1. Linux file system "On a UNIX system, everything is a file;
if something is not a file, it is a process."
Sorts of files (on a Linux system)
Directories: files that are lists of other files.
Special files: the mechanism used for input and output. Most special files are in /dev.
Links
Sockets and Named pipes: files are used to pass information
between applications amongst other applications

2. The -l option to ls displays the file type
File types in a long list
Symbol Meaning
Regular file
d Directory
l Link
c Special file
s Socket
p Named pipe

3. File system layout /
bin
boot
dev
etc
home
lib
mnt
opt
proc
root
sbin
tmp
usr
var

4. each subdirectory of root have special roles(By tradition):
/bin: executable commands
/sbin: sys adm commands
/etc: system configuration files
/lib shared libraries
/dev peripheral devices
/tmp temporary files
/mnt to mount external devices
/var Storage for all variable files and temporary files
created by users, such as log files
/proc system information
/usr/bin further exe files
/usr/sbin further system-important exe files
/usr/lib further libraries
User-installed programs typically go under the /usr/local hierarchy

5. The df command
~]$ df -h
Using the df command with a dot (.) as an option shows the partition
the current directory belongs to,
~]$ df -h .

6. The file system in reality
In a file system, a file is represented by an inode, a kind of serial number containing information about the actual data that makes up the file: to whom
this file belongs, and where is it located on the hard disk.
use ls -i to disply indoe numbers

7. Orientation in the file system
The PATH environment variable lists those directories in the system
where executable files can be found
~]$ echo $PATH
Linux searches for required program in paths and as soon as a match is
found, the search is stopped
The which command
The export command
The env command

8. Absolute and relative paths
A path, which is the way you need to follow in the tree structure to reach a given file, can be described as starting from the trunk of the tree (the / or root directory). In that case, the path starts with a slash and is called an absolute path
In the other case, the path doesn't start with a slash and confusion is possible between ~/bin/wc (in the user's home directory) and bin/wc in /usr. Paths that don't start with a slash are always relative. In relative paths we also use the . and .. indications for the current and the parent directory
~ means usr home directory

9. Manipulating files
more about ls
ls -l
ls -ltr
In most UNIX commands, options can be combined
Color ls default color scheme
Color File type
blue directories
red compressed archives
white text files
pink images
cyan links
yellow devices
green executables
flashing red broken links

10. Crating and deleting files and directories
nautilus: The default file manager in Gnome, the GNU desktop.
konqueror: The file manager typically used on a KDE desktop.
mc: Midnight Commander, the Unix file manager

11. creating directories mkdir
mkdir -p : creating directories and subdirectories in one step
~]$ mkdir -p project/iut/1
~]$ mkdir 1 2 5

12. mv can rename files or directories, or move
Copying files with cp
-i, interactively prompt before overwriting files
-R, -r, --recursive copy directories recursively
Moving Files with mv
mv can rename files or directories, or move
them to different directories
-f, force overwrite, even if target already exists
-i, ask user interactively before overwriting files

13. -f, delete write-protected files without prompting
Deleting Files with rm
-f, delete write-protected files without prompting
-i, interactive ask the user before deleting files
-r, recursively delete files and directories
There is no recycle bin in shell so s file is really gone when you use rm

14. Finding Files with locate
The locate command is a simple and fast way to find files
The locate command searches a database of filenames
The database needs to be updated regularly
Usually this is done automatically with cron
But locate will not find files created since the last update
Options:
-i option makes the search case-insensitive
-r treats the pattern as a regular expression, rather than a simple string

15. Finding Files More Flexibly: find
find <path> name <searchstring>
~]$ find . -name test
~]$
find . size +5000k

16. The grep command
The diff command
The tkdiff command

17. File Security
Owner
Group
Others d
r w x
r w x
r w x
Execute
File type
Write
.= file
d= directory
l= link
Read

18. Each type of permission is assigned a access mode code: read = 4 or r
write = 2 or w
execute = 1 or x
User group codes
user = u
group= g
others= o

19. The id command
Print information for USERNAME, or the current user.
The chmod command
Operation
+ add
- remove
= set exactly

20. Before: -rwxr-xr-x  archive.sh
Command: chmod o=r archive.sh
After: rwxr-xr--  archive.sh
Before: rw-r-----   topsecret.inf
Command: chmod g= topsecret.inf
After: rw     topsecret.inf
Before: rw-r--r--    publicity.html
Command: chmod og=rw publicity.html
After: rw-rw-rw-   publicity.html

21. $ chmod u=rw,g=r,o=r test_file -rw-r--r-- 1 eric users
$ ls -l test_file
-rw-r--r eric users
$ chmod o-r test_file
$ ls -l test_file
-rw-r eric users
$ chmod g+wx test_file
$ ls -l test_file
-rw-rwx eric users
$ chmod u=rw,g=r,o=r test_file
-rw-r--r eric users
For changing file permissions in directory trees use -R.
$ chmod -R g-w test_dir

22. 6 = read + write (but not execute) 5 = read + execute (but not write)
$ chmod 754 test_file
-rwxr-xr-x 1 eric users
Owner = = 7
Group = = 5
World = = 4
The permission numbers are determined by adding the values of the allowed permissions:
7 = read + write + execute
6 = read + write (but not execute)
5 = read + execute (but not write)
3 = write + execute (but not read)
4 = read (but not write and execute)
2 = write (but not read or execute)
1 = execute (but not read or write)

23. The newgrp , chown and chgrp commands