[리눅스마스터 1급 실기] crontab 설정

crontab 설정

crontab 설정 확인

• crontab 설정은 'man 5 crontab' 명령 및  /etc/crontab 파일에서 확인할 수 있다.

$ man 5 crontab

CRONTAB(5)                                     File Formats                                    CRONTAB(5)



NAME
       crontab - files used to schedule the execution of programs

DESCRIPTION
       A  crontab  file  contains instructions for the cron(8) daemon in the following simplified manner:
       "run this command at this time on this date".  Each user can define their own  crontab.   Commands
       defined  in  any given crontab are executed under the user who owns that particular crontab.  Uucp
       and News usually have their own crontabs, eliminating the need for  explicitly  running  su(1)  as
       part of a cron command.

       Blank lines, leading spaces, and tabs are ignored.  Lines whose first non-white space character is
       a pound-sign (#) are comments, and are not processed.  Note that comments are not allowed  on  the
       same  line as cron commands, since they are considered a part of the command.  Similarly, comments
       are not allowed on the same line as environment variable settings.

       An active line in a crontab is either an environment setting or a cron  command.   An  environment
       setting is of the form:

          name = value

       where  the  white  spaces  around  the equal-sign (=) are optional, and any subsequent non-leading
       white spaces in value is a part of the value assigned to name.  The value string may be placed  in
       quotes (single or double, but matching) to preserve leading or trailing white spaces.

       Several  environment  variables  are  set up automatically by the cron(8) daemon.  SHELL is set to
       /bin/sh, and LOGNAME and HOME are set from the /etc/passwd line of the crontab´s owner.  HOME  and
       SHELL can be overridden by settings in the crontab; LOGNAME can not.

       (Note:  the  LOGNAME  variable  is  sometimes called USER on BSD systems and is also automatically
       set).

       In addition to LOGNAME, HOME, and SHELL, cron(8) looks at the MAILTO variable if a mail  needs  to
       be send as a result of running any commands in that particular crontab.  If MAILTO is defined (and
       non-empty), mail is sent to the specified address.  If MAILTO is defined but empty (MAILTO=""), no
       mail  is sent.  Otherwise, mail is sent to the owner of the crontab.  This option is useful if you
       decide to use /bin/mail instead of /usr/lib/sendmail as your mailer.  Note that /bin/mail does not
       provide aliasing and UUCP usually does not read its mail.  If MAILFROM is defined (and non-empty),
       it is used as the envelope sender address, otherwise, ``root'' is used.

       By default, cron sends a mail using the 'Content-Type:' header of of the locale in which  crond(8)
       is  started  up, i.e., either the default system locale, if no LC_* environment variables are set,
       or the locale specified by the LC_* environment variables (see  locale(7)).   Different  character
       encodings  can be used for mailing cron job outputs by setting the CONTENT_TYPE and CONTENT_TRANS‐
       FER_ENCODING variables in a crontab to the correct values of the mail headers of those names.

       The CRON_TZ variable specifies the time zone specific for the cron table.  The user should enter a
       time  according  to  the specified time zone into the table.  The time used for writing into a log
       file is taken from the local time zone, where the daemon is running.

       The MLS_LEVEL environment variable provides support for multiple per-job SELinux security contexts
       in  the  same crontab.  By default, cron jobs execute with the default SELinux security context of
       the user that created the crontab file.  When using multiple security levels and roles,  this  may
       not  be  sufficient, because the same user may be running in different roles or in different secu‐
       rity levels.  For more information about roles and SELinux MLS/MCS, see selinux(8) and the crontab
       example  mentioned later on in this text.  You can set the MLS_LEVEL variable to the SELinux secu‐
       rity context string specifying the particular SELinux security context in which you want  jobs  to
       be  run.   crond will then set the execution context of those jobs that meet the specifications of
       the particular security context.  For more information, see crontab(1) -s option.

       The RANDOM_DELAY variable allows delaying job startups by random  amount  of  minutes  with  upper
       limit  specified  by  the variable. The random scaling factor is determined during the cron daemon
       startup so it remains constant for the whole run time of the daemon.

       The format of a cron command is similar to the V7 standard, with  a  number  of  upward-compatible
       extensions.  Each line has five time-and-date fields followed by a username (if this is the system
       crontab file), and followed by a command.  Commands are executed by  cron(8)  when  the  'minute',
       'hour',  and  'month of the year' fields match the current time, and at least one of the two 'day'
       fields ('day of month', or 'day of week') match the current time (see "Note" below).

       Note that this means that non-existent times, such as the "missing hours" during the daylight sav‐
       ings  time  conversion, will never match, causing jobs scheduled during the "missing times" not to
       be run.  Similarly, times that occur more than once (again, during the daylight savings time  con‐
       version) will cause matching jobs to be run twice.

       cron(8) examines cron entries every minute.

       The time and date fields are:

              field          allowed values
              -----          --------------
              minute         0-59
              hour           0-23
              day of month   1-31
              month          1-12 (or names, see below)
              day of week    0-7 (0 or 7 is Sunday, or use names)

       A field may contain an asterisk (*), which always stands for "first-last".

       Ranges  of  numbers  are  allowed.  Ranges are two numbers separated with a hyphen.  The specified
       range is inclusive.  For example, 8-11 for an

       Lists are allowed.  A list is a set  of  numbers  (or  ranges)  separated  by  commas.   Examples:
       "1,2,5,9", "0-4,8-12".

       Step  values can be used in conjunction with ranges.  Following a range with "/<number>" specifies
       skips of the number's value through the range.  For example, "0-23/2" can be used in  the  'hours'
       field to specify command execution for every other hour (the alternative in the V7 standard is "0,
       2,4,6,8,10,12,14,16,18,20,22").  Step values are also permitted after an asterisk, so if  specify‐
       ing a job to be run every two hours, you can use "*/2".

       Names  can  also be used for the 'month' and 'day of week' fields.  Use the first three letters of
       the particular day or month (case does not matter).  Ranges or lists of names are not allowed.

       The "sixth" field (the rest of the line) specifies the command to be run.  The entire command por‐
       tion  of the line, up to a newline or a "%" character, will be executed by /bin/sh or by the shell
       specified in the SHELL variable of the cronfile.  A "%" character in the command,  unless  escaped
       with a backslash (\), will be changed into newline characters, and all data after the first % will
       be sent to the command as standard input.

       Note: The day of a command's execution can be specified in the following  two  fields  —  'day  of
       month',  and  'day  of week'.  If both fields are restricted (i.e., do not contain the "*" charac‐
       ter), the command will be run when either field matches the current time.  For example,
       "30 4 1,15 * 5" would cause a command to be run at 4:30 am on the 1st and 15th of each month, plus
       every Friday.

EXAMPLE CRON FILE
       # use /bin/sh to run commands, no matter what /etc/passwd says
       SHELL=/bin/sh
       # mail any output to `paul', no matter whose crontab this is
       MAILTO=paul
       #
       CRON_TZ=Japan
       # run five minutes after midnight, every day
       5 0 * * *       $HOME/bin/daily.job >> $HOME/tmp/out 2>&1
       # run at 2:15pm on the first of every month -- output mailed to paul
       15 14 1 * *     $HOME/bin/monthly
       # run at 10 pm on weekdays, annoy Joe
       0 22 * * 1-5    mail -s "It's 10pm" joe%Joe,%%Where are your kids?%
       23 0-23/2 * * * echo "run 23 minutes after midn, 2am, 4am ..., everyday"
       5 4 * * sun     echo "run at 5 after 4 every sunday"

Jobs in /etc/cron.d/
       The  jobs  in  cron.d  and  /etc/crontab are system jobs, which are used usually for more than one
       user, thus, additionaly the username is needed.  MAILTO on the first line is optional.

EXAMPLE OF A JOB IN /etc/cron.d/job
       #login as root
       #create job with preferred editor (e.g. vim)
       MAILTO=root
       * * * * * root touch /tmp/file

SELinux with multi level security (MLS)
       In a crontab, it is important to specify a security level by crontab -s or specifying the required
       level on the first line of the crontab.  Each level is specified in /etc/selinux/targeted/seusers.
       When using crontab in the MLS mode, it is especially important to:
       - check/change the actual role,
       - set correct role for directory, which is used for input/output.

EXAMPLE FOR SELINUX MLS
       # login as root
       newrole -r sysadm_r
       mkdir /tmp/SystemHigh
       chcon -l SystemHigh /tmp/SystemHigh
       crontab -e
       # write in crontab file
       MLS_LEVEL=SystemHigh
       0-59 * * * * id -Z > /tmp/SystemHigh/crontest

FILES
       /etc/crontab main system crontab file.  /var/spool/cron/ a directory for storing crontabs  defined
       by users.  /etc/cron.d/ a directory for storing system crontabs.

SEE ALSO
       cron(8), crontab(1)

EXTENSIONS
       These special time specification "nicknames" which replace the 5 initial time and date fields, and
       are prefixed with the '@' character, are supported:

       @reboot    :    Run once after reboot.
       @yearly    :    Run once a year, ie.  "0 0 1 1 *".
       @annually  :    Run once a year, ie.  "0 0 1 1 *".
       @monthly   :    Run once a month, ie. "0 0 1 * *".
       @weekly    :    Run once a week, ie.  "0 0 * * 0".
       @daily     :    Run once a day, ie.   "0 0 * * *".
       @hourly    :    Run once an hour, ie. "0 * * * *".

CAVEATS
       crontab files have to be regular files or symlinks to regular files, they must not  be  executable
       or  writable  for  anyone  else but the owner.  This requirement can be overridden by using the -p
       option on the crond command line.  If inotify support is in use, changes in the symlinked crontabs
       are not automatically noticed by the cron daemon.  The cron daemon must receive a SIGHUP signal to
       reload the crontabs.  This is a limitation of the inotify API.

AUTHOR
       Paul Vixie ⟨vixie@isc.org⟩



cronie                                          2012-11-22                                     CRONTAB(5)

 

$ cat /etc/crontab

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root

# For details see man 4 crontabs

# Example of job definition:
# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  * user-name  command to be executed

 

 

설정 항목

• 설정 항목은 총 7개의 필드로 구성되어 있다.
  • 날짜 관련 필드(5)
    • 앞부분부터 띄어쓰기로 구분해서 5개의 필드
    • 순서 : '분 시 일 월 요일'
    • 기본 설정 값 이외에 '*', '-', ',', '/' 사용 가능
      • * : 모든(all)
      • - : 연결된 설정 값을 지정할 때 사용
      • , : 연결되지 않는 값을 사용할 때 사용
      • / : 연결된 설정 값 범위에서 특정 주기로 나눌 때 사용
    • 요일 : 0(sun) ~ 6(sat)
  • 사용자 이름(1)
  • 실행할 명령어(1)

 

 

TIP) 'cat /etc/crontab' 명령어를 입력하면 설정 항목에 관한 내용이 간단히 출력된다.