seconds past a whole minute mark), wait until the
next whole minute before beginning to schedule cron
events.
-o<opt> Remainder of word contains options to be passed to
the C shell with any commands. E.g., -oF means
C shell should do fast startup when running
commands.
-d <dir> Output files should be created in the specified
directory, not the directory where the .crt files
were found.
-! Install a interrupt handler to shield cron and its
children from Ctrl-C and Ctrl-Break interrupts.
(Remember to type this option at the end of the
list so the space afterward will prevent it from
being confused as a history reference.)
-W Open a new console window rather than using the
current one. Only meaningful for cron run as an
ordinary process, but useful if the -! option is
specified.
-h Help. (This screen.)
-- End of options.
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