Linux System Administration

226

|

Chapter 10: Scripting

• month is a number between 1 and 12 or a name such as February.

•

day_of_week is a number between 0 and 7 (0 or 7 is Sunday, 6 is Saturday) or a

name such as

Tuesday.

•

day_of_month and day_of_week are ORed together, which may cause surprises.

For instance, if each field contains a

1, cron will execute the command in Janu-

ary as well as on Mondays. Usually, the crontab line puts a specific value in only

one of these fields.

• In any field, a value means an exact match; for instance, a

1 in the month field

means only January.

• An asterisk (

*) means any value.

• Two values separated by a hyphen indicate a range. Thus,

11-12 in the month

field means November through December.

• To specify more than one value, separate the values with commas. A

month list of

2,3,5-6 means February, March, and May through June.

•Astep modifier may follow values and a slash (

/), and it indicates how many

units to increment between values. A

month value of */3 means every third

month. A

month value of 4-9/2 means months 4, 6, and 8.

The shell executes the command, so it can use the features mentioned in this chap-

ter. Some examples using direct commands rather than scripts are:

5 * * * * rm /tmp/*.gif # remove all GIF files every 5 minutes

5 * * * * rm -v /tmp/*.gif >> /tmp/gif.log # the same, logged

When cron runs the command, it emails its standard output and standard error to

the owner of the crontab ...