Modifying file permissions with chmod command in GNU/Linux


In any multi-user operating system the files have a series of features that allow us to see them, modify them or execute them for users that we define. Although there are several alternatives for achieving this, GNU/Linux uses the traditional users and groups system, allowing us to configure the file permission in various ways. By executing ls -l we can list the permissions over the various files inside the directory we are in. Look at the folowing example line displayed on executing ls -l

-rwxr-xr-x 1 user1 group1 128931 Feb 19 2000 test.txt

From the above line, first 10 characters corresponds to file permissions and we concentrate on them for the same reason.
Character 1        : It tells us whether it is a file or a directory. ‘-‘ for file and ‘d’ for directory.
Characters 2,3,4: These are the so called rwx permissions i.e, read,write and execute permissions. And 2,3 and 4 characters include the Read More »

Recover hidden GNU/Linux after installing Windows with live ISO


Most often GNU/Linux becomes hidden after installing/re-installing windows. It is because windows overrides the traditional GRUB from GNU/Linux with
its own boot loader which cannot detect other operating systems. Whereas GRUB is capable of recognizing other operating systems(including Macintosh,
windows etc). So the solution is cut and clear, just re-install the GRUB. But it requires a live GNU/Linux distro. Since I use Ubuntu here a provide a link to download the ISO for Ubuntu because it contains GRUB(don’t think that Ubuntu is the only one with GRUB). If you choose another distro, make
sure that it includes GRUB.

1. Download the live ISO.
2. Use the ISO to make a bootable USB or CD/DVD(better to use unetbootin to make a bootable flash drive).
3. Boot the same before getting into windows by making the first boot device to CD/DVD or any external device in BIOS.
4. Choose the option Try Ubuntu Without Installing.
5. Access the terminal by pressing Ctrl+Alt+T.
Read More »

Protect your GRUB from unauthorized editing with passwords


Here I consider GRUB 2 and in the following post GRUB means GRUB 2.

If you are a GNU/Linux user, GRUB will be quite familiar to you. It is the first thing you see after you switch on the system. As the expansion of GRUB (GRand Unified Bootloader) suggests it is a boot loader. This is very helpful in various situations. Earlier I had written a post regarding GRUB with its ability to modify the way it appears to the user. The other side is about the security which is of greater importance. After the installation of GRUB it is open to all i.e, anybody can edit the GRUB entries and gain the superuser privilege. So protecting the grub with passwords restricts this attack.

First of all let me introduce to you some of the grub scripts included under /etc/grub.d/


These are used to generate the grub.cfg file under /boot/grub/ on running update-grub. Among these we will consider only 00_header, 10_linux,
20_memtest86+ and 30_os-prober. A superuser must be designated. This user can select all menu entries, edit any items in the GRUB menu during the
boot process, and access the GRUB terminal.The superuser is identified as
set superusers=”<user>”
Ex:- set superusers=”root”
The format for adding the superuser password and any additional users and passwords
password <user> <password>
Ex:- password root 123456
If you wish more users to be added append those with the above line
Ex:- password root 123456
      password tom tom123
      password alen alen

Note: Encryption of passwords will be explained later.

Open /etc/grub.d/00_header and add the following at the bottom of the file.
cat << EOF
        set superusers=”root”
        password root 123456
Each type of OS entries are protected through different scripts.

For linux entries, find the following line in /etc/grub.d/10_linux
printf “menuentry ‘${title}’ ${CLASS} {\n” “${os}” “${version}”
Add –users to allow permission to superuser
printf “menuentry ‘${title}’ ${CLASS} –users {\n” “${os}” “${version}”
Add –users tom to allow permission to superuser+tom
        printf “menuentry ‘${title}’ ${CLASS} –users tom {\n” “${os}” “${version}”
Add –users tom,alen to allow permission to superuser+tom+alen
printf “menuentry ‘${title}’ ${CLASS} –users tom,alen {\n” “${os}” “${version}”

For other OS entries, find lines starting with menu entry in /etc/grub.d/30_os-prober and add –users after –class os to requires entries.
For memory test entry, modification is to be done inside /etc/grub.d/20_memtest86+ in the following lines
 menuentry “Memory test (memtest86+)” {
    menuentry “Memory test (memtest86+, serial console 115200)” {

Run sudo update-grub after all modifications.