Originally Posted by Rajesh

I am just adding to your data base of knowledge

Suppose you want to know which file was last edited out of hundreds files then use below command

ls -ltrh

The last modified file will be at bottom show , enjoy the result

Thanks

Originally Posted by sandeepgurung7

Permission Groups are

u - Owner - owns the file
g - Group - a member of the group that owns the file
o - Other - neither the owner, nor belonging to the group that owns the file
a - All Users

Permission Types are

read
write
execute

Read permission

Folder - one can check the content of a folder (ls command) but can not change directory (cd command)
File - one can only able to read the file (cat command)

Read and Write permission

Folder - one can only do the same as mention above in read permission
File - one can modify the content of file

Read and Execute permission

Folder - one can change the directory (cd command) but can not able to create folder (mkdir command) or file (touch command) under it.
File - file became executable

ALL permission

Folder - one can check the content, change directory and create file & folder.
File - one can read, modify and execute


ls -l commond will display permissions of file and folder, something like the following

-rwxr-xr-x

- flag (start from left side) used for regular file and d flag used for directory. Next 3 flag is used to show the permission of owner. Next 3 flag for group permission and remaing 3 flag indicating the permission for other.

To set/modify a file's permissions we need to use the chmod command and chmod command will be used to set permissions in either of two modes:

Absolute Mode – Use numbers to add permissions or remove permissions. Absolute mode is the method most commonly used to set permissions.
Symbolic Mode – Use combinations of letters and symbols to add permissions or remove permissions.

Abolute mode is already explain by harry and sachin. so I am only describing symbolic mode through some example

Add execute permissions for owner
chmod u+x sandy

Remove the execute permission from both the file's owner and group
chmod ug-x sandy

remove all the permissions but add read permission for everybody
chmod a=r sandy