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Candidates should be thoroughly familiar with the Filesystem Hierarchy Standard (FHS), including typical file locations and directory classifications.
- Understand the correct locations of files under the FHS.
- Find files and commands on a Linux system.
- Know the location and purpose of important file and directories as defined in the FHS.
- find
- locate
- updatedb
- whereis
- which
- type
- /etc/updatedb.conf
Filesystem Hierarchy Standard (FHS) is a document describing the Linux / Unix file hierarchy. It is very useful to know these because it lets you easily find what you are looking for:
| directory | usage |
|---|---|
| bin | Essential command binaries |
| boot | Static files of the boot loader |
| dev | Device files |
| etc | Host-specific system configuration |
| lib | Essential shared libraries and kernel modules |
| media | Mount point for removable media |
| mnt | Mount point for mounting a filesystem temporarily |
| opt | Add-on application software packages |
| sbin | Essential system binaries |
| srv | Data for services provided by this system |
| tmp | Temporary files |
| usr | Secondary hierarchy |
| var | Variable data |
| home | User home directories (optional) |
| lib | Alternate format essential shared libraries (optional) |
| root | Home directory for the root user (optional) |
The /usr filesystem is the second major section of the filesystem, containing shareable, read-only data. It can be shared between systems, although present practice does not often do this.
The /var filesystem contains variable data files, including spool directories and files, administrative and logging data, and transient and temporary files. Some portions of /var are not shareable between different systems, but others, such as /var/mail, /var/cache/man, /var/cache/fonts, and /var/spool/news, may be shared.
A general linux install has a lot of files; 741341 files in my case. So how it find out where to look when you type a command? This is done by a variable called PATH:
$ echo $PATH
/home/jadi/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games;/home/jadi/bin/
And for root user:
# echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
As you can see, this is the list of directories separated with a colon. Obviously you can change your path with export PATH=$PATH:/usr/new/dir or put this in .bashrc to make it permanent.
The which command shows the first appearance of the command given in path:
$ which mkfd
$ which mkfs
/sbin/mkfs
use the
-aswitch to show all appearance in the path and not only the first one.
But what happens if you which for ?
$ which for
$ type for
for is a shell keyword
As you can see, which did not found anything for for and we used type.
$ type type
type is a shell builtin
$ type for
for is a shell keyword
$ type mkfs
mkfs is /sbin/mkfs
$ type mkfd
bash: type: mkfd: not found
The type command is more general that which and also understand and shows the bash keywords.
Another useful command in this category is whereis. Unlike which, whereis shows man pages and source codes of programs alongside their binary location.
$ whereis mkfs
mkfs: /sbin/mkfs.bfs /sbin/mkfs.ext3 /sbin/mkfs.ext4 /sbin/mkfs.vfat /sbin/mkfs.cramfs /sbin/mkfs.minix /sbin/mkfs.ext2 /sbin/mkfs.msdos /sbin/mkfs.fat /sbin/mkfs.ntfs /sbin/mkfs.ext4dev /sbin/mkfs /usr/share/man/man8/mkfs.8.gz
$ whereis ping
ping: /bin/ping /usr/share/man/man8/ping.8.gz
$ whereis chert
chert:
$
We have already seen this command in detail but lets see a couple of new switches.
- The
-userand-groupspecifies a specific user & group - The
-maxdepthtells the find how deep it should go into the directories.
$ find /tmp/ -maxdepth 1 -user jadi | head
$ find /tmp/ -maxdepth 1 -user jadi | head
/tmp/asheghloo.png
/tmp/tmpAN6Drb
/tmp/wrapper-24115-2-out
/tmp/sni-qt_goldendict_20048-sRlmvN
/tmp/asheghloo.gif
/tmp/zim-jadi
/tmp/3la.txt
/tmp/unity_support_test.0
/tmp/batman.jpg
Or even find the files not belonging to any user / group with -nouser and -nogroup.
Like other tests, you can add a
!just before any phrase to negate it. So this will find files not belonging to jadi:find . ! -user jadi
You tries find and know that it is slowwwww... It searches the file system on each run but lets see the fastest command:
$ locate happy
/home/jadi/.Spark/xtra/emoticons/Default.adiumemoticonset/happy.png
/home/jadi/.Spark/xtra/emoticons/sparkEmoticonSet/happy.png
/home/jadi/Downloads/jadi-net_radio-geek_040_antihappy.mp3
/usr/share/emoticons/kde4/unhappy.png
/usr/share/pixmaps/fvwm/mini.happy.xpm
/usr/share/pixmaps/pidgin/emotes/default/happy.png
/usr/share/pixmaps/pidgin/emotes/small/happy.png
/usr/src/linux-headers-3.13.0-40-generic/include/config/happymeal.h
/usr/src/linux-headers-3.16.0-25-generic/include/config/happymeal.h
/usr/src/linux-headers-3.16.0-28-generic/include/config/happymeal.h
/usr/src/linux-headers-3.16.0-29-generic/include/config/happymeal.h
And it is fast:
$ time locate kernel | wc -l
11235
real 0m0.341s
user 0m0.322s
sys 0m0.015s
This is fast because its data comes from a database created with updatedb command which is usually run on a daily basis with a cron job. Its configuration file is /etc/updatedb.conf or /etc/sysconfig/locate:
$ cat /etc/updatedb.conf
PRUNE_BIND_MOUNTS="yes"
# PRUNENAMES=".git .bzr .hg .svn"
PRUNEPATHS="/tmp /var/spool /media /home/.ecryptfs"
PRUNEFS="NFS nfs nfs4 rpc_pipefs afs binfmt_misc proc smbfs autofs iso9660 ncpfs coda devpts ftpfs devfs mfs shfs sysfs cifs lustre tmpfs usbfs udf fuse.glusterfs fuse.sshfs curlftpfs ecryptfs fusesmb devtmpfs"
Please note that you can update the db by running updatedb as root and get some info about it by -S switch of locate command:
$ locate -S
Database /var/lib/mlocate/mlocate.db:
73,602 directories
711,894 files
46,160,154 bytes in file names
18,912,999 bytes used to store database
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