At Your Service—Job Scheduling for Linux
If your attempt to use one of the at programs produces the following diagnostic: “You do not have permission to use at”, you need to contact your local SA. Linux SAs can manage at with a fair amount of flexibility.
As you might expect, the root user has absolute permission to use the at utility and can grant the same permission to nonroot users. Two system files, /etc/at.allow and /etc/at.deny, control access to the at utility. Table 3 shows how their presence and content determines users' permission on a given system.
Note that if the /etc/at.allow files exist, /etc/at.deny is completely ignored. Users are identified in both files by their Linux login, each appearing on a separate line. at does not provide a command-line utility to control the content of these files. SAs generally select their favorite text editor and manually edit the files as needed. This hardly can be considered a shortcoming, though, given the likely infrequency of change.
To summarize, nonroot users can be explicitly or implicitly assigned or denied permission to use at. SAs either can choose to manage access to at by exclusion or inclusion. Select the approach that makes the most sense for your particular installation. For example, a highly sensitive production site probably should be managed based on inclusion (i.e., nonroot users do not have permission unless it's explicitly granted—the /etc/at.allow file exists). Conversely, the Linux default configuration might be fine for most development/test environments (i.e., nonroot users have permission unless it's explicitly denied—/etc/at.allow does not exist and /etc/at.deny has zero or more entries).
Collectively, the at programs offer an intuitive way to manage the deferred execution of applications. Despite its simplicity and usefulness, the at utility is often ignored by Linux administrators and developers. Other less frequently used at command-line options exist that I chose not to cover here. I encourage you to review the at manual page by typing man at at your favorite shell's prompt to review all details. Also, most Linux overview books provide some coverage of at and similar programs, such as O'Reilly's Linux in a Nutshell by Ellen Siever, et. al.
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