11.6. 設定 `cron`
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## 11.6. 設定 `cron`

Contributed by Tom Rhodes.

FreeBSD 最好用的工具之一就是 cron(8)。 `cron` 會在背景下運作，並不斷檢查 `/etc/crontab` 檔以及 `/var/cron/tabs` 目錄，來搜尋是否有新 `crontab` 檔案。 這些 `crontab` 檔會存放一些排程工作的設定，來給 `cron` 執行。

`cron` 程式，可同時採用兩種不同類型的設定檔：系統本身的 crontab 及使用者本身的 crontab。而兩種格式唯一差別在於第六欄的不同；In the system crontab, the sixth field is the name of a user for the command to run as. This gives the system crontab the ability to run commands as any user. In a user crontab, the sixth field is the command to run, and all commands run as the user who created the crontab; this is an important security feature.

### 注意:

User crontabs allow individual users to schedule tasks without the need for `root` privileges. Commands in a user's crontab run with the permissions of the user who owns the crontab.

The `root` user can have a user crontab just like any other user. This one is different from `/etc/crontab` (the system crontab). Because of the system crontab, there is usually no need to create a user crontab for `root`.

Let us take a look at the `/etc/crontab` file (the system crontab):

# /etc/crontab - root's crontab for FreeBSD # # $FreeBSD: src/etc/crontab,v 1.32 2002/11/22 16:13:39 tom Exp $ # # SHELL=/bin/sh PATH=/etc:/bin:/sbin:/usr/bin:/usr/sbin HOME=/var/log # # #minute hour mday month wday who command # # */5 * * * * root /usr/libexec/atrun

| Like most FreeBSD configuration files, the `#` character represents a comment. A comment can be placed in the file as a reminder of what and why a desired action is performed. Comments cannot be on the same line as a command or else they will be interpreted as part of the command; they must be on a new line. Blank lines are ignored.
| First, the environment must be defined. The equals (`=`) character is used to define any environment settings, as with this example where it is used for the `SHELL`, `PATH`, and `HOME` options. If the shell line is omitted, `cron` will use the default, which is `sh`. If the `PATH` variable is omitted, no default will be used and file locations will need to be absolute. If `HOME` is omitted, `cron` will use the invoking users home directory.
| This line defines a total of seven fields. Listed here are the values `minute`, `hour`, `mday`, `month`, `wday`, `who`, and `command`. These are almost all self explanatory. `minute` is the time in minutes the command will be run. `hour` is similar to the `minute` option, just in hours. `mday` stands for day of the month. `month` is similar to `hour` and `minute`, as it designates the month. The `wday` option stands for day of the week. All these fields must be numeric values, and follow the twenty-four hour clock. The `who` field is special, and only exists in the `/etc/crontab` file. This field specifies which user the command should be run as. When a user installs his or her `crontab` file, they will not have this option. Finally, the `command` option is listed. This is the last field, so naturally it should designate the command to be executed.
| This last line will define the values discussed above. Notice here we have a `*/5` listing, followed by several more `*` characters. These `*` characters mean “first-last”, and can be interpreted as every time. So, judging by this line, it is apparent that the `atrun` command is to be invoked by `root` every five minutes regardless of what day or month it is. For more information on the `atrun` command, see the atrun(8) manual page. Commands can have any number of flags passed to them; however, commands which extend to multiple lines need to be broken with the backslash “\” continuation character.

This is the basic set up for every `crontab` file, although there is one thing different about this one. Field number six, where we specified the username, only exists in the system `/etc/crontab` file. This field should be omitted for individual user `crontab` files.

### 11.6.1. 工作排程(Crontab)的排定與管理

### 重要:

You must not use the procedure described here to edit/install the system crontab. Simply use your favorite editor: the `cron` utility will notice that the file has changed and immediately begin using the updated version. See this FAQ entry for more information.

To install a freshly written user `crontab`, first use your favorite editor to create a file in the proper format, and then use the `crontab` utility. The most common usage is:

`%` `crontab crontab-file`

In this example, `crontab-file` is the filename of a `crontab` that was previously created.

There is also an option to list installed `crontab` files: just pass the `-l` option to `crontab` and look over the output.

For users who wish to begin their own crontab file from scratch, without the use of a template, the `crontab -e` option is available. This will invoke the selected editor with an empty file. When the file is saved, it will be automatically installed by the `crontab` command.

If you later want to remove your user `crontab` completely, use `crontab` with the `-r` option.

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