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Crontab用户命令

可执行文件

crontab命令的可执行文件在哪儿？

$ which -a crontab
/usr/bin/crontab
/bin/crontab

crontab命令的可执行文件有2个：/usr/bin/crontab 和 /bin/crontab

$ diff /usr/bin/crontab /bin/crontab
$

diff 发现这两个文件并无区别。那么，执行时使用的是哪个文件呢？

$ echo $PATH
/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games

打印环境变量PATH，发现/usr/bin排在/bin之前，所以，执行的是/usr/bin/crontab吗？

$ strace crontab -l
execve("/usr/bin/crontab", ["crontab", "-l"], 0x7ffdf7c19ce8 /* 21 vars */) = 0
...

strace 跟踪一下crontab的执行过程，发现实际执行的的确是/usr/bin/crontab。

cronatb命令使用手册

man 1 cronatb

CRONTAB(1)                                             General Commands Manual                                            CRONTAB(1)NAMEcrontab - maintain crontab files for individual users (Vixie Cron)SYNOPSIScrontab [ -u user ] filecrontab [ -u user ] [ -i ] { -e | -l | -r }DESCRIPTIONcrontab  is  the  program used to install, deinstall or list the tables used to drive the cron(8) daemon in Vixie Cron.  Eachuser can have their own crontab, and though these are files in /var/spool/cron/crontabs, they are not intended to  be  editeddirectly.If  the  /etc/cron.allow  file exists, then you must be listed (one user per line) therein in order to be allowed to use thiscommand.  If the /etc/cron.allow file does not exist but the /etc/cron.deny file does exist, then you must not be  listed  inthe /etc/cron.deny file in order to use this command.If  neither of these files exists, then depending on site-dependent configuration parameters, only the super user will be al‐lowed to use this command, or all users will be able to use this command.If both files exist then /etc/cron.allow takes precedence.  Which means that /etc/cron.deny is not considered and  your  usermust be listed in /etc/cron.allow in order to be able to use the crontab.Regardless  of  the  existence of any of these files, the root administrative user is always allowed to setup a crontab.  Forstandard Debian systems, all users may use this command.If the -u option is given, it specifies the name of the user whose crontab is to be used (when  listing)  or  modified  (whenediting).   If  this option is not given, crontab examines "your" crontab, i.e., the crontab of the person executing the com‐mand.  Note that su(8) can confuse crontab and that if you are running inside of su(8) you should always use  the  -u  optionfor safety's sake.The first form of this command is used to install a new crontab from some named file or standard input if the pseudo-filename``-'' is given.The -l option causes the current crontab to be displayed on standard output.  See the note under DEBIAN SPECIFIC below.The -r option causes the current crontab to be removed.The -e option is used to edit the current crontab using the editor specified by the VISUAL or EDITOR  environment  variables.After  you  exit  from the editor, the modified crontab will be installed automatically.  If neither of the environment vari‐ables is defined, then the default editor /usr/bin/editor is used.The -i option modifies the -r option to prompt the user for a 'y/Y' response before actually removing the crontab.DEBIAN SPECIFICThe "out-of-the-box" behaviour for crontab -l is to display the three line "DO NOT EDIT THIS FILE" header that is  placed  atthe beginning of the crontab when it is installed.  The problem is that it makes the sequencecrontab -l | crontab -non-idempotent  —  you keep adding copies of the header.  This causes pain to scripts that use sed to edit a crontab.  There‐fore, the default behaviour of the -l option has been changed to not output such header.  You may obtain the original  behav‐iour  by setting the environment variable CRONTAB_NOHEADER to 'N', which will cause the crontab -l command to emit the extra‐neous header.SEE ALSOcrontab(5), cron(8)FILES/etc/cron.allow/etc/cron.deny/var/spool/cron/crontabsThe files /etc/cron.allow and /etc/cron.deny if, they exist, must be either world-readable, or readable by group ``crontab''.If they are not, then cron will deny access to all users until the permissions are fixed.There  is  one  file for each user's crontab under the /var/spool/cron/crontabs directory.  Users are not allowed to edit thefiles under that directory directly to ensure that only users allowed by the system to run periodic tasks can add  them,  andonly  syntactically  correct  crontabs  will be written there.  This is enforced by having the directory writable only by thecrontab group and configuring crontab command with the setgid bid set for that specific group.STANDARDSThe crontab command conforms to IEEE Std1003.2-1992 (``POSIX'').  This new command syntax differs from previous  versions  ofVixie Cron, as well as from the classic SVR3 syntax.DIAGNOSTICSA fairly informative usage message appears if you run it with a bad command line.cron  requires  that  each entry in a crontab end in a newline character.  If the last entry in a crontab is missing the new‐line, cron will consider the crontab (at least partially) broken and refuse to install it.The files under /var/spool/cron/crontabs are named based on the user's account name.  Crontab jobs will not be run for  userswhose accounts have been renamed either due to changes in the local system or because they are managed through a central userdatabase (external to the system, for example an LDAP directory).AUTHORPaul Vixie <[e-mail]> is the author of cron and original creator of this manual page.  This page has also  been  modifiedfor Debian by Steve Greenland, Javier Fernandez-Sanguino and Christian Kastner.4th Berkeley Distribution                                   19 April 2010                                                 CRONTAB(1)

crontab配置文件

man 5 crontab

CRONTAB(5)                                               File Formats Manual                                              CRONTAB(5)NAMEcrontab - tables for driving cronDESCRIPTIONA  crontab  file  contains  instructions  to  the cron(8) daemon of the general form: ``run this command at this time on thisdate''.  Each user has their own crontab, and commands in any given crontab will  be  executed  as  the  user  who  owns  thecrontab.   Uucp and News will usually have their own crontabs, eliminating the need for explicitly running su(1) as part of acron command.Blank lines and leading spaces and tabs are ignored.  Lines whose first non-space character is a hash-sign (#) are  comments,and  are  ignored.  Note that comments are not allowed on the same line as cron commands, since they will be taken to be partof the command.  Similarly, comments are not allowed on the same line as environment variable settings.An active line in a crontab will be either an environment setting or a cron command.  The crontab file is parsed from top  tobottom,  so any environment settings will affect only the cron commands below them in the file.  An environment setting is ofthe form,name = valuewhere the spaces around the equal-sign (=) are optional, and any subsequent non-leading spaces in value will be part  of  thevalue  assigned  to  name.   The value string may be placed in quotes (single or double, but matching) to preserve leading ortrailing blanks.  To define an empty variable, quotes must be used.The value string is not parsed for environmental substitutions or replacement of variables or tilde(~) expansion, thus  lineslikePATH = $HOME/bin:$PATHPATH = ~/bin:/usr/bin:/binwill not work as you might expect. And neither will this workA=1B=2C=$A $BThere will not be any substitution for the defined variables in the last value.Several  environment variables are set up automatically by the cron(8) daemon.  SHELL is set to /bin/sh, and LOGNAME and HOMEare set from the /etc/passwd line of the crontab's owner.  PATH is set to "/usr/bin:/bin".  HOME,  SHELL,  and  PATH  may  beoverridden by settings in the crontab; LOGNAME is the user that the job is running from, and may not be changed.(Another note: the LOGNAME variable is sometimes called USER on BSD systems...  on these systems, USER will be set also.)In  addition to LOGNAME, HOME, and SHELL, cron(8) will look at MAILTO and MAILFROM if it has any reason to send mail as a re‐sult of running commands in ``this'' crontab.If MAILTO is defined (and non-empty), mail is sent to the user so named.  MAILTO may also be used to direct mail to  multiplerecipients  by  separating  recipient  users with a comma.  If MAILTO is defined but empty (MAILTO=""), no mail will be sent.Otherwise mail is sent to the owner of the crontab.If MAILFROM is defined, the sender email address is set to MAILFROM. Otherwise mail is sent as "root (Cron Daemon)".On the Debian GNU/Linux system, cron supports the pam_env module, and loads the environment specified by /etc/environment and/etc/security/pam_env.conf.   It  also  reads  locale information from /etc/default/locale.  However, the PAM settings do NOToverride the settings described above nor any settings in the crontab file itself.  Note in particular that  if  you  want  aPATH other than "/usr/bin:/bin", you will need to set it in the crontab file.By  default,  cron  will send mail using the mail "Content-Type:" header of "text/plain" with the "charset=" parameter set tothe charmap / codeset of the locale in which crond(8) is started up – i.e. either the default system locale, if no LC_* envi‐ronment variables are set, or the locale specified by the LC_* environment variables ( see locale(7)).  You can use differentcharacter encodings for mailed cron job output  by  setting  the  CONTENT_TYPE  and  CONTENT_TRANSFER_ENCODING  variables  incrontabs, to the correct values of the mail headers of those names.The format of a cron command is very much the V7 standard, with a number of upward-compatible extensions.  Each line has fivetime and date fields, followed by a command, followed by a newline character ('\n').  The system crontab (/etc/crontab)  usesthe same format, except that the username for the command is specified after the time and date fields and before the command.The fields may be separated by spaces or tabs.  The maximum permitted length for the command field is 998 characters.Commands are executed by cron(8) when the minute, hour, and month of year fields match the current time, and  when  at  leastone  of the two day fields (day of month, or day of week) match the current time (see ``Note'' below).  cron(8) examines cronentries once every minute.  The time and date fields are:field          allowed values-----          --------------minute         0–59hour           0–23day of month   1–31month          1–12 (or names, see below)day of week    0–7 (0 or 7 is Sun, or use names)A field may be 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 exam‐ple, 8-11 for an ``hours'' entry specifies execution at hours 8, 9, 10 and 11.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'svalue through the range.  For example, ``0-23/2'' can be used in the hours field to specify  command  execution  every  otherhour (the alternative in the V7 standard is ``0,2,4,6,8,10,12,14,16,18,20,22'').  Steps are also permitted after an asterisk,so if you want to say ``every two hours'', just use ``*/2''.Names can also be used for the ``month'' and ``day of week'' fields.  Use the first three letters of the  particular  day  ormonth (case doesn't 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 portion of the line, up to anewline or % character, will be executed by /bin/sh or by the shell specified in the SHELL  variable  of  the  crontab  file.Percent-signs  (%)  in  the command, unless escaped with backslash (\), will be changed into newline characters, and all dataafter the first % will be sent to the command as standard input.  There is no way to split a single command line onto  multi‐ple lines, like the shell's trailing "\".Note:  The  day  of a command's execution can be specified by two fields — day of month, and day of week.  If both fields arerestricted (i.e., don't start with *), 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.  One  can,however, achieve the desired result by adding a test to the command (see the last example in EXAMPLE CRON FILE below).Instead of the first five fields, one of eight special strings may appear:string         meaning------         -------@reboot        Run once, at startup.@yearly        Run once a year, "0 0 1 1 *".@annually      (same as @yearly)@monthly       Run once a month, "0 0 1 * *".@weekly        Run once a week, "0 0 * * 0".@daily         Run once a day, "0 0 * * *".@midnight      (same as @daily)@hourly        Run once an hour, "0 * * * *".Please note that startup, as far as @reboot is concerned, is the time when the cron(8) daemon startup.  In particular, it maybe before some system daemons, or other facilities, were startup.  This is due to the boot order sequence of the machine.EXAMPLE CRON FILEThe following lists an example of a user crontab file.# use /bin/bash to run commands, instead of the default /bin/shSHELL=/bin/bash# mail any output to `paul', no matter whose crontab this isMAILTO=paul## run five minutes after midnight, every day5 0 * * *       $HOME/bin/daily.job >> $HOME/tmp/out 2>&1# run at 2:15pm on the first of every month — output mailed to paul15 14 1 * *     $HOME/bin/monthly# run at 10 pm on weekdays, annoy Joe0 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"0 */4 1 * mon   echo "run every 4th hour on the 1st and on every Monday"0 0 */2 * sun   echo "run at midn on every Sunday that's an uneven date"# Run on every second Saturday of the month0 4 8-14 * *    test $(date +\%u) -eq 6 && echo "2nd Saturday"All the above examples run non-interactive programs.  If you wish to run a program that interacts with the user's desktop youhave to make sure the proper environment variable DISPLAY is set.# Execute a program and run a notification every day at 10:00 am0 10 * * *  $HOME/bin/program | DISPLAY=:0 notify-send "Program run" "$(cat)"EXAMPLE SYSTEM CRON FILEThe  following  lists the content of a regular system-wide crontab file.  Unlike a user's crontab, this file has the usernamefield, as used by /etc/crontab.# /etc/crontab: system-wide crontab# Unlike any other crontab you don't have to run the `crontab'# command to install the new version when you edit this file# and files in /etc/cron.d.  These files also have username fields,# that none of the other crontabs do.SHELL=/bin/shPATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin# 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# |  |  |  |  |# m h dom mon dow usercommand17 * * * *  root  cd / && run-parts --report /etc/cron.hourly25 6 * * *  root  test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )47 6 * * 7  root  test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )52 6 1 * *  root  test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )#Note that all the system-wide tasks will run, by default, from 6 am to 7 am.  In the case of systems that are not powered  onduring that period of time, only the hourly tasks will be executed unless the defaults above are changed.SEE ALSOcron(8), crontab(1)EXTENSIONSWhen specifying day of week, both day 0 and day 7 will be considered Sunday.  BSD and AT&T seem to disagree about this.Lists  and  ranges are allowed to co-exist in the same field.  "1-3,7-9" would be rejected by AT&T or BSD cron — they want tosee "1-3" or "7,8,9" ONLY.Ranges can include "steps", so "1-9/2" is the same as "1,3,5,7,9".Months or days of the week can be specified by name.Environment variables can be set in the crontab.  In BSD or AT&T, the environment handed to child processes is basically  theone from /etc/rc.Command  output  is  mailed  to the crontab owner (BSD can't do this), can be mailed to a person other than the crontab owner(SysV can't do this), or the feature can be turned off and no mail will be sent at all (SysV can't do this either).All of the `@' commands that can appear in place of the first five fields are extensions.LIMITATIONSThe cron daemon runs with a defined timezone.  It currently does not support per-user timezones.  All the tasks: system's anduser's  will  be  run  based on the configured timezone.  Even if a user specifies the TZ environment variable in his crontabthis will affect only the commands executed in the crontab, not the execution of the crontab tasks themselves.POSIX specifies that the day of month and the day of week fields both need to match the current time if either of them  is  a*.   However,  this  implementation only checks if the first character is a *.  This is why "0 0 */2 * sun" runs every Sundaythat's an uneven date while the POSIX standard would have it run every Sunday and on every uneven date.The crontab syntax does not make it possible to define all possible  periods  one  can  imagine.   For  example,  it  is  notstraightforward  to  define  the  last  weekday of a month.  To have a task run in a time period that cannot be defined usingcrontab syntax, the best approach would be to have the program itself check the date and time information and continue execu‐tion only if the period matches the desired one.If the program itself cannot do the checks then a wrapper script would be required.  Useful tools that could be used for dateanalysis are ncal or calendar For example, to run a program the last Saturday of every month  you  could  use  the  followingwrapper code:0 4 * * Sat   [ "$(date +\%e)" = "$(LANG=C ncal | sed -n 's/^Sa .* \([0-9]\+\) *$/\1/p')" ] && echo "Last Saturday" && program_to_runDIAGNOSTICScron  requires  that each entry in a crontab end in a newline character.  If the last entry in a crontab is missing a newline(i.e. terminated by EOF), cron will consider the crontab (at least partially) broken.  A warning will be written to syslog.AUTHORPaul Vixie <[e-mail]> is the author of cron and original creator of this manual page.  This page has also  been  modifiedfor Debian by Steve Greenland, Javier Fernandez-Sanguino, Christian Kastner and Christian Pekeler.4th Berkeley Distribution                                   19 April 2010                                                 CRONTAB(5)

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