16.1.1 The bash Package and /etc/profile

Bash is the default system shell. When used as a login shell, it reads several initialization files. Bash processes them in the order they appear in this list:

Make custom settings in ~/.profile or ~/.bashrc. To ensure the correct processing of these files, it is necessary to copy the basic settings from /etc/skel/.profile or /etc/skel/.bashrc into the home directory of the user. It is recommended to copy the settings from /etc/skel after an update. Execute the following shell commands to prevent the loss of personal adjustments:

mv ~/.bashrc ~/.bashrc.old
cp /etc/skel/.bashrc ~/.bashrc
mv ~/.profile ~/.profile.old
cp /etc/skel/.profile ~/.profile

Then copy personal adjustments back from the *.old files.

16.1.2 The cron Package

If you want to run commands regularly and automatically in the background at predefined times, cron is the tool to use. cron is driven by specially formatted time tables. Some of of them come with the system and users can write their own tables if needed.

The cron tables are located in /var/spool/cron/tabs. /etc/crontab serves as a systemwide cron table. Enter the username to run the command directly after the time table and before the command. In Example 16-1, root is entered. Package-specific tables, located in /etc/cron.d, have the same format. See the cron man page (man cron).

1-59/5 * * * *   root   test -x /usr/sbin/atrun && /usr/sbin/atrun

You cannot edit /etc/crontab by calling the command crontab -e. This file must be loaded directly into an editor, modified, then saved.

A number of packages install shell scripts to the directories /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, and /etc/cron.monthly, whose execution is controlled by /usr/lib/cron/run-crons. /usr/lib/cron/run-crons is run every 15 minutes from the main table (/etc/crontab). This guarantees that processes that may have been neglected can be run at the proper time.

To run the hourly, daily, or other periodic maintenance scripts at custom times, remove the time stamp files regularly using /etc/crontab entries (see Example 16-2, which removes the hourly one before every full hour, the daily one once a day at 2:14 a.m., etc.).

59 *  * * *     root  rm -f /var/spool/cron/lastrun/cron.hourly
14 2  * * *     root  rm -f /var/spool/cron/lastrun/cron.daily
29 2  * * 6     root  rm -f /var/spool/cron/lastrun/cron.weekly
44 2  1 * *     root  rm -f /var/spool/cron/lastrun/cron.monthly

Alternatively, set DAILY_TIME in /etc/sysconfig/cron to the time at which cron.daily should start. The setting of MAX_NOT_RUN ensures that the daily jobs get triggered to run, even if the user did not turn on the computer at the specified DAILY_TIME for a longer period of time. The maximum value of MAX_NOT_RUN is 14 days.

The daily system maintenance jobs are distributed to various scripts for reasons of clarity. They are contained in the package aaa_base. /etc/cron.daily contains, for example, the components suse.de-backup-rpmdb, suse.de-clean-tmp, or suse.de-cron-local.

16.1.3 Log Files: Package logrotate

There are a number of system services (daemons) that, along with the kernel itself, regularly record the system status and specific events to log files. This way, the administrator can regularly check the status of the system at a certain point in time, recognize errors or faulty functions, and troubleshoot them with pinpoint precision. These log files are normally stored in /var/log as specified by FHS and grow on a daily basis. The logrotate package helps control the growth of these files.

Configure logrotate with the file /etc/logrotate.conf. In particular, the include specification primarily configures the additional files to read. Programs that produce log files install individual configuration files in /etc/logrotate.d. For example, such files ship with the packages, e.g. apache2 (/etc/logrotate.d/apache2) and syslogd (/etc/logrotate.d/syslog).

# see "man logrotate" for details
# rotate log files weekly
weekly

# keep 4 weeks worth of backlogs
rotate 4

# create new (empty) log files after rotating old ones
create

# uncomment this if you want your log files compressed
#compress

# RPM packages drop log rotation information into this directory
include /etc/logrotate.d

# no packages own lastlog or wtmp - we'll rotate them here
#/var/log/wtmp {
#    monthly
#    create 0664 root utmp
#    rotate 1
#}

# system-specific logs may be also be configured here.

logrotate is controlled through cron and is called daily by /etc/cron.daily/logrotate.

16.1.4 The locate Command

locate, a command for quickly finding files, is not included in the standard scope of installed software. If desired, install the package findutils-locate. The updatedb process is started automatically every night or about 15 minutes after booting the system.

16.1.5 The ulimit Command

With the ulimit (user limits) command, it is possible to set limits for the use of system resources and to have these displayed. ulimit is especially useful for limiting the memory available for applications. With this, an application can be prevented from using too much memory on its own, which could bring the system to a standstill.

ulimit can be used with various options. To limit memory usage, use the options listed in Table 16-1.

Systemwide entries can be made in /etc/profile. There, enable creation of core files, needed by programmers for debugging. A normal user cannot increase the values specified in /etc/profile by the system administrator, but can make special entries in ~/.bashrc.

# Limits maximum resident set size (physical memory):
ulimit -m 98304
 
# Limits of virtual memory:
ulimit -v 98304

Memory amounts must be specified in KB. For more detailed information, see man bash.

16.1.6 The free Command

The free command is somewhat misleading if your goal is to find out how much RAM is currently being used. That information can be found in /proc/meminfo. These days, users with access to a modern operating system, such as Linux, should not really need to worry much about memory. The concept of available RAM dates back to before the days of unified memory management. The slogan free memory is bad memory applies well to Linux. As a result, Linux has always made the effort to balance out caches without actually allowing free or unused memory.

Basically, the kernel does not have direct knowledge of any applications or user data. Instead, it manages applications and user data in a page cache. If memory runs short, parts of it are written to the swap partition or to files, from which they can initially be read with the help of the mmap command (see man mmap).

The kernel also contains other caches, such as the slab cache, where the caches used for network access are stored. This may explain differences between the counters in /proc/meminfo. Most, but not all of them, can be accessed via /proc/slabinfo.

16.1.7 The /etc/resolv.conf File

Domain name resolution is handled through the file /etc/resolv.conf. Refer to Section 22.0, The Domain Name System.

This file is updated by the script /sbin/modify_resolvconf exclusively, with no other program having permission to modify /etc/resolv.conf directly. Enforcing this rule is the only way to guarantee that the system's network configuration and the relevant files are kept in a consistent state.

16.1.8 Man Pages and Info Pages

For some GNU applications (such as tar), the man pages are no longer maintained. For these commands, use the --help option to get a quick overview of the info pages, which provide more in-depth instructions. Info is GNU's hypertext system. Read an introduction to this system by entering info info. Info pages can be viewed with Emacs by entering emacs -f info or directly in a console with info. You can also use tkinfo, xinfo, or the help system to view info pages.

16.1.9 Settings for GNU Emacs

GNU Emacs is a complex work environment. The following sections cover the configuration files processed when GNU Emacs is started. More information is available at https://www.gnu.org/software/emacs/.

On start-up, Emacs reads several files containing the settings of the user, system administrator, and distributor for customization or preconfiguration. The initialization file ~/.emacs is installed to the home directories of the individual users from /etc/skel. .emacs, in turn, reads the file /etc/skel/.gnu-emacs. To customize the program, copy .gnu-emacs to the home directory (with cp /etc/skel/.gnu-emacs ~/.gnu-emacs) and make the desired settings there.

.gnu-emacs defines the file ~/.gnu-emacs-custom as custom-file. If users make settings with the customize options in Emacs, the settings are saved to ~/.gnu-emacs-custom.

With openSUSE, the emacs package installs the file site-start.el in the directory /usr/share/emacs/site-lisp. The file site-start.el is loaded before the initialization file ~/.emacs. Among other things, site-start.el ensures that special configuration files distributed with Emacs add-on packages, such as psgml, are loaded automatically. Configuration files of this type are located in /usr/share/emacs/site-lisp, too, and always begin with suse-start-. The local system administrator can specify systemwide settings in default.el.

More information about these files is available in the Emacs info file under Init File: info:/emacs/InitFile. Information about how to disable loading these files (if necessary) is also provided at this location.

The components of Emacs are divided into several packages: