Micron 1 Overview 2 Crontabs 2.1 Crontab Groups 2.2 Crontab Format 2.2.1 Cronjob Definition 2.2.2 Variable Settings 2.3 Cronjob Output 2.3.1 Mailing the cronjob output 2.3.2 Logging output to syslog 2.3.3 Sending output to file 3 micrond 3.1 micrond Invocation 4 The 'crontab' Utility 4.1 crontab Invocation 5 Key Features 5.1 User Group Crontabs 5.2 Long Crontab Lines 5.3 Built-in Variables 5.4 Day Field Semantics 5.5 Variable Assignments in Crontabs 5.6 Job Output Report 5.7 Simultaneous Job Execution 5.8 Detection of Crontab Modifications 6 Downloads and Installation 7 Bug Reports Appendix A GNU Free Documentation License Index Micron ****** This edition of the 'Micron Manual', last updated 3 January 2024, documents 'micron' Version 1.3.93. 1 Overview ********** 'Micron' is an implementation of the UNIX 'cron' daemon, a program that executes periodically various tasks. It provides a flexible job scheduler that offers complete control over execution of the scheduled commands as well as additional organizational features. Cron daemons have a long history, dating back to Seventh Edition Unix. The most widespread cron implementations today are "Vixie cron" and its derivatives, originally written by Paul Vixie, and "Dillon's cron", written by Matt Dillon. 'Micron' is a clean, multi-threaded implementation, written from scratch, which does not share any code with its predecessors. Some of the new features provided by 'micron' are: flexible control over cronjob's standard output and error streams, which can be redirected to syslog, mailed to arbitrary list of emails, appended to a file, or simply ignored, control over the number of cronjob instances that are allowed to run simultaneously, and user-group crontabs. For a complete list, see *note Features::. While providing new features, 'micron' tries to preserve backward compatibility with the two above-mentioned implementations. In particular, existing crontabs from "Vixie" cron can be used with 'micron' without changes. The same holds true for per-user crontabs from "Dillon's" cron. System-wide crontabs from "Dillon's" cron will require a minor editing: the user name must be added between the schedule and command. The implementation consists of two binaries: the main daemon 'micrond' and the 'crontab' utility. 2 Crontabs ********** A set of "crontab" files contains instructions specifying what commands to run and defining schedules for them. 'micrond' reads its crontabs at startup and loads them to memory. When running, it keeps track of crontab modifications and updates its in-memory tables as soon as such are detected. 2.1 Crontab Groups ================== Crontabs are stored in several locations, collectively known as "crontab groups" or "crongroups", for short. These are: "master crontab" The file '/etc/crontab'. It is used for site-wide cronjobs, i.e. tasks necessary for the normal functioning of the system. "system crontabs" A collection of crontab files in the '/etc/cron.d' directory. These are used for per-package cronjobs. "user crontabs" Crontab files located in '/var/spool/cron/crontabs' contain per-user cronjobs and are editable by users. "user group crontabs" A special crontab group intended for use with pseudo-accounts, such as 'apache' or 'bind'. Crontabs of this group are located in subdirectories of '/var/spool/cron/crongroups' named by the corresponding account. This crontab group is described in detail in *note User Group Crontabs::. Each _active_ (i.e. non-empty and non-comment) line in a crontab specifies a schedule and a command line to be run according to that schedule. Active lines in master and system crontabs specify also login name of the user on behalf of whom the command must be run. Both master and system crontabs are writable only by the super-user. User and user group crontabs belong to particular users, and instructions they contain are executed on behalf of their owners. To enable users to manipulate their crontabs, the 'crontab' command is provided (*note crontab::). 2.2 Crontab Format ================== Crontabs used by 'micrond' are mostly compatible with "Vixie" crontabs. They are plain text, line-oriented files. When parsing a crontab, the leading and trailing whitespace on each line is ignored. Comments are introduced by a hash sign (#) appearing as the first non-whitespace character in a line. Comments and empty lines are ignored. Very long lines can be split across several physical lines using backslash as continuation character. Total length of a valid crontab line after removing continuation characters cannot exceed 1024 characters. Each crontab line is either a cronjob definition or a variable setting. 2.2.1 Cronjob Definition ------------------------ A "cronjob definition" is a line in crontab that defines a running schedule, a user on whose behalf the job will be run (in system crontabs), and a command to be run. The fields are delimited by arbitrary amount of whitespace. Cronjob definition begins with a "cron expression", which defines schedule for running the command. It consists of five fields in this order: field allowed values -------------------------------------------------------------------------- minute 0-59 hour 0-23 day of month 1-31 month 1-12, or abbreviated month names day of week 0-7 (0 or 7 is Sunday), or abbreviated day names A field may contain a single asterisk '*', which stands for "each time unit". For example, it means "each minute" when used in the first field, "each hour" in second one, and so on. Asterisk can be followed by a slash and a decimal number, which defines a step within the interval. E.g. '*/2' in second field stands for "each second hour". The field may also be a comma-delimited list of the following constructs: N A single number. E.g. '2' in first field means "second minute of an hour". I-J Two numbers delimited by a dash define a range. E.g '2-5' in first field is "minutes 2,3,4, and 5 (inclusive)". The order is important. For example, the following schedule: 55-5 * * * * means "minutes 55 through 59 and 0 through 5 of the next hour". I-J/N Same as above, but with step of N units, e.g.: 10-25/5 * * * * which stands for "minutes 10,15,20,25". Names can be used in "month" and "day of week" fields. Day and week names are abbreviated to their first three characters (case-insensitive). Ranges or lists of names are allowed as well. The day of command's execution is determined by two fields: day of month, and day of week. If both are supplied (i.e. are not '*'), the result depends on the selected "day field semantics". There are three cases: "Strict semantics" The command will be run only when both fields match the current time. For example, 30 4 1,15 * 5 means "run the command at 4:30 am each Friday between the 1st and 15th of each month". This semantics is the default for 'micrond'. "Vixie semantics" The command will be run when either field matches the current time. Thus, the previous example would cause a command to be run at 4:30 am on the 1st and 15th of each month, plus every Friday. "Dillon semantics" If both day of the month and day of week are supplied, the former must be an integer number in range 1-5. Its meaning is Nth such weekday in the month. For example, to run the date command at 11 am on the second and third Monday, Tuesday and Wednesday of each month: 0 11 2,3 * mon-wed date To request the last Monday, etc. in a month, use '5'. This will always match the last Monday, etc., even if there are only four Mondays in the month: 0 11 1,5 * mon-wed date When the fourth Monday in a month is the last, it will match against both 4 and 5, but will only run once if both are specified. The semantics to use is selected by setting the '_MICRON_DAY_SEMANTICS' (*note _MICRON_DAY_SEMANTICS::) variable to one of the following values: 'strict', 'vixie', 'dillon' (case-insensitive). Instead of the first five fields, the following macro-definitions can be used: '@reboot' Run once, at 'micrond' startup. '@yearly' '@annually' Run once a year, i.e. '0 0 1 1 *'. '@monthly' Run once a month, '0 0 1 * *'. '@weekly' Run once a week, '0 0 * * 0'. '@daily' '@midnight' Run once a day, '0 0 * * *'. '@hourly' Run once an hour, '0 * * * *'. In master crontab ('/etc/crontab') and system crontabs (located in '/etc/cron.d'), the cron expression is followed by "user name" field, which holds a login name of the user on whose behalf the command will be run. This field is absent in user personal crontabs, since these are always run on behalf of the user that owns them, The remainder of line is taken as a shell command to be run when the time matches the schedule. The percent sign in the command line is special: the first occurrence of unescaped and unquoted '%' delimits the command line proper and the text that will be piped to its standard input. Any unescaped occurrences of '%' to the right of it will be replaced with newlines. To use a literal percent sign in the job command line, either precede it with a backslash or quote it using single or double quotes. For example, the following command line: mail hostmaster%Hello,%%This is a daily notification.%--%Regards from cron.% will cause 'mirond' to start the 'mail' command and to supply the following text to its standard input: Hello, This is a daily notification. -- Regards from cron. Before running the command, 'micrond' changes the directory to the home directory of the user on whose behalf the command is run (as defined by the 'HOME' environment variable, *note HOME::), modifies the environment as requested by variable settings (*note Variable Settings::) and runs the command as $SHELL -c "COMMAND" Default 'SHELL' is '/bin/sh' (*note SHELL::). 2.2.2 Variable Settings ----------------------- Variable settings modify execution environment for subsequent cron commands. Some variables are internal to 'micrond' and modify its behavior. A variable setting has the form NAME = VALUE where NAME is the variable name and VALUE is the value to be assigned to it. Notice several important differences from the similar construct in shell. First of all, optional whitespace is allowed on either side of the equals sign. Secondly, the value is assigned verbatim, after removing any trailing and leading whitespace. No expansions or substitutions take place. If you need this kind of functionality, move environment manipulations to a shell script and invoke it from the cron command. You don't need to quote VALUE even if it contains embedded whitespace. There are only two cases when quoting becomes necessary: to define an empty variable or to preserve leading or trailing whitespace. Both single and double quotes can be used, the only requirement being that both quotes match. Inside a quoted string, a backslash character can be used to escape a quote character or another backslash. To define an empty variable, use a pair of quotes. The special construct NAME = unsets the variable NAME. Each variable setting applies to all cron commands that follow it, until another setting overwrites it or the end of file is encountered. The following are "built-in variables". They modify the handling of cron commands that follow them, but are not copied to the command environment. Variables prefixed with '_JOB_' affect only the first cron command that appears after them, whereas variables prefixed with '_MICRON_' affect all commands that follow them, until another assignment of the same variable is encountered or the end of the file is reached. A built-in variable can be assigned a default value in the command line using the '-v' option (*note initial value::). -- Built-in variable: _MICRON_OUTFILE_FACILITY -- Built-in variable: _JOB_OUTFILE_FACILITY The value of this variable is the name of a file. If the variable is set, any output the cronjob produces will be appended to that file. *Note Sending output to file::. -- Built-in variable: _MICRON_SYSLOG_FACILITY -- Built-in variable: _JOB_SYSLOG_FACILITY If this variable is set to a meaningful syslog facility, the cronjob output will be logged to that facility (priority 'INFO'), instead of mailing it the usual way. *Note Logging output to syslog::. Allowed values for this variable are 'auth', 'authpriv', 'cron', 'daemon', 'ftp', 'lpr', 'mail', 'news', 'syslog', 'user', 'uucp', and 'local0' through 'local7'. The value 'default' means to use the default syslog facility ('cron'), and 'off' or 'none' disables syslog and reverts to mailing the program output as directed by the 'MAILTO' variable (*note MAILTO::). -- Built-in variable: _JOB_SYSLOG_TAG Sets the syslog tag for the next cron command. The tag is used when logging cronjob output to syslog (*note Logging output to syslog::), or appending it to a file (*note Sending output to file::). If not specified, the tag will be constructed as FILE:LINE(PROG) where FILE and LINE are the file name and line number of the line where the cron command appeared and PROG is the first word of the command. The global counterpart of this variable, '_MICRON_SYSLOG_TAG', is provided for completeness. It is probably of little use, since it sets the same tag for all crontab entries. -- Built-in variable: _MICRON_MAXINSTANCES -- Built-in variable: _JOB_MAXINSTANCES An integer value defining how many instances of the same cron job can be running simultaneously. Default is 1, i.e. 'micrond' will refuse to start a job if its previous run has not yet terminated. -- Built-in variable: _JOB_MAILTO This variable temporarily masks the 'MAILTO' environment variable and disables the variables '_MICRON_SYSLOG_FACILITY' and '_MICRON_OUTFILE' for the next crontab entry. After that, the previous values are restored. Use it to redirect output of a single crontab entry to a particular email address. The global version of this variable, '_MICRON_MAILTO', is mostly equivalent to the traditional 'MAILTO' variable, except that any existing value of 'MAILTO' is retained in the environment. *Note Mailing the cronjob output::, for the detailed discussion of how the output is mailed. *Note Cronjob Output:: -- Built-in variable: _MICRON_DAY_SEMANTICS -- Built-in variable: _JOB_DAY_SEMANTICS Defines the day semantics. Allowed values are: 'strict', 'vixie', and 'dillon' (case-insensitive). *Note Day Field Semantics::. The following are environment variables that modify the behavior of 'micrond' itself. -- Environment: MAILTO Defines an email address or a comma-delimited list of email addresses. If any of the cron commands that follow this setting produces anything on its standard output or standard error, the output will be collected and mailed to the emails listed in the 'MAILTO' variable. If the variable is undefined, the output will be sent to the owner of the crontab. If 'MAILTO' is set to an empty string, no mail will be sent. *Note Mailing the cronjob output::, for the detailed discussion of how the output is mailed. -- Environment: HOME Defines the working directory from which the subsequent commands will be run. Defaults to home directory of the user on whose behalf the command is run. -- Environment: SHELL Defines the shell used to start commands. Defaults to '/bin/sh'. 2.3 Cronjob Output ================== When run, a cronjob can print something on its standard output or error streams. The output it produces can be sent via email to a given list of addresses, appended to a file, or reported via syslog. This "output redirection" can be configured either globally, or individually for each job. It is controlled by the following variables: 'MAILTO' '_MICRON_MAILTO' '_JOB_MAILTO' Send the output via email. '_MICRON_SYSLOG_FACILITY' '_JOB_SYSLOG_FACILITY' Report each output line via syslog. '_MICRON_OUTFILE' '_JOB_OUTFILE' Append the output to a file. These settings are discussed in detail in the following subsections. 2.3.1 Mailing the cronjob output -------------------------------- The default behavior is to mail the cronjob output to the user on whose behalf the cronjob is run. The message 'From:' header is constructed as follows: From: (Cron daemon) where OWNER is the login name of the job owner, and HOST is the name of the host where it was run. The 'Subject:' header contains the owner login name, hostname of the server where the command was run and the command itself, in the following format: Cron COMMAND A copy of execution environment is included in the message in the form of additional 'X-Cron-Env:' headers, each containing a single environment variable. The recipient of the message can be altered by setting the 'MAILTO', '_JOB_MAILTO', and '_MICRON_MAILTO' variables in the crontab. The 'MAILTO' variable is the traditional way of defining the recipients for the job output. It affects all cronjobs in crontab that appear after it, until next definition of any of the three variables discussed, or end of the crontab file, whichever occurs first. It is also reflected in the environment of the job itself. *Note MAILTO::. If 'MAILTO' is unset, the default behavior (mailing to the owner) is restored. If it is set to an empty string, cronjob output is discarded. _Notice_ the difference: 'MAILTO =' Restores default behavior. 'MAILTO = ""' Discards the output. The two built-in variables '_JOB_MAILTO' and '_MICRON_MAILTO' behave as 'MAILTO', with the following differences: The '_JOB_MAILTO' variable, if set, affects only the cronjob that immediately follows it. It is not reflected in the environment. *Note _JOB_MAILTO::. The '_MICRON_MAILTO' variable is rarely used. It works exactly as 'MAILTO', except that the actual value of the latter remains unchanged. It is not reflected in the environment, either. 2.3.2 Logging output to syslog ------------------------------ To send the output of all the cronjobs to the syslog, assign the name of the syslog facility to the built-in variable '_MICRON_SYSLOG_FACILITY', e.g.: _MICRON_SYSLOG_FACILITY = local0 Valid facility names are 'auth', 'authpriv', 'cron', 'daemon', 'ftp', 'lpr', 'mail', 'news', 'syslog', 'user', 'uucp', and 'local0' through 'local7'. Similarly, to log only a single cronjob output, define the variable '_JOB_SYSLOG_FACILITY' immediately before that cronjob. In the produced log, each line of a cronjob output will be tagged with a fully-qualified name of the cronjob, constructed as: FILE:LINE(PROG) You can alter this using the '_JOB_SYSLOG_TAG' variable. The following example illustrates the use of the two per-job syslog variables: _JOB_SYSLOG_FACILITY = daemon _JOB_SYSLOG_TAG = daily_cleanup 0 1 * * * root /usr/local/bin/cleanup 2.3.3 Sending output to file ---------------------------- To redirect cronjob output to a file, use the '_MICRON_OUTFILE' or '_JOB_OUTFILE' built-in variables. The value of these variables is the name of file to append the output to. For example, to redirect output of all cronjobs in a crontab to file '/var/log/cronout', place the following at the beginning of the crontab: _MICRON_OUTFILE = /var/log/cronout For example, when running 'micrond' in a docker container, you can redirect the output of all cronjobs to the container log as follows: _MICRON_OUTFILE = /proc/1/fd/1 The '_JOB_OUTFILE' variable affects only the cronjob that follows it. Output from each job is appended to the output file in a single chunk, which appears between the following two delimiter lines: TS_START: TAG output begins and TS_END: TAG output ends where TS_START and TS_END are job start and termination timestamps in ISO 8601 format, and TAG is the cronjob tag, formatted as described in *note cronjob tag::. If the '_SYSLOG_TAG' variable is defined, its value will be used instead. For example: 2021-10-23T03:00:00: /etc/cron.d/backup:16(daily_backup) output begins . . . 2021-10-23T03:07:15: /etc/cron.d/backup:16(daily_backup) output ends 3 micrond ********* 'micrond' executes commands periodically as directed by one or more "crontabs". *Note Crontabs::, for a discussion of crontab locations and format. Normally, a cron job is not run if its previous instance is still running. For instance, if a cron job scheduled to run each minute takes three minutes to finish, it will be actually run once in three minutes. This behavior can be altered by setting the '_MICRON_MAXINSTANCES' variable to the desired number of cron job instances that can be run simultaneously (use '_JOB_MAXINSTANCES' to change the setting for one job only, *note _MICRON_MAXINSTANCES::). Depending on cronjob settings, any data the running job produces on its standard output and standard error can be either logged via syslog in real time, or captured and, upon termination of the job, mailed to the job owner, or appended to a file (*note Cronjob Output::). On GNU/Linux systems, 'micrond' monitors each opened crontab for modifications and re-reads it as soon as it is written to disk. On other systems, it checks crontab modification times each minute and re-reads those crontabs for which this value has changed. 3.1 micrond Invocation ====================== The 'micrond' daemon takes no command line arguments. Its behavior is controlled by command line options. Unless instructed otherwise, upon startup 'micrond' disconnects from the controlling terminal and remains in the background. During normal execution it logs its diagnostic messages (if any) via 'syslog' facility 'cron'. Eventual output and error diagnostics from the invoked cron jobs are mailed to the job owner, as described in *note Mailing the cronjob output::. The following options modify the 'micrond' behavior: '-f' Remain in foreground. '-h' Display a short help summary and exit. '-g GROUP=DIR' Define the directory or file name for crontab group GROUP. Valid group names are: 'master', 'system', 'user', and 'group'. *Note Crongroups::. '-g GROUP' '-g noGROUP' Enable or disable crontab group GROUP. Note, that the 'group' crontab group is disabled by default. '-l PRI' Log only messages with syslog priority PRI or higher. Valid arguments are, in order of increasing priority: 'debug', 'info', 'notice', 'warning', 'err', 'crit', 'alert', and 'emerg'. '-m MAILER' Set the mailer command. Default is '/usr/sbin/sendmail -oi -t'. '-P FILE' Write PID of the program to FILE. '-p SOCKET' Send messages to syslog via this socket. The argument is either an absolute file name of a UNIX socket, or a host name or IPv4 address optionally followed by a colon and port number or service name. '-S' When running in foreground (see the '-f' option), log messages from 'micrond' to the syslog facility 'cron', instead of printing them to the standard error. Not to be confused with the '-s' option, described below. '-s' Log output from cronjobs to syslog. By default, the 'cron' facility is used. Use the '-v syslog_facility=F' option to change it to facility F. See below, for the discussion of the '-v' option. '-T N' In foreground mode, when logs go to stderr, precede each line with a timestamp in ISO 8601 format ('-T0'). N can be 1 to skip seconds and 2 to skip both minutes and seconds from the timestamp. '-t N' Before exiting, 'micrond' checks if any of the cronjobs are still running. If so, it sends them the 'SIGTERM' signal and waits N seconds for them to terminate. The cronjobs that fail to exit within that amount of time are terminated forcibly by sending them the 'SIGKILL' signal. The default timeout is 60 seconds. '-V' Print program version, licensing information and exit. '-v NAME=VALUE' Assigns initial value for the 'micrond' internal variable NAME (*note built-in variables::). The variable name must be used without prefix. Comparison is case-insensitive. For example: -v syslog_facility=daemon is equivalent to _MICRON_SYSLOG_FACILITY = daemon '-W OPT' Set internal 'micrond' option. As of version 1.3.93, only one such option is implemented: 'paranoid_memfree'. When '-Wparanoid_memfree' is given, 'micrond' will meticulously free all allocated memory before terminating. Normally this isn't needed, as the operating system will reclaim that memory anyway. Use this option if you run 'micrond' under 'valgrind' or another memory leak detector, in order to avoid spurious warnings. 4 The 'crontab' Utility *********************** The 'crontab' utility allows the user to list, edit or remove personal or group crontabs. The 'micrond' cron daemon reads crontabs from several crontab groups, two of which contain crontabs for particular system users. The "user crongroup" contains per-user crontabs, and the "group crongroup" contains user crontabs editable by a group of users (*note Crongroups::). By default, 'crontab' operates on per-user crontabs. To edit group crontabs, the '-g' option must be specified. When run without arguments, 'crontab' enters "copy mode", in which it copies the content of the supplied file to the user personal crontab, overwriting its prior content. For example crontab newfile overwrites the current crontab schedule of the user with the content of the file 'newfile'. To list the content of the crontab, use the '-l' option. It will be displayed on standard output. To edit it, run 'crontab -e'. A temporary copy of the crontab will be created and loaded to the editor specified by the 'VISUAL' environment variable. If it is unset, the 'EDITOR' variable is consulted. If this variable is unset as well, the built-in default 'vi' will be used. Once you quit the editor, the edited crontab will be atomically moved to your personal crontab and re-read by 'micrond'. The '-r' option removes the crontab. Use it with caution. No backup copies are preserved. When using destructive operations (such as copying or removal), it is safer to use the '-i' option which instructs the program to ask for the user consent before undertaking the modification. The super-user can address the crontab of a particular user by supplying the user login name with the '-u' option. The use of this option is restricted for super-user, except if used together with the '-g' option. User crontab groups contain multiple files for each system user. They are useful for certain pseudo-accounts. For example, a site running multiple web services may need to install separate crontabs for each of them and to allow users who run these services to edit their crontabs. This is done using the '-g' option. The name of the account for which the crontab is edited is supplied with the '-u' option. For example, to edit a crontab 'portal' in account 'www-data', one would use: crontab -g -u www-data -e portal The use of group crontabs for account X is allowed only for users who are members of the primary group of X. 4.1 crontab Invocation ====================== The crontab usage is: 'crontab [-u NAME] FILE' Copy mode. Replace the crontab of the user who invoked the command with the content of FILE. The '-u' is allowed only for super-user. With this option, the crontab of the user NAME is replaced. 'crontab [-eilr] [-u NAME]' List ('-l'), edit ('-e') or remove ('-r') the user's crontab. The '-u' is allowed only for super-user. With this option, the command operates on the crontab of the user NAME. 'crontab -g [-u NAME] [-eilr] FILE' List ('-l'), edit ('-e') or remove ('-r') the crontab FILE in the crongroup of the current user (or user NAME). (*note User Group Crontabs::) The '-u' option is allowed for super-user and any user whose primary group matches that of the user NAME. The following table summarizes command line options of the 'crontab' utility: '-e' Edit crontab. '-g' Operate on user cron group files. '-h' Print a short command line usage summary and exit. '-i' Interactively ask before removing or replacing. '-l' List crontab content. When used with the '-g' option, this option lists the content of the crontab FILE in the group. If FILE is not supplied, displays the list of available crontabs in the group, along with their owners. '-r' Remove the crontab. '-u NAME' Operate on crontab of user NAME. '-V' Print program version, licensing information and exit. 5 Key Features ************** The following sections describe features specific to 'micrond'. 5.1 User Group Crontabs ======================= User group crontabs are an experimental feature designed to facilitate maintenance of per-service crontabs. Consider, for example, a web server that runs multiple web sites maintained by various users who need to run periodic backend jobs on behalf of the account the 'httpd' server runs as. User group crontabs make it possible without intervention of the system administrator. Let's assume 'httpd' runs as the user 'apache'. The system administrator creates a directory '/var/spool/cron/crongroups/apache', and sets 'apache' as its owner: mkdir /var/spool/cron/crongroups/apache chown apache: /var/spool/cron/crongroups/apache Then, he adds login names of the users who are allowed to edit apache cronjobs to the primary group of the 'apache' user. Once done, these users become able to create and edit crontabs in this directory using the 'crontab' command with the '-g' option (short for 'group'). For example, the command crontab -u apache -g -e myproject edits the file 'myproject' in this directory. User group crontabs are disabled by default. To enable them, run 'micrond' with the '-g group' option. 5.2 Long Crontab Lines ====================== Very long crontab lines can be split across several physical lines using the familiar backslash continuation technique: a backslash appearing immediately before the ending newline character is removed along with the newline and the content of the next line is appended in its place. Multiple line continuations are allowed, as long as total line length does not exceed 1024 characters. 5.3 Built-in Variables ====================== A number of built-in variables control the interpretation of crontab entries and execution of commands. Each built-in variable has two name variants: setting the name prefixed with '_JOB_' affects only the cron job definition that immediately follows (with optional variable assignments in between), whereas setting the name prefixed with '_MICRON_' affects all commands that follow, until another assignment of the same variable (or its '_JOB_' counterpart) is encountered or the end of file is reached. For example, the following fragment instructs 'micrond' to log all output produced by the command 'run-periodic' to syslog facility 'daemon' using the tag 'hourly'. These two settings affect only this particular command: _JOB_SYSLOG_FACILITY = daemon _JOB_SYSLOG_TAG = hourly 15 * * * * root run-periodic Built-in variables are described in detail in *note built-in variables::. 5.4 Day Field Semantics ======================= In a crontab schedule, the day of a command's execution can be specified by two fields: day of month (field 3), and day of week (field 5). If both fields are restricted (i.e. are not '*'), their interpretation differs between various 'cron' implementations. "Vixie" cron will run the command when either field matches the current time (the fields are thus joined by a logical OR). "Dillon's" cron interprets the 3rd field as an ordinal number of weekday in month (so that allowed numeric values of the 3rd field in this case are 1-5). Consider for example the following schedule: 0 11 1,4 * 1-3 For "Vixie" cron, this means "run the command on each 1st and 4th day of the month as well as on each Monday, Tuesday and Wednesday". The meaning of this schedule for "Dillon's" cron is: "run the command on each first and fourth Monday, Tuesday and Wednesday in the month". The semantics used by 'micron' is configurable. By default it assumes the two fields to be joined by a logical AND, i.e. the example above would mean "each first and fourth day of the month _if_ the day of week is Monday, Tuesday or Wednesday". The use of "Vixie" or "Dillon" semantics can be requested by setting the '_MICRON_DAY_SEMANTICS' variable in the crontab. For example, the line: _MICRON_DAY_SEMANTICS = Vixie requests the semantics used by "Vixie" cron. *Note _MICRON_DAY_SEMANTICS::, for a detailed description of this variable. 5.5 Variable Assignments in Crontabs ==================================== Variable assignments can appear anyplace in a crontab. The modified environment remains in effect for all subsequent commands until changed by another assignment or the end of file is reached, whichever happens first. For example, the output of the following two example entries is mailed to two different users: MAILTO=one * * * * * command one MAILTO=two * * * * * command two 5.6 Job Output Report ===================== Output of a crontab job can be either mailed to its owner (a traditional behavior), or reported via 'syslog' to arbitrary facility, or appended to a disk file. The default behavior is to send the output to the user on whose behalf the cronjob was run. This in discussed in *note Mailing the cronjob output::. When syslog is requested, each line of the cronjob output is sent to the syslog in real time. This setting can be enabled either globally by the use '-s' command line option, individually in a crontab (using the '_MICRON_SYSLOG_FACILITY' variable), or individually for a cronjob using the '_JOB_SYSLOG_FACILITY' variable. Syslog tag can be supplied using the '_JOB_SYSLOG_TAG' variable (*note _JOB_SYSLOG_TAG::). In its absence, syslog tag is constructed from the location of the job in the crontab file and the first word of the command. For a detailed discussion of syslog redirection, *Note Logging output to syslog::. Finally, the output can be captured and appended to a given disk file upon termination of the cronjob. This can be useful, e.g. when running 'micrond' in a Docker container. *Note Logging output to syslog::. 5.7 Simultaneous Job Execution ============================== 'Micrond' controls the number of simultaneously running instances of a cron job. It is set by the '_MICRON_MAXINSTANCES' variable. The default value is 1, which means that the job won't be started until its previous instance terminates. *Note _MICRON_MAXINSTANCES::. 5.8 Detection of Crontab Modifications ====================================== On GNU/Linux systems, 'micrond' uses inotify (http://man.gnu.org.ua/manpage/?7+inotify) to track crontab modifications, which means that any change to a crontab is noticed as soon as the crontab file is saved. On other systems, 'micrond' relies on checking the crontab modification times each minute, which is less effective. The use of the 'kqueue' interface on *BSD systems is planned in future versions. 6 Downloads and Installation **************************** The program can be downloaded from . Before installation, create a group which will be used as owner for the user and user group crontab directories. The 'crontab' binary will be installed as set-GID to that group. By default, the group is named 'crontab'. Assuming this, the usual build sequence is ./configure make make install If you chose another group name, supply it to 'configure' using the '--with-crontab-gid' option. The above commands will install the package under '/usr/local'. That is, the server will be installed as '/usr/local/sbin/micron', the 'crontab' utility as '/usr/local/bin/crontab', etc. If that's not what you want, use the '--prefix' option to specify the installation prefix, e.g. ./configure --prefix=/usr Please refer to the 'INSTALL' document in the source directory for a discussion of available options to configure and their effect. 7 Bug Reports ************* If you think you found a bug in 'micron' or in its documentation, please send a mail to (Sergey Poznyakoff) or use the bug tracker at (requires authorization). Appendix A GNU Free Documentation License ***************************************** Version 1.3, 3 November 2008 Copyright (C) 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. 0. PREAMBLE The purpose of this License is to make a manual, textbook, or other functional and useful document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others. This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software. We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. 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ADDENDUM: How to use this License for your documents ==================================================== To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page: Copyright (C) YEAR YOUR NAME. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''. If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the "with...Texts." line with this: with the Invariant Sections being LIST THEIR TITLES, with the Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST. If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation. If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software. Index ***** * Menu: * -e: crontab Invocation. (line 781) * -f: micrond invocation. (line 609) * -g: micrond invocation. (line 615) * -g <1>: micrond invocation. (line 620) * -g <2>: crontab Invocation. (line 784) * -h: micrond invocation. (line 612) * -h <1>: crontab Invocation. (line 787) * -i: crontab Invocation. (line 790) * -l: micrond invocation. (line 625) * -l <1>: crontab Invocation. (line 793) * -m: micrond invocation. (line 630) * -P: micrond invocation. (line 633) * -p: micrond invocation. (line 636) * -r: crontab Invocation. (line 799) * -S: micrond invocation. (line 641) * -s: micrond invocation. (line 647) * -T: micrond invocation. (line 653) * -t: micrond invocation. (line 658) * -u: crontab Invocation. (line 802) * -V: micrond invocation. (line 667) * -v: micrond invocation. (line 670) * -V <1>: crontab Invocation. (line 805) * -W: micrond invocation. (line 681) * @annually: Cronjob Definition. (line 232) * @daily: Cronjob Definition. (line 242) * @hourly: Cronjob Definition. (line 246) * @midnight: Cronjob Definition. (line 242) * @monthly: Cronjob Definition. (line 236) * @reboot: Cronjob Definition. (line 229) * @weekly: Cronjob Definition. (line 239) * @yearly: Cronjob Definition. (line 232) * _JOB_DAY_SEMANTICS: Variable Settings. (line 396) * _JOB_MAILTO: Variable Settings. (line 379) * _JOB_MAXINSTANCES: Variable Settings. (line 374) * _JOB_OUTFILE: Sending output to file. (line 545) * _JOB_OUTFILE_FACILITY: Variable Settings. (line 334) * _JOB_SYSLOG_FACILITY: Variable Settings. (line 342) * _JOB_SYSLOG_TAG: Variable Settings. (line 357) * _MICRON_DAY_SEMANTICS: Variable Settings. (line 395) * _MICRON_MAILTO: Variable Settings. (line 386) * _MICRON_MAXINSTANCES: Variable Settings. (line 373) * _MICRON_OUTFILE: Sending output to file. (line 532) * _MICRON_OUTFILE_FACILITY: Variable Settings. (line 333) * _MICRON_SYSLOG_FACILITY: Variable Settings. (line 341) * _MICRON_SYSLOG_FACILITY <1>: Logging output to syslog. (line 503) * _MICRON_SYSLOG_TAG: Variable Settings. (line 369) * built-in variable, initial value: micrond invocation. (line 670) * built-in variables: Variable Settings. (line 324) * day field semantics: Cronjob Definition. (line 183) * day field semantics <1>: Day Field Semantics. (line 877) * default editor: crontab. (line 717) * define a variable: Variable Settings. (line 292) * dillon day semantics: Cronjob Definition. (line 205) * Dillon, Matt: Overview. (line 42) * editor, default: crontab. (line 717) * EDITOR, environment variable: crontab. (line 717) * email, job output: Mailing the cronjob output. (line 450) * empty variable: Variable Settings. (line 307) * From: header, job output mail: Mailing the cronjob output. (line 450) * group crontab: Crongroups. (line 95) * HOME: Variable Settings. (line 415) * initial value of a built-in variable: micrond invocation. (line 670) * input, cronjob: Cronjob Definition. (line 257) * MAILTO: Variable Settings. (line 403) * master crontab: Crongroups. (line 83) * Matt Dillon: Overview. (line 42) * paranoid_memfree: micrond invocation. (line 682) * Paul Vixie: Overview. (line 42) * PID file: micrond invocation. (line 633) * SHELL: Variable Settings. (line 420) * standard input for a cronjob: Cronjob Definition. (line 257) * strict day semantics: Cronjob Definition. (line 189) * Subject: header, job output mail: Mailing the cronjob output. (line 459) * system crontab: Crongroups. (line 87) * unset a variable: Variable Settings. (line 315) * user crontab: Crongroups. (line 91) * variable, built-in: Variable Settings. (line 324) * variable, define: Variable Settings. (line 292) * variable, empty value: Variable Settings. (line 307) * variable, undefine: Variable Settings. (line 315) * variable, unset: Variable Settings. (line 315) * variables in crontab: Variable Settings. (line 292) * vi: crontab. (line 717) * VISUAL, environment variable: crontab. (line 717) * vixie day semantics: Cronjob Definition. (line 200) * Vixie, Paul: Overview. (line 42) * X-Cron-Env:, job output mail: Mailing the cronjob output. (line 465)