tmpfs 514208 269136 245072 53% /dev/shm
/dev/sda5 44552904 36315896 7784380 83% /home
The id command displays the identity of the user running the session, along with the list of groups they belong to. Since access to some files or devices may be limited to group members, checking available group membership may be useful.
$ id
uid=1000(rhertzog) gid=1000(rhertzog) groups=1000(rhertzog),24(cdrom),25(floppy),27(sudo),29(audio),30(dip),44(video),46(plugdev),108(netdev),109(bluetooth),115(scanner)
B.2. Organization of the Filesystem Hierarchy
B.2.1. The Root Directory
A Debian system is organized along the File Hierarchy Standard (FHS). This standard defines the purpose of each directory. For instance, the top-level directories are described as follows:
/bin/: basic programs;
/boot/: Linux kernel and other files required for its early boot process;
/dev/: device files;
/etc/: configuration files;
/home/: user's personal files;
/lib/: basic libraries;
/media/*: mount points for removable devices (CD-ROM, USB keys and so on);
/mnt/: temporary mount point;
/opt/: extra applications provided by third parties;
/root/: administrator's (root's) personal files;
/sbin/: system programs;
/srv/: data used by servers hosted on this system;
/tmp/: temporary files; this directory is often emptied at boot;
/usr/: applications; this directory is further subdivided into bin, sbin, lib (according to the same logic as in the root directory). Furthermore, /usr/share/ contains architecture-independent data. /usr/local/ is meant to be used by the administrator for installing applications manually without overwriting files handled by the packaging system (dpkg).
/var/: variable data handled by daemons. This includes log files, queues, spools, caches and so on.
/proc/ and /sys/ are specific to the Linux kernel (and not part of the FHS). They are used by the kernel for exporting data to user-space.
B.2.2. The User's Home Directory
The contents of a user's home directory is not standardized, but there are still a few noteworthy conventions. One is that a user's home directory is often referred to by a tilde (“~”). That is useful to know because command interpreters automatically replace a tilde with the correct directory (usually /home/user/).
Application configuration files are often stored directly under the user's home directory, but their names usually start with a dot (for instance, the mutt email client stores its configuration in ~/.muttrc). Filenames that start with a dot are hidden by default, and ls only lists them when the -a option is used.
Some programs use multiple configuration files organized in one directory (for instance, ~/.evolution/). Some applications (such as the Iceweasel web browser) also use their directory to store a cache of downloaded data. This means that those directories can end up using a lot of disk space.
Graphical desktops usually display the contents of the ~/Desktop/ directory (or ~/Bureau/ or whatever the appropriate translation is for systems not configured in English) on the desktop (ie, what's visible on screen once all applications are closed or iconized).
Finally, the email system sometimes stores incoming emails into a ~/Mail/ directory.
B.3. Inner Workings of a Computer: the Different Layers Involved
A computer is often considered as something rather abstract, and the externally visible interface is much simpler than its internal complexity. Such complexity comes in part from the number of pieces involved. However, these pieces can be viewed in layers, where a layer only interacts with those immediately above or below.
An end-user can get by without knowing these details… as long as everything works. When confronting a problem such as, “The internet doesn't work!”, the first thing to do is to identify in which layer the problem originates. Is the network card (hardware) working? Is it recognized by the computer? Does the Linux kernel see it? Are the network parameters properly configured? All these questions isolate an appropriate layer and focus on a potential source of the problem.
B.3.1. The Deepest Layer: the Hardware
Let us start with a basic reminder that a computer is, first and foremost, a set of hardware elements. There is generally a main board, with one (or more) processor(s), some RAM, device controllers, and extension slots for option boards (for other device controllers). Most noteworthy among these controllers are IDE (Parallel ATA), SCSI and Serial ATA, for connecting to storage devices such as hard disks. Other controllers include USB, which is able to host a great variety of devices (ranging from webcams to thermometers, from keyboards to home automation systems) and IEEE_1394 (Firewire). These controllers often allow connecting several devices so the complete subsystem handled by a controller is therefore usually known as a “bus”. Option boards include graphics cards (where monitor screens will be plugged in to), sound cards, network interface cards, and so on. Some main boards are pre-built with these features, and don't need option boards.
IN PRACTICE Checking that the hardware works
Checking that a piece of hardware works can be tricky. On the other hand, proving that it doesn't work is sometimes quite simple.
A hard disk drive is made of spinning platters and moving magnetic heads. When a hard disk is powered up, the platter motor makes a characteristic whir. It also dissipates energy as heat. Consequently, a hard disk drive that stays cold and silent when powered up is broken.
Network cards often include LEDs displaying the state of the link. If a cable is plugged in and leads to a working network hub or switch, at least one LED will be on. If no LEDs lights, either the card itself, the network device, or the cable between them, is faulty. The next step is therefore testing each component individually.
Some option boards — especially 3D video cards — include cooling devices, such as heat sinks and/or fans. If the fan does not spin even though the card is powered up, a plausible explanation is the card overheated. This also applies to the main processor(s) located on the main board.
B.3.2. The Starter: the BIOS
Hardware, on its own, is unable to perform useful tasks without a corresponding piece of software driving it. Controlling and interacting with the hardware is the purpose of the operating system and applications. These, in turn, require functional hardware to run.
This symbiosis between hardware and software does not happen on its own. When the computer is first powered up, some initial setup is required. This role is assumed by the BIOS, a tiny piece of software embedded into the main board that runs automatically upon power-up. Its primary task is searching for software it can hand over control to. Usually, this involves looking for the first hard disk with a boot sector (also known as the master boot record or MBR), loading that boot sector, and running it. From then on, the BIOS is usually not involved (until the next boot).
TOOL Setup, the BIOS configuration tool
The BIOS also contains a piece of software called Setup, designed to allow configuring aspects of the computer. In particular, it allows choosing which boot device is preferred (for instance, the floppy disk or CD-ROM drive), setting the system clock, and so on. Starting Setup usually involves pressing a key very soon after the computer is powered on. This key is often Del or Esc, sometimes F2 or F10. Most of the time, the choice is flashed on screen while booting.