System administrator

SSH One Computer From Another On Same Newtork

If you have authorized username and password for a different machine(computer) connected on the same network, you can easily ssh into it to access its contents.

For clarity, machine we are using to ssh is referred Machine 1 and machine we have to ssh into is referred Machine 2.

Now here’s how to do it.

First get the IP of the Machine 2. To find IP run the following(in Machine 2):
ifconfig                       #ipconfig for windows

It will output something like:
eth0 Link encap:Ethernet HWaddr 5x:xx:xx:xx:xx:xc
UP BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)
Interrupt:19

lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:65536 Metric:1
RX packets:4309 errors:0 dropped:0 overruns:0 frame:0
TX packets:4309 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:539900 (539.9 KB) TX bytes:539900 (539.9 KB)

wlan0 Link encap:Ethernet HWaddr 28:e3:47:6d:29:2c
inet addr:192.168.0.105 Bcast:192.168.0.255 Mask:255.255.255.0
inet6 addr: fe80::2ae3:47ff:fe6d:292c/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:5906878 errors:0 dropped:0 overruns:0 frame:0
TX packets:4779402 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:5143419880 (5.1 GB) TX bytes:619868581 (619.8 MB)

As we can see the IP of the machine for wlan0 is 192.168.0.105

Now run the following from Machine 1:
ssh <usser>@<ip-address>

For eg:
ssh shivam@192.168.0.105

When asked enter the password for the user and you can now remotely access Machine 2 from Machine 1.

Standard
System administrator

Reading File Permissions in Linux

Linux is based on the idea that the same system is going to be used by different users. With different users accessing the same system, there arises the need of permission. Think of it like this, you are sharing a room with different person/people in a hostel. There are a few things that are common to all, say table, chair, etc. But you still need some personal space (most likely your cupboard in this analogy) where you keep things that are accessible only by you.

Same concept can be applied in case of files present in your linux system. There are different users who can locate these files, but depending upon the type of permission those users can read them or modify them.

To check the permissions of files in a particular folder, run this:
ls -l
You will get something like this as output:
-rw-rw-r-- 1 shivam users 4004 May 8 20:19 CONTRIBUTING.md

Lets understand it backwards i.e right to left.
CONTRIBUTING.md: is the name of the file.
May 8 20:19: represents the time this file was last modified/created.
4004: is the size of the file in bytes.
users: is the group of users to which this file belongs.
shivam: is the owner of the file.
1: is the number of files.

Now comes the file permissions.
-rw-rw-r--
starting ‘-‘ means its a file. Had it been a directory, you would have found a ‘d’.
Now, ‘rw’ represents the permissions of the owner. In this case the owner has both read and write permissions.
Second ‘rw’ represents the permission of group of users. Here as you can see even the group users have both read and write permissions(not quite secure, eh?).
Finally the ‘r’ is the permission for everyone else, i.e. they can only read the file and cannot edit it.

Changing the permissions
You can change the permissions of the file using the chmod command which changes the permission for the site.
For example.
sudo chmod 0600 CONTRIBUTING.md

Now if you do ls -l, you will get
-rw------- 1 shivam shivam 4005 May 24 22:47 CONTRIBUTING.md
meaning only the owner had read and write permissions. Other cant even read it.

Different file permissions and there meaning are as follows:

Value Meaning
777 (rwxrwxrwx) No restrictions on permissions. Anybody may do anything. Generally not a desirable setting.
755 (rwxr-xr-x) The file’s owner may read, write, and execute the file. All others may read and execute the file. This setting is common for programs that are used by all users.
700 (rwx——) The file’s owner may read, write, and execute the file. Nobody else has any rights. This setting is useful for programs that only the owner may use and must be kept private from others.
666 (rw-rw-rw-) All users may read and write the file.
644 (rw-r–r–) The owner may read and write a file, while all others may only read the file. A common setting for data files that everybody may read, but only the owner may change.
600 (rw——-) The owner may read and write a file. All others have no rights. A common setting for data files that the owner wants to keep private.
Standard
System administrator

Grub2: Boot in recovery mode, Like a boss!

There are 1000 things that can go wrong ultimately leading to server crash/freeze. Some of the most common reason are : network problems, system overload, configuration errors, hardware issues, out of disk space etc.

When things go wrong we restart server in recover mode. Sometimes by default on reboot, grubs menu shows up with no timeout. This can prevent you from automatically rebooting your servers (as grub will infinitely wait for a user input).

You certainly in most of the cases want to avoid such situation. This can easily be done in a few steps.

Open the file /etc/default/grub with an editor and add the variable GRUB_RECORDFAIL_TIMEOUT as following:

GRUB_RECORDFAIL_TIMEOUT     = N

where N =  desired timeout in seconds. This will make the grub wait for N secs and then continue with default choice.

In case you do not want any timeout at all, set it to -1.

GRUB_RECORDFAIL_TIMEOUT     = -1

If you want to completely disable the menu (even for failed startup), set it to 0

GRUB_RECORDFAIL_TIMEOUT     = 0

Now save the file and update grub by:

sudo update-grub

This will apply your changes to the grub as desired.

Standard