Print readable writable and executable files
Write a shell script which displays a list of all files in the current
directory to which you have read, write and executer permission.
#!/bin/bash if [[ ! -d $1 || $# -lt 1 ]]; then echo "Usage: $0 dir" exit 1 fi for file in * do if test -f $file -a -r $file -a -w $file -a -x $file; then echo $file fi done
from wiki Linux we know that:-
Understanding Linux File Permissions
Although there are already a lot of good security features built into Linux-based systems, one very important potential vulnerability can exist when local access is granted – – that is file permission based issues resulting from a user not assigning the correct permissions to files and directories. So based upon the need for proper permissions, I will go over the ways to assign permissions and show you some examples where modification may be necessary.
Each file and directory has three user based permission groups:
owner – The Owner permissions apply only the owner of the file or directory, they will not impact the actions of other users.
group – The Group permissions apply only to the group that has been assigned to the file or directory, they will not effect the actions of other users.
all users – The All Users permissions apply to all other users on the system, this is the permission group that you want to watch the most.
Each file or directory has three basic permission types:
if we have to Print readable writable and executable files.
read – The Read permission refers to a user’s capability to read the contents of the file.
write – The Write permissions refer to a user’s capability to write or modify a file or directory.
execute – The Execute permission affects a user’s capability to execute a file or view the contents of a directory.
Viewing the Permissions
You can view the permissions by checking the file or directory permissions in your favorite GUI File Manager (which I will not cover here) or by reviewing the output of the \”ls -l\” command while in the terminal and while working in the directory which contains the file or folder.
The permission in the command line is displayed as: _rwxrwxrwx 1 owner:group
The first character that I marked with an underscore is the special permission flag that can vary.
The following set of three characters (rwx) is for the owner permissions.
The second set of three characters (rwx) is for the Group permissions.
The third set of three characters (rwx) is for the All Users permissions.
Following that grouping since the integer/number displays the number of hardlinks to the file.
The last piece is the Owner and Group assignment formatted as Owner:Group.
Modifying the Permissions
When in the command line, the permissions are edited by using the command chmod. You can assign the permissions explicitly or by using a binary reference as described below.
Explicitly Defining Permissions
To explicity define permissions you will need to reference the Permission Group and Permission Types.
The Permission Groups used are:
u – Owner
g – Group
o or a – All Users
The potential Assignment Operators are + (plus) and – (minus); these are used to tell the system whether to add or remove the specific permissions.
The Permission Types that are used are:
r – Read
w – Write
x – Execute
So for an example, lets say I have a file named file1 that currently has the permissions set to _rw_rw_rw, which means that the owner, group and all users have read and write permission. Now we want to remove the read and write permissions from the all users group.
To make this modification you would invoke the command: chmod a-rw file1
To add the permissions above you would invoke the command: chmod a+rw file1
As you can see, if you want to grant those permissions you would change the minus character to a plus to add those permissions.
Using Binary References to Set permissions
Now that you understand the permissions groups and types this one should feel natural. To set the permission using binary references you must first understand that the input is done by entering three integers/numbers.
A sample permission string would be chmod 640 file1, which means that the owner has read and write permissions, the group has read permissions, and all other user have no rights to the file.
The first number represents the Owner permission; the second represents the Group permissions; and the last number represents the permissions for all other users. The numbers are a binary representation of the rwx string.
r = 4
w = 2
x = 1
You add the numbers to get the integer/number representing the permissions you wish to set. You will need to include the binary permissions for each of the three permission groups.
So to set a file to permissions on file1 to read _rwxr_____, you would enter chmod 740 file1.
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