Which of the following file types are commonly used by scripts in a linux command line environment?

1.1.1 Bash Configuration Files #

A shell can be invoked as an:

1. Interactive login shell.  This is used when logging in to a machine, invoking Bash with the --login option or when logging in to a remote machine with SSH.

2. “Ordinary” interactive shell.  This is normally the case when starting xterm, konsole, gnome-terminal, or similar command-line interface (CLI) tools.

3. Non-interactive shell.  This is invoked when invoking a shell script at the command line.

Depending on the type of shell you use, different configuration files will be read. The following tables show the login and non-login shell configuration files.

Note that the login shell also sources the configuration files listed under Table 1.2, “Bash Configuration Files for Non-Login Shells”.

Additionally, Bash uses some more files:

No-Login Shells#

There are special shells that block users from logging into the system: /bin/false and /sbin/nologin. Both fail silently when the user attempts to log into the system. This was intended as a security measure for system users, though modern Linux operating systems have more effective tools for controlling system access, such as PAM and AppArmor.

The default on SUSE Linux Enterprise Server is to assign /bin/bash to human users, and /bin/false or /sbin/nologin to system users. The nobody user has /bin/bash for historical reasons, as it is a minimally-privileged user that used to be the default for system users. However, whatever little bit of security gained by using nobody is lost when multiple system users use it. It should be possible to change it to /sbin/nologin; the fastest way to test it is change it and see if it breaks any services or applications.

Use the following command to list which shells are assigned to all users, system and human users, in /etc/passwd. The output varies according to the services and users on your system:

> sort -t: -k 7 /etc/passwd | awk -F: '{print $1"\t" $7}' | column -t tux /bin/bash nobody /bin/bash root /bin/bash avahi /bin/false chrony /bin/false dhcpd /bin/false dnsmasq /bin/false ftpsecure /bin/false lightdm /bin/false mysql /bin/false postfix /bin/false rtkit /bin/false sshd /bin/false tftp /bin/false unbound /bin/false bin /sbin/nologin daemon /sbin/nologin ftp /sbin/nologin lp /sbin/nologin mail /sbin/nologin man /sbin/nologin nscd /sbin/nologin polkitd /sbin/nologin pulse /sbin/nologin qemu /sbin/nologin radvd /sbin/nologin rpc /sbin/nologin statd /sbin/nologin svn /sbin/nologin systemd-coredump /sbin/nologin systemd-network /sbin/nologin systemd-timesync /sbin/nologin usbmux /sbin/nologin vnc /sbin/nologin wwwrun /sbin/nologin messagebus /usr/bin/false scard /usr/sbin/nologin

1.1.2 The Directory Structure #

The following table provides a short overview of the most important higher-level directories that you find on a Linux system. Find more detailed information about the directories and important subdirectories in the following list.

The following list provides more detailed information and gives some examples of which files and subdirectories can be found in the directories:

/bin

Contains the basic shell commands that may be used both by root and by other users. These commands include ls, mkdir, cp, mv, rm and rmdir. /bin also contains Bash, the default shell in SUSE Linux Enterprise Server.

Contains data required for booting, such as the boot loader, the kernel, and other data that is used before the kernel begins executing user-mode programs.

Holds device files that represent hardware components.

Contains local configuration files that control the operation of programs like the X Window System. The /etc/init.d subdirectory contains LSB init scripts that can be executed during the boot process.

Holds the private data of every user who has an account on the system. The files located here can only be modified by their owner or by the system administrator. By default, your e-mail directory and personal desktop configuration are located here in the form of hidden files and directories, such as .gconf/ and .config.

If you are working in a network environment, your home directory may be mapped to a directory in the file system other than /home.

Contains the essential shared libraries needed to boot the system and to run the commands in the root file system. The Windows equivalent for shared libraries are DLL files.

Contains mount points for removable media, such as CD-ROMs, flash disks, and digital cameras (if they use USB). /media generally holds any type of drive except the hard disk of your system. When your removable medium has been inserted or connected to the system and has been mounted, you can access it from here.

This directory provides a mount point for a temporarily mounted file system. root may mount file systems here.

Reserved for the installation of third-party software. Optional software and larger add-on program packages can be found here.

Home directory for the root user. The personal data of root is located here.

A tmpfs directory used by systemd and various components. /var/run is a symbolic link to /run.

As the s indicates, this directory holds utilities for the superuser. /sbin contains the binaries essential for booting, restoring and recovering the system in addition to the binaries in /bin.

Holds data for services provided by the system, such as FTP and HTTP.

This directory is used by programs that require temporary storage of files.

Data stored in /tmp is not guaranteed to survive a system reboot. It depends, for example, on settings made in /etc/tmpfiles.d/tmp.conf.

/usr has nothing to do with users, but is the acronym for Unix system resources. The data in /usr is static, read-only data that can be shared among various hosts compliant with the Filesystem Hierarchy Standard (FHS). This directory contains all application programs including the graphical desktops such as GNOME and establishes a secondary hierarchy in the file system. /usr holds several subdirectories, such as /usr/bin, /usr/sbin, /usr/local, and /usr/share/doc.

Contains generally accessible programs.

Contains programs reserved for the system administrator, such as repair functions.

In this directory the system administrator can install local, distribution-independent extensions.

Holds various documentation files and the release notes for your system. In the manual subdirectory find an online version of this manual. If more than one language is installed, this directory may contain versions of the manuals for different languages.

Under packages find the documentation included in the software packages installed on your system. For every package, a subdirectory /usr/share/doc/packages/PACKAGENAME is created that often holds README files for the package and sometimes examples, configuration files or additional scripts.

If HOWTOs are installed on your system /usr/share/doc also holds the howto subdirectory in which to find additional documentation on many tasks related to the setup and operation of Linux software.

Whereas /usr holds static, read-only data, /var is for data which is written during system operation and thus is variable data, such as log files or spooling data. For an overview of the most important log files you can find under /var/log/, refer to Table 44.1, “Log Files”.