Linux administration

• Many common directory names are abbreviated versions of real words
• Traditional structure which has developed over many years
  • Most system files have their proper place
  • Programs rely on them being in the correct place
  • Users familiar with Unix directory structure can find their way around any Unix or Linux system
• But a user's home directory can be structured however they want

• Started as an attempt to standardise Linux filesystem layouts
  • Called the FSSTND when the first version was published in 1994
• Widely accepted by distributors
  • But few Linux systems are 100% compliant yet
• Intended to avoid fragmentation of Linux distributions
• Renamed to the File Hierarchy Standard, or FHS
• Now intended to apply to all Unix-like operating systems

• Some files can be shared between multiple computers, using networked filesystems such as NFS
  • This can save space, although cheap hard drives mean that this is not so important now
  • More importantly, it can help to centralise administration for a network
• Usually programs, email and home directories are all shareable
• Log files and machine-specific configuration files are not shareable

• Some files hardly ever need to be changed, while others change all the time
• It can help to store static files separately from those which regularly change:
  • The static files can be on a partition mounted read-only (such as a CD-ROM)
• Programs and libraries are usually static (except when new software is installed)
• Home directories and status files are usually more variable

• Programs are usually found in the bin and sbin directories
  • These are found in /, /usr and /usr/local

• sbin is used for programs likely to be useful to system administrators rather than to general users (mail daemon, web server, etc.)

• These directories are named after binaries

  • Most programs in them are binaries (compiled programs), although some are human-readable scripts

• Libraries are stored in directories called lib, found in the same places as bin

  • These directories should be listed in /etc/ld.so.conf

• /usr/include contains header files used by C/C++ programs

• /usr/X11R6 contains files used by the X Window system, including programs, libraries, configuration files and documentation

• /usr/local is where software is installed when it is compiled from source code rather than installed as a package

• /usr/share contains files which are not specific to the architecture of a machine, e.g., fonts and icons

  • Theoretically could be shared between different types of machine over a network

• /usr/src often contains the source code for the Linux kernel

• /var/run contains pid files (process-id files for currently-running daemon programs)
  • Also contains utmp, a record of user logins
• /var/mail or /var/spool/mail is where each user's email is queued up until it is deleted or saved
• /var/log contains logs produced by various programs, including syslog
• /var/cache contains data generated by programs which is cached to save time
  • Cached data can be regenerated if deleted

• /etc contains configuration files
• /mnt is used to mount other filesystems temporarily
• /boot contains files used by GRUB to boot the system
• /dev contains device files, which provide access to hardware devices such as disk drives or serial ports
• /tmp is used by many programs for temporary files
• /opt can contain packages of software from third parties (i.e., not in the native package management format)

• /proc provides access to information from the kernel, particularly about running processes
• /home contains directories which belong to each user
  • Use echo ˜ to find out where your home directory is
• /root is the home directory of the root user

• Searches for programs which can be run
• Looks in the same directories as the shell
  • Determined by the $PATH environment variable
  • Use echo $PATH to see what directories are searched
• For example, to find out where gnumeric is:

$ which gnumeric

• This is useful if different versions of the same program are installed in different places

• type is like which, but is built into the shell
  • It tells you about shell aliases and functions
  • Not available in the C Shell
• type -p is just like which
• type -a shows all the commands of the name given
  • Useful for detecting duplicate programs, or aliases which are hiding real programs
• See help type for full details

• Some commands are built into the shell
  • Examples include cd, test, pwd and ulimit
• The Bash shell has a builtin called type which reports on whether a command is a builtin
• For example, to see whether the test command will run a shell builtin, or a real program:

$ type test

• The example shows that test will run a shell builtin, even though there is a real program with the same name
• type will also identify shell aliases and functions

• Use the updatedb program to refresh the database of files used by locate
• Modern versions are configured by giving options to updatedb
  • -e provides a list of directories which will not be searched
  • -f gives the names of filesystem types to miss out
  • See the manpage for full details
• updatedb is usually run by cron every night
  • Look in /etc/cron.daily for the script which runs it

• Older versions of GNU updatedb used the configuration file /etc/updatedb.conf
  • For compatibility, some modern versions still read it
• The configuration is done by setting environment variables
• For example, to ignore certain filesystems:

$ PRUNEPATHS="/tmp /usr/tmp /var/tmp /mnt /var/spool"
$ export PRUNEPATHS

• The $PRUNEFS variable lists the names for filesystems which should be ignored (e.g., nfs, iso9660, etc.)
• These variables are equivalent to the -e and -f options

• whatis finds manpages with the given name and lists them
  • Usually only useful when the name of a command is already known
• For example, to find manpages about bash:

$ whatis bash

• The database searched by whatis is updated with the makewhatis command
  • This should be run when new manpages are installed
  • Debian instead has /etc/cron.daily/man-db, which also expunges old cached man pages

• The apropos command is similar to whatis
  • The difference is that any word in the title line of a manpage can match the word given
• apropos word is identical to man -k word
• For example, to find commands relating to directories:

$ apropos directories
$ man -k directories

• apropos also uses the database built by makewhatis

• The FHS — http://www.pathname.com/fhs/

• a. Find out whether the ls command runs a program directly, or is a shell alias or function.
• b. Locate the binary of the traceroute program.
• c. Use whatis to find out what the watch command does.
• d. Use apropos to find programs for editing the partition table of disks.
• e. See if the Linux installation you are using has an updatedb.conf, and look at the current configuration if it has.
• f. Log on as root and update the locate database with the updatedb command.