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Annotated edit history of cron(8) version 1, including all changes. View license author blame.
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1 perry 1 CRON
2 !!!CRON
3 NAME
4 SYNOPSIS
5 DESCRIPTION
6 DEBIAN SPECIFIC
7 SEE ALSO
8 AUTHOR
9 ----
10 !!NAME
11
12
13 cron - daemon to execute scheduled commands (Vixie Cron)
14 !!SYNOPSIS
15
16
17 cron
18 !!DESCRIPTION
19
20
21 ''cron'' is started automatically from /etc/init.d on
22 entering multi-user runlevels.
23
24
25 ''cron'' searches its spool area
26 (/var/spool/cron/crontabs) for crontab files (which are
27 named after accounts in /etc/passwd); crontabs found are
28 loaded into memory. Note that crontabs in this directory
29 should not be accessed directly - the ''crontab'' command
30 should be used to access and update them.
31
32
33 ''cron'' also reads /etc/crontab, which is in a slightly
34 different format (see ''crontab(5)).'' Additionally, cron
35 reads the files in /etc/cron.d; see the __DEBIAN
36 SPECIFIC__ section below for more details.
37
38
39 ''cron'' then wakes up every minute, examining all stored
40 crontabs, checking each command to see if it should be run
41 in the current minute. When executing commands, any output
42 is mailed to the owner of the crontab (or to the user named
43 in the MAILTO environment variable in the crontab, if such
44 exists). The children copies of cron running these processes
45 has its name coerced to uppercase, as will be seen in the
46 syslog and ps output.
47
48
49 Additionally, ''cron'' checks each minute to see if its
50 spool directory's modtime (or the modtime on
51 ''/etc/crontab)'' has changed, and if it has, ''cron''
52 will then examine the modtime on all crontabs and reload
53 those which have changed. Thus ''cron'' need not be
54 restarted whenever a crontab file is modified. Note that the
55 crontab(1) command updates the modtime of the spool
56 directory whenever it changes a crontab.
57
58
59 Special considerations exist when the clock is changed by
60 less than 3 hours, for example at the beginning and end of
61 daylight savings time. If the time has moved forwards, those
62 jobs which would have run in the time that was skipped will
63 be run soon after the change. Conversely, if the time has
64 moved backwards by less than 3 hours, those jobs that fall
65 into the repeated time will not be re-run.
66
67
68 Only jobs that run at a particular time (not specified as
69 @hourly, nor with '*' in the hour or minute specifier) are
70 affected. Jobs which are specified with wildcards are run
71 based on the new time immediately.
72
73
74 Clock changes of more than 3 hours are considered to be
75 corrections to the clock, and the new time is used
76 immediately.
77 !!DEBIAN SPECIFIC
78
79
80 ''cron'' treats the files in /etc/cron.d as extensions to
81 the /etc/crontab file (they follow the special format of
82 that file, i.e. they include the ''user'' field). The
83 intended purpose of this feature is to allow packages that
84 require finer control of their scheduling than the
85 /etc/cron.{daily,weekly,monthly} directories allow to add a
86 crontab file to /etc/cron.d. Such files should be named
87 after the package that supplies them. Files must conform to
88 the same naming convention as used by run-parts(8): they
89 must consist solely of upper- and lower-case letters,
90 digits, underscores, and hyphens. Like /etc/crontab, the
91 files in the /etc/cron.d directory are monitored for
92 changes.
93 !!SEE ALSO
94
95
96 crontab(1), crontab(5)
97 !!AUTHOR
98
99
100 Paul Vixie
101 ----
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