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Introduction

In this session, we have covered the most common directories in the Linux file tree. It also shows that on Unix everything is a file.

filesystem hierarchy standard

Many Linux distributions partially follow the Filesystem Hierarchy Standard. The FHS (Filesystem Hierarchy Standard) may help make more Unix/Linux file system trees conform better in the future. It is known from the FHS that The filesystem hierarchy standard has been designed to be used by Unix distribution developers, package developers, and system implementers. However, it is primarily intended to be a reference and is not a tutorial on how to manage a Unix filesystem or directory hierarchy.

man hier

There are some differences in the filesystems between Linux distributions. For help about your machine, enter man hier to find information about the file system hierarchy. This manual will explain the directory structure on your computer.

the root directory /

All Linux systems have a directory structure that starts at the root directory (the home directory of the root directory). The root directory is represented by a forward slash, like this: /. Everything that exists on your Linux system can be found below this root directory. Let's take a brief look at the contents of the root directory.

[email protected]:~$ ls /
bin   cdrom  etc   initrd.img  lost+found  mnt  proc  run   srv  tmp  var
boot  dev    home  lib         media       opt  root  sbin  sys  usr  vmlinuz

binary directories

Binaries are files that contain compiled source code (or machine code). Binaries can be executed on the computer. Sometimes binaries are called executables.

/bin

The /bin directory contains standard system utilities. According to the FHS the /bin directory should contain /bin/cat and /bin/date (among others).
The following information shows the common Unix/Linux commands like cat, cp, cpio, date, dd, echo, grep, and so on.

[email protected]:~$ ls /bin
bash                  fgconsole   nc                       sed
bunzip2               fgrep       nc.openbsd               setfacl
busybox               findmnt     netcat                   setfont
bzcat                 fuser       netstat                  setupcon
bzcmp                 fusermount  nisdomainname            sh
bzdiff                getfacl     ntfs-3g                  sh.distrib
bzegrep               grep        ntfs-3g.probe            sleep
bzexe                 gunzip      ntfs-3g.secaudit         ss
bzfgrep               gzexe       ntfs-3g.usermap          static-sh
bzgrep                gzip        ntfscat                  stty
bzip2                 hostname    ntfsck                   su
bzip2recover          ip          ntfscluster              sync
bzless                kbd_mode    ntfscmp                  tailf
bzmore                kill        ntfsdump_logfile         tar
cat                   kmod        ntfsfix                  tempfile
chacl                 less        ntfsinfo                 touch
...

other /bin directories

You can find a /bin subdirectory in many other directories. A user named serena could put her own programs in /home/serena/bin.
Some applications, often when installed directly from source will put themselves in /opt. A samba server installation can use /opt/samba/bin to store its binaries.

/sbin

T/sbin contains binaries to configure the operating system. Many of the system binaries require root privilege to perform certain tasks.
Below a screenshot containing system binaries to change the ip address, partition a disk and create an ext4 file system.

[email protected]:~$ ls -l /sbin/ifconfig /sbin/fdisk /sbin/mkfs.ext4
-rwxr-xr-x 1 root root 105444 Apr 16 21:37 /sbin/fdisk
-rwxr-xr-x 1 root root  69484 Dec  8  2013 /sbin/ifconfig
lrwxrwxrwx 1 root root      6 Jun 20 11:30 /sbin/mkfs.ext4 -> mke2fs

/lib

Binaries found in /bin and /sbin often use shared libraries located in /lib. Below is a screenshot of the partial contents of /lib.

[email protected]:/$ cd lib
[email protected]:/lib$ ls
apparmor                              libip4tc.so.0         modules
brltty                                libip4tc.so.0.1.0     modules-load.d
cpp                                   libip6tc.so.0         plymouth
crda                                  libip6tc.so.0.1.0     recovery-mode
firmware                              libiptc.so.0          resolvconf
hdparm                                libiptc.so.0.0.0      systemd
i386-linux-gnu                        libxtables.so.10      terminfo
ifupdown                              libxtables.so.10.0.0  udev
init                                  linux-sound-base      ufw
klibc-SDKhWJaiUdo4OxxZ-mvprY1CZus.so  lsb                   xtables
ld-linux.so.2                         modprobe.d

/lib/modules

Typically, the Linux kernel loads kernel modules from /lib/modules/$kernel-version/. This directory is discussed in detail in the Linux kernel chapter.

/lib32 and /lib64

We currently are in a transition between 32-bit and 64-bit systems. Therefore, you may encounter directories named /lib32 and /lib64 which clarify the register size used during compilation time of the libraries. A 64-bit computer may have some 32-bit binaries and libraries for compatibility with legacy applications. This screenshot uses the file utility to demonstrate the difference.

The ELF (Executable and Linkable Format) is used in almost every Unix-like operating system since System V.

/opt

The purpose of /opt is to store optional software. In many cases, this is software from outside the distribution repository. You may find an empty /opt directory on many systems.

A large package can install all its files in /bin, /lib, /etc subdirectories within /opt/ $packagename/. If for example the package is called wp, then it installs in /opt/wp, putting binaries in /opt/wp/bin and manpages in /opt/wp/man.

configuration directories

/boot

The /boot directory contains all files needed to boot the computer. These files don't change very often. On Linux systems, you typically find the /boot/grub directory here. /boot/grub contains /boot/grub/grub.cfg (older systems may still have /boot/grub/grub.conf) which defines the boot menu that is displayed before the kernel starts.

/etc

All the administrative files and configuration files should be located in /etc. Many times the name of a configuration files is the same as the application, daemon, or protocol with .conf added as the extension.

[email protected]:/$ ls /etc/*.conf
/etc/adduser.conf          /etc/insserv.conf     /etc/pnm2ppa.conf
/etc/apg.conf              /etc/kernel-img.conf  /etc/popularity-contest.conf
/etc/blkid.conf            /etc/kerneloops.conf  /etc/resolv.conf
/etc/brltty.conf           /etc/ld.so.conf       /etc/rsyslog.conf
/etc/ca-certificates.conf  /etc/libaudit.conf    /etc/sensors3.conf
/etc/colord.conf           /etc/logrotate.conf   /etc/signond.conf
/etc/debconf.conf          /etc/ltrace.conf      /etc/sysctl.conf
/etc/deluser.conf          /etc/mke2fs.conf      /etc/ts.conf
/etc/fuse.conf             /etc/mtools.conf      /etc/ucf.conf
/etc/gai.conf              /etc/netscsid.conf    /etc/updatedb.conf
/etc/hdparm.conf           /etc/nsswitch.conf    /etc/usb_modeswitch.conf
/etc/host.conf             /etc/pam.conf         /etc/wodim.conf
[email protected]:/$

There is much more to be found in /etc.

/etc/init.d/

A lot of Unix/Linux distributions have an /etc/init.d directory that contains scripts to start and stop daemons. This directory could disappear as Linux migrates to systems that replace the old init way of starting all daemons.

/etc/X11/

The graphical display (aka X Window System or just X) is driven by software from the X.org foundation. The configuration file for your graphical display is /etc/X11/xorg.conf.

/etc/skel/

The skeleton directory /etc/skel is copied to the home directory of a newly created user. It usually contains hidden files like a .bashrc script.

/etc/sysconfig/

This directory, which is not mentioned in the FHS, contains a lot of Red Hat Enterprise Linux configuration files. We will discuss some of them in greater detail. The screenshot below is the /etc/sysconfig directory from RHELv4u4 with everything installed.

[email protected]:/etc$ ls /etc | more
acpi
adduser.conf
alternatives
anacrontab
apg.conf
apm
apparmor
apparmor.d
apport
apt
aptdaemon
at-spi2
avahi
bash.bashrc
bash_completion
bash_completion.d
bindresvport.blacklist
blkid.conf
blkid.tab
bluetooth
brlapi.key
brltty
brltty.conf
--More--

The file /etc/sysconfig/firstboot tells the Red Hat Setup Agent not to run at boot time. If you want to run the Red Hat Setup Agent at the next reboot, then simply remove this file, and run chkconfig --level 5 firstboot on. The Red Hat Setup Agent allows you to install the latest updates, create a user account, join the Red Hat Network and more. It will then create the /etc/sysconfig/firstboot file again.

The /etc/sysconfig/harddisks file contains some parameters to tune the hard disks. The file explains itself.

You can see hardware detected by kudzu in /etc/sysconfig/hwconf. Kudzu is software from Red Hat for automatic discovery and configuration of hardware.

The keyboard type and keymap table are set in the /etc/sysconfig/keyboard file. For more console keyboard information, check the manual pages of keymaps(5), dumpkeys(1), loadkeys(1) and the directory /lib/kbd/keymaps/.

We will discuss networking files in this directory in the networking chapter.

data directories

/home

Users' home directories are stored here, users can store personal or project data under /home. It is common (but not mandatory by the fhs) practice to name the users home directory after the user name in the format /home/ $USERNAME. For example:

[email protected]:/$ ls /home
datasoft

Besides giving every user (or every project or group) a location to store personal files, the home directory of a user also serves as a location to store the user profile. A typical Unix user profile contains many hidden files (files whose file name starts with a dot). The hidden files of the Unix user profiles contain settings specific for that user.

/root

On many systems /root is the default location for personal data and profile of the root user. If it does not exist by default, then some administrators create it.

/srv

You may use /srv for data that is served by your system. The FHS allows locating cvs, rsync, ftp and www data in this location. The FHS also approves administrative naming in / srv, like /srv/project55/ftp and /srv/sales/www.

On Sun Solaris (or Oracle Solaris) /export is used for this purpose.

/media

The /media directory serves as a mount point for removable media devices such as CDROM's, digital cameras, and various usb-attached devices. Since /media is rather new in the Unix world, you could very well encounter systems running without this directory. Solaris 9 does not have it, Solaris 10 does. Most Linux distributions today mount all removable media in /media.

[email protected]:/$ ls /home
datasoft

/mnt

The /mnt directory should be empty and should only be used for temporary mount points (according to the FHS).

Unix and Linux administrators used to create many directories here, like /mnt/something/. You likely will encounter many systems with more than one directory created and/or mounted inside /mnt to be used for various local and remote filesystems.

/tmp

Applications and users should use /tmp to store temporary data when needed. Data stored in /tmp may use either disk space or RAM. Both of which are managed by the operating system. Never use /tmp to store data that is important or which you wish to archive.

in memory directories

/dev

Device files in /dev appear to be ordinary files, but are not actually located on the hard disk. The /dev directory is populated with files as the kernel is recognizing hardware.

common physical devices

Common hardware such as hard disk devices are represented by device files in /dev. Below a screenshot of SATA device files on a laptop and then IDE attached drives on a desktop. (The detailed meaning of these devices will be discussed later.)

[email protected]:/$ ls /dev/sd*
/dev/sda   /dev/sda10  /dev/sda5  /dev/sda7  /dev/sda9
/dev/sda1  /dev/sda2   /dev/sda6  /dev/sda8

Besides representing physical hardware, some device files are special. These special devices can be very useful.

/dev/tty and /dev/pts

For example, /dev/tty1 represents a terminal or console attached to the system. (Don't break your head on the exact terminology of 'terminal' or 'console', what we mean here is a command line interface.) When typing commands in a terminal that is part of a graphical interface like Gnome or KDE, then your terminal will be represented as /dev/pts/1 (1 can be another number).

/dev/null

On Linux you will find other special devices such as /dev/null which can be considered a black hole; it has unlimited storage, but nothing can be retrieved from it. Technically speaking, anything written to /dev/null will be discarded. /dev/null can be useful to discard unwanted output from commands. /dev/null is not a good location to store your backups ;-).

/proc conversation with the kernel

/proc is another special directory, appearing to be ordinary files, but not taking up disk space. It is actually a view of the kernel, or better, what the kernel manages, and is a means to interact with it directly. /proc is a proc filesystem.

[email protected]:/$ mount -t proc
proc on /proc type proc (rw,noexec,nosuid,nodev)

When listing the /proc directory you will see many numbers (on any Unix) and some interesting files (on Linux)

[email protected]:/$ ls /proc
1     1632  19    2195  34   751        cpuinfo        mounts
10    1635  1901  22    346  754        crypto         mtrr
1014  1643  1904  2201  35   761        devices        net
1063  1647  1909  2202  36   78         diskstats      pagetypeinfo
11    1656  1927  2261  37   79         dma            partitions
1107  1660  1931  2272  417  8          driver         sched_debug
1130  1664  1935  23    481  80         execdomains    schedstat
12    1667  1949  2302  49   802        fb             scsi
13    1686  1954  2308  495  846        filesystems    self
1323  1695  1958  2315  5    885        fs             slabinfo
136   17    1962  2324  50   888        interrupts     softirqs
137   1709  1977  24    52   889        iomem          stat
1498  1710  2     2481  53   896        ioports        swaps
15    1712  20    25    54   9          ipmi           sys
1500  1714  2012  2503  55   949        irq            sysrq-trigger
1563  1723  2057  2551  565  954        kallsyms       sysvipc
1571  1724  2067  26    577  957        kcore          timer_list
1580  1725  2082  27    592  975        key-users      timer_stats
1586  1730  2090  2726  596  984        kmsg           tty
1598  1744  21    276   597  986        kpagecount     uptime
16    1760  2105  28    618  995        kpageflags     version
1614  1772  2110  282   638  acpi       latency_stats  version_signature
1618  1794  2117  29    671  asound     loadavg        vmallocinfo
1622  18    2129  3     679  buddyinfo  locks          vmstat
1623  1804  2143  31    7    bus        mdstat         zoneinfo
1625  1813  2152  319   740  cgroups    meminfo
1626  1818  2159  32    744  cmdline    misc
1631  1898  2182  33    750  consoles   modules

Let's investigate the file properties inside /proc. Looking at the date and time will display the current date and time showing the files are constantly updated (a view on the kernel).

[email protected]:/$ date
Wed Jul 30 16:13:52 IST 2014
[email protected]:/$ ls -al /proc/cpuinfo
-r--r--r-- 1 root root 0 Jul 30 16:14 /proc/cpuinfo
[email protected]:/$ 
[email protected]:~$ date
Wed Jul 30 17:28:27 IST 2014
[email protected]:~$ ls -al /proc/cpuinfo
-r--r--r-- 1 root root 0 Jul 30 17:28 /proc/cpuinfo

Most files in /proc are 0 bytes, yet they contain data--sometimes a lot of data. You can see this by executing cat on files like /proc/cpuinfo, which contains information about the CPU.

[email protected]:~$ file /proc/cpuinfo
/proc/cpuinfo: empty 
[email protected]:~$ cat /proc/cpuinfo
processor	: 0
vendor_id	: GenuineIntel
cpu family	: 6
model		: 42
model name	: Intel(R) Pentium(R) CPU G620 @ 2.60GHz
stepping	: 7
microcode	: 0x25
cpu MHz		: 1600.000
cache size	: 3072 KB
physical id	: 0
siblings	: 2
core id		: 0
cpu cores	: 2
apicid		: 0
initial apicid	: 0
fdiv_bug	: no
f00f_bug	: no
coma_bug	: no
fpu		: yes
fpu_exception	: yes
cpuid level	: 13
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx rdtscp lm constant_tsc arch_perfmon pebs bts xtopology nonstop_tsc aperfmperf eagerfpu pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr pdcm pcid sse4_1 sse4_2 popcnt tsc_deadline_timer xsave lahf_lm arat epb xsaveopt pln pts dtherm tpr_shadow vnmi flexpriority ept vpid
bogomips	: 5188.25
clflush size	: 64
cache_alignment	: 64
address sizes	: 36 bits physical, 48 bits virtual
power management:

processor	: 1
vendor_id	: GenuineIntel
cpu family	: 6
model		: 42
model name	: Intel(R) Pentium(R) CPU G620 @ 2.60GHz
stepping	: 7
microcode	: 0x25
cpu MHz		: 1600.000
cache size	: 3072 KB
physical id	: 0
siblings	: 2
core id		: 1
cpu cores	: 2
apicid		: 2
initial apicid	: 2
fdiv_bug	: no
f00f_bug	: no
coma_bug	: no
fpu		: yes
fpu_exception	: yes
cpuid level	: 13
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe nx rdtscp lm constant_tsc arch_perfmon pebs bts xtopology nonstop_tsc aperfmperf eagerfpu pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr pdcm pcid sse4_1 sse4_2 popcnt tsc_deadline_timer xsave lahf_lm arat epb xsaveopt pln pts dtherm tpr_shadow vnmi flexpriority ept vpid
bogomips	: 5188.25
clflush size	: 64
cache_alignment	: 64
address sizes	: 36 bits physical, 48 bits virtual
power management:

[email protected]:~$

Just for fun, here is /proc/cpuinfo on a Sun Sunblade 1000...

[email protected]:~$ cat /proc/cpuinfo | more
processor	: 0
vendor_id	: GenuineIntel
cpu family	: 6
model		: 42
model name	: Intel(R) Pentium(R) CPU G620 @ 2.60GHz
stepping	: 7
microcode	: 0x25
cpu MHz		: 1600.000
cache size	: 3072 KB
physical id	: 0
siblings	: 2
core id		: 0
cpu cores	: 2
apicid		: 0
initial apicid	: 0
fdiv_bug	: no
f00f_bug	: no
coma_bug	: no
fpu		: yes
fpu_exception	: yes
cpuid level	: 13
wp		: yes
flags		: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov 
--More--

Most of the files in /proc are read only, some require root privileges, some files are writable, and many files in /proc/sys are writable. Let's discuss some of the files in /proc.

/proc/interrupts

On the x86 architecture, /proc/interrupts displays the interrupts.

[email protected]:~$ cat /proc/interrupts
           CPU0       CPU1       
  0:         16          0   IO-APIC-edge      timer
  1:          2          0   IO-APIC-edge      i8042
  7:          0          0   IO-APIC-edge      parport0
  8:          1          0   IO-APIC-edge      rtc0
  9:          3          0   IO-APIC-fasteoi   acpi
 12:          4          0   IO-APIC-edge      i8042
 16:         29          0   IO-APIC-fasteoi   ehci_hcd:usb1
 19:      17283      26337   IO-APIC-fasteoi   ata_piix, ata_piix
 23:      55734      64589   IO-APIC-fasteoi   ehci_hcd:usb2
 40:         13          0   PCI-MSI-edge      mei_me
 41:     500032          0   PCI-MSI-edge      i915
 42:        470          0   PCI-MSI-edge      snd_hda_intel
 43:      43010          0   PCI-MSI-edge      eth0
NMI:         24         26   Non-maskable interrupts
LOC:     359598     369234   Local timer interrupts
SPU:          0          0   Spurious interrupts
PMI:         24         26   Performance monitoring interrupts
IWI:       9565      11827   IRQ work interrupts
RTR:          0          0   APIC ICR read retries
RES:     101165      71652   Rescheduling interrupts
CAL:        202        596   Function call interrupts
TLB:       3370       6154   TLB shootdowns
TRM:          0          0   Thermal event interrupts
THR:          0          0   Threshold APIC interrupts
MCE:          0          0   Machine check exceptions
MCP:         42         42   Machine check polls
ERR:          0
MIS:          0

On a machine with two CPU's, the file looks like this.

[email protected]:~$ cat /proc/interrupts
           CPU0       CPU1       
  0:         16          0   IO-APIC-edge      timer
  1:          2          0   IO-APIC-edge      i8042
  7:          0          0   IO-APIC-edge      parport0
  8:          1          0   IO-APIC-edge      rtc0
  9:          3          0   IO-APIC-fasteoi   acpi
 12:          4          0   IO-APIC-edge      i8042
 16:         29          0   IO-APIC-fasteoi   ehci_hcd:usb1
 19:      17283      26337   IO-APIC-fasteoi   ata_piix, ata_piix
 23:      55734      64589   IO-APIC-fasteoi   ehci_hcd:usb2
 40:         13          0   PCI-MSI-edge      mei_me
 41:     500032          0   PCI-MSI-edge      i915
 42:        470          0   PCI-MSI-edge      snd_hda_intel
 43:      43010          0   PCI-MSI-edge      eth0
NMI:         24         26   Non-maskable interrupts
LOC:     359598     369234   Local timer interrupts
SPU:          0          0   Spurious interrupts
PMI:         24         26   Performance monitoring interrupts
IWI:       9565      11827   IRQ work interrupts
RTR:          0          0   APIC ICR read retries
RES:     101165      71652   Rescheduling interrupts
CAL:        202        596   Function call interrupts
TLB:       3370       6154   TLB shootdowns
TRM:          0          0   Thermal event interrupts
THR:          0          0   Threshold APIC interrupts
MCE:          0          0   Machine check exceptions
MCP:         42         42   Machine check polls
ERR:          0

/proc/kcore

The physical memory is represented in /proc/kcore. Do not try to cat this file, instead use a debugger. The size of /proc/kcore is the same as your physical memory, plus four bytes.

[email protected]:~$ ls -lh /proc/kcore
-r-------- 1 root root 1020M Jul 30 17:41 /proc/kcore
[email protected]:~$ 

/sys Linux 2.6 hot plugging

The /sys directory was created for the Linux 2.6 kernel. Since 2.6, Linux uses sysfs to support usb and IEEE 1394 (FireWire) hot plug devices. See the manual pages of udev(8) (the successor of devfs) and hotplug(8) for more info (or visit http://linuxhotplug. sourceforge.net/ ).

Basically the /sys directory contains kernel information about hardware.

/usr Unix System Resources

Although /usr is pronounced like user, remember that it stands for Unix System Resources. The /usr hierarchy should contain shareable, read only data. Some people choose to mount /usr as read only. This can be done from its own partition or from a read only NFS share.

/usr/bin

The /usr/bin directory contains a lot of commands.

[email protected]:~$ ls /usr/bin | wc -l
1421

/usr/include

The /usr/include directory contains general use include files for C.

[email protected]:~$ ls /usr/include/
aio.h          fcntl.h         math.h           pwd.h          sysexits.h
aliases.h      features.h      mcheck.h         python2.7      syslog.h
alloca.h       fenv.h          memory.h         rdma           tar.h
argp.h         fmtmsg.h        mntent.h         re_comp.h      termio.h
argz.h         fnmatch.h       monetary.h       regex.h        termios.h
ar.h           fstab.h         mqueue.h         regexp.h       tgmath.h
arpa           fts.h           mtd              resolv.h       thread_db.h
asm-generic    ftw.h           nautilus-sendto  rpc            time.h
assert.h       _G_config.h     net              rpcsvc         ttyent.h
...

/usr/lib

The /usr/lib directory contains libraries that are not directly executed by users or scripts.

[email protected]:~$ ls /usr/lib | head -7
2013.com.canonical.certification:checkbox
2013.com.canonical.certification:plainbox-resources
accountsservice
aisleriot
apg
apt
aspell

/usr/local

The /usr/local directory can be used by an administrator to install software locally.

[email protected]:~$ ls /usr/local/
bin  etc  games  include  lib  man  sbin  share  src
[email protected]:~$ du -sh /usr/local
11M	/usr/local

/usr/share

The /usr/share directory contains architecture independent data. As you can see, this is a fairly large directory.

[email protected]:~$ ls /usr/share/ | wc -l
283
[email protected]:~$ du -sh /usr/share/
1.1G	/usr/share/

This directory typically contains /usr/share/man for manual pages.

[email protected]:~$ ls /usr/share/man
ar   [email protected]  en_GB  fr            hr  ko    man5  nb  pt_BR  sq  uk
ast  cs           eo     fr.ISO8859-1  hu  ku    man6  nl  ro     sr  uz
be   cy           es     fr.UTF-8      id  lt    man7  nn  ru     sv  vi
bg   da           et     fy            io  lv    man8  oc  se     ta  zh_CN
bn   de           eu     gd            it  man1  mhr   pa  shn    te  zh_HK
bo   el           fa     gl            ja  man2  ml    pl  si     th  zh_TW
bs   en_AU        fi     he            kk  man3  ms    ps  sk     tr
ca   en_CA        fo     hi            km  man4  my    pt  sl     ug

And it contains /usr/share/games for all static game data (so no high-scores or play logs).

[email protected]:~$ ls /usr/share/games/
ls: cannot access /usr/share/games/: No such file or directory

/usr/src

The /usr/src directory is the recommended location for kernel source files.

[email protected]:~$ ls -l /usr/src/
total 8
drwxr-xr-x 24 root root 4096 Apr 17 06:54 linux-headers-3.13.0-24
drwxr-xr-x  7 root root 4096 Apr 17 06:55 linux-headers-3.13.0-24-generic

/var variable data

Files that are unpredictable in size, such as log, cache and spool files, should be located in /var.

/var/log

The /var/log directory serves as a central point to contain all log files.

[email protected]:~$ ls /var/log
alternatives.log  dist-upgrade    fsck               syslog
apport.log        dmesg           gpu-manager.log    syslog.1
apport.log.1      dmesg.0         hp                 syslog.2.gz
apport.log.2.gz   dmesg.1.gz      installer          udev
apt               dmesg.2.gz      kern.log           unattended-upgrades
auth.log          dmesg.3.gz      lastlog            upstart
boot.log          dmesg.4.gz      lightdm            wtmp
bootstrap.log     dpkg.log        pm-powersave.log   Xorg.0.log
btmp              faillog         samba              Xorg.0.log.old
cups              fontconfig.log  speech-dispatcher

/var/log/messages

A typical first file to check when troubleshooting on Red Hat (and derivatives) is the /var/ log/messages file. By default this file will contain information on what just happened to the system. The file is called /var/log/syslog on Debian and Ubuntu.

/var/cache

The /var/cache directory can contain cache data for several applications.

[email protected]:/$ ls /var/cache/
apparmor          cups                   fontconfig  pm-utils
apt               debconf                ldconfig    pppconfig
apt-xapian-index  dictionaries-common    lightdm     samba
cracklib          flashplugin-installer  man         software-center

/var/spool

The /var/spool directory typically contains spool directories for mail and cron, but also serves as a parent directory for other spool files (for example print spool files).

/var/lib

The /var/lib directory contains application state information.

Red Hat Enterprise Linux for example keeps files pertaining to rpm in /var/lib/rpm/.

/var/...

/var also contains Process ID files in /var/run (soon to be replaced with /run) and temporary files that survive a reboot in /var/tmp and information about file locks in /var/lock. There will be more examples of /var usage further in this tutorial.

Exercise, Practice and Solution :

1. Does the file /bin/cat exist ? What about /bin/dd and /bin/echo. What is the type of these files ?

Code:

ls /bin/cat ; file /bin/cat
ls /bin/dd ; file /bin/dd
ls /bin/echo ; file /bin/echo

2. What is the size of the Linux kernel file(s) (vmlinu*) in /boot ?

Code:

ls -lh /boot/vm*

3. Create a directory ~/test. Then issue the following commands:

cd ~/test
dd if=/dev/zero of=zeroes.txt count=1 bs=100
od zeroes.txt

dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file /dev/zero to ~/test/zeroes.txt. Can you describe the functionality of /dev/zero ?

Code:

cd ~/test
dd if=/dev/zero of=zeroes.txt count=1 bs=100
od zeroes.txt

dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file /dev/zero
to ~/test/zeroes.txt. Can you describe the functionality of /dev/zero ?
/dev/zero is a Linux special device. It can be considered a source of zeroes. You cannot send
something to /dev/zero, but you can read zeroes from it.

4. Now issue the following command:

dd if=/dev/random of=random.txt count=1 bs=100 ; od random.txt

dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file /dev/ random to ~/test/random.txt. Can you describe the functionality of /dev/random ?

Code:

dd if=/dev/random of=random.txt count=1 bs=100 ; od random.txt

dd will copy one times (count=1) a block of size 100 bytes (bs=100) from the file /dev/
random to ~/test/random.txt. Can you describe the functionality of /dev/random ?
/dev/random acts as a random number generator on your Linux machine.

5. Issue the following two commands, and look at the first character of each output line.

ls -l /dev/sd* /dev/hd*
ls -l /dev/tty* /dev/input/mou*

The first ls will show block(b) devices, the second ls shows character(c) devices. Can you tell the difference between block and character devices ?

Code:

ls -l /dev/sd* /dev/hd*
ls -l /dev/tty* /dev/input/mou*

The first ls will show block(b) devices, the second ls shows character(c) devices. Can you
tell the difference between block and character devices ?
Block devices are always written to (or read from) in blocks. For hard disks, blocks of 512
bytes are common. Character devices act as a stream of characters (or bytes). Mouse and
keyboard are typical character devices.

6. Use cat to display /etc/hosts and /etc/resolv.conf. What is your idea about the purpose of these files?

Code:

/etc/hosts contains hostnames with their ip address
/etc/resolv.conf should contain the ip address of a DNS name server.

7. Are there any files in /etc/skel/ ? Check also for hidden files.

Code:

Issue "ls -al /etc/skel/". Yes, there should be hidden files there.

8. Display /proc/cpuinfo. On what architecture is your Linux running?

Code:

The file should contain at least one line with Intel or other cpu.

9. Display /proc/interrupts. What is the size of this file ? Where is this file stored ?

Code:

The size is zero, yet the file contains data. It is not stored anywhere because /proc is a
virtual file system that allows you to talk with the kernel. (If you answered "stored in RAMmemory,
that is also correct...).

10. Can you enter the /root directory ? Are there (hidden) files?

Code:

Try "cd /root". Yes there are (hidden) files there.

11. Are ifconfig, fdisk, parted, shutdown and grub-install present in /sbin ? Why are these binaries in /sbin and not in /bin ?

Code:

Because those files are only meant for system administrators.

12. Is /var/log a file or a directory ? What about /var/spool ?

Code:

Both are directories.

13. Open two command prompts (Ctrl-Shift-T in gnome-terminal) or terminals (Ctrl-Alt-F1, Ctrl-Alt-F2, ...) and issue the who am i in both. Then try to echo a word from one terminal to the other.

Code:

tty-terminal: echo Hello > /dev/tty1
pts-terminal: echo Hello > /dev/pts/1

14. Read the man page of random and explain the difference between /dev/random and / dev/urandom.

Code:

man 4 random


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