Linux for freshers: October 2016

Tuesday, October 4, 2016

who command examples in linux ?

You need to use who command to display users who are currently logged in your server

Purpose

Display who is on the system

Syntax

The basic syntax is as follows:

who

who am i

who [options] [File]

who --help

who --version

who | grep 'userNameHere'

who command examples

To show a list of all the users currently logged in to the system, type:

$ who

root@linuxforfreshers.com:~$ who

root :0 2016-10-03 07:46 (:0)

root pts/61 2016-10-04 08:08 (:0)

root pts/46 2016-10-04 07:58 (:0)

To show only hostname and user associated with stdin (usually keyboard), enter:

$ who -m

root@linuxforfreshers.com:~$ who -m

root pts/61 2016-10-04 08:08 (:0)

To show active processes spawned by init:

$ who -p

To show user’s message status as +, – or ?, enter:

$ who -T

root@linuxforfreshers.com:~$ who -T

root ? :0 2016-10-03 07:46 (:0)

root + pts/61 2016-10-04 08:08 (:0)

root + pts/46 2016-10-04 07:58 (:0)

Show or list users logged in

Type the command:

$ who -u

root@linuxforfreshers.com:~$ who -u

root :0 2016-10-03 07:46 ? 3431 (:0)

root pts/61 2016-10-04 08:08 . 21700 (:0)

root pts/46 2016-10-04 07:58 01:46 21700 (:0)

Show time of last system boot

To display time of last system boot pass the -b option to who command:

$ who -b

root@linuxforfreshers.com:~$ who -b

system boot 2016-10-03 07:44

Show dead processes on the system

You need pass the -d option to who command:

$ who -d

$ who -d -H

root@linuxforfreshers.com:~$ who -d

pts/9 2016-10-03 16:56 0 id=/9 term=0 exit=0

pts/58 2016-10-03 16:56 0 id=/58 term=0 exit=0

pts/0 2016-10-03 16:56 0 id=/0 term=0 exit=0

pts/60 2016-10-04 15:55 0 id=/60 term=0 exit=0

pts/62 2016-10-04 15:55 0 id=/62 term=0 exit=0

Show system login processes

To just display system login processes pass the -l option:

$ who -l

$ who -l -H

root@linuxforfreshers.com:~$ who -l

Count all login names and number of users logged on the system

To count all login names and number of users logged on:

$ who -q

root@linuxforfreshers.com:~$ who -q

root root root

# users=3

Display the current runlevel

To display the current system runlevel, type:

$ who -r

root@linuxforfreshers.com:~$ who -r

run-level 2 2016-10-03 07:44

You can combine -r and -b options as follows:

$ who -r -b

root@linuxforfreshers.com:~$ who -r -b

system boot 2016-10-03 07:44

run-level 2 2016-10-03 07:44

root@linuxforfreshers.com:~$ who am i

root pts/61 2016-10-04 08:08 (:0)

Displays "all" information, and headers above each column of data, for example:

root@linuxforfreshers.com:~$ who -aH

NAME LINE TIME IDLE PID COMMENT EXIT

system boot 2016-10-03 07:44

run-level 2 2016-10-03 07:44

root ? :0 2016-10-03 07:46 ? 3431 (:0)

pts/9 2016-10-03 16:56 0 id=/9 term=0 exit=0

pts/58 2016-10-03 16:56 0 id=/58 term=0 exit=0

root + pts/61 2016-10-04 08:08 . 21700 (:0)

pts/0 2016-10-03 16:56 0 id=/0 term=0 exit=0

root + pts/46 2016-10-04 07:58 01:39 21700 (:0)

pts/60 2016-10-04 15:55 0 id=/60 term=0 exit=0

pts/62 2016-10-04 15:55 0 id=/62 term=0 exit=0

Sunday, October 2, 2016

lsblk command examples in linux ?

lsblk command is used to list information about all available block devices, however, it does not list information about RAM disks. Examples of block devices are hard disk, flash drives, CD-ROM…

Install lsblk

Ubuntu and Linux Mint installation

The command lsblk comes in the package util-linux.

sudo apt-get install util-linux -y

Fedora and CentOS installation

sudo yum install util-linux-ng

How to use lsblk command

lsblk command by default will list all block devices in a tree-like format.

Example: Type lsblk in your terminal:

root@Linuxforfreshers ~ $ lsblk

NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT

sda 8:0 0 465,8G 0 disk

├─sda1 8:1 0 100M 0 part

├─sda2 8:2 0 80G 0 part

├─sda3 8:3 0 297,9G 0 part

├─sda4 8:4 0 1K 0 part

├─sda5 8:5 0 28G 0 part /

├─sda6 8:6 0 3,7G 0 part [SWAP]

└─sda7 8:7 0 56,2G 0 part /home

sr0 11:0 1 1024M 0 rom

There are seven columns namely:

NAME: This is the device name.

MAJ:MIN: This column shows the major and minor device number.

RM: This column shows whether the device is removable or not.

SIZE: This is column give information on the size of the device.

RO: This indicates whether a device is read-only.

TYPE:This column shows information whether the block device is a disk or a partition(part) within a disk.

MOUNTPOINT: This column indicates mount point on which the device is mounted.

List all devices

To show list all devices including empty devices use following command:

lsblk -a

Example output:

root@Linuxforfreshers ~ $ lsblk -a

NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT

sda 8:0 0 465,8G 0 disk

├─sda1 8:1 0 100M 0 part

├─sda2 8:2 0 80G 0 part

├─sda3 8:3 0 297,9G 0 part

├─sda4 8:4 0 1K 0 part

├─sda5 8:5 0 28G 0 part /

├─sda6 8:6 0 3,7G 0 part [SWAP]

└─sda7 8:7 0 56,2G 0 part /home

sr0 11:0 1 1024M 0 rom

ram0 1:0 0 64M 0 disk

ram1 1:1 0 64M 0 disk

ram2 1:2 0 64M 0 disk

ram3 1:3 0 64M 0 disk

ram4 1:4 0 64M 0 disk

ram5 1:5 0 64M 0 disk

ram6 1:6 0 64M 0 disk

ram7 1:7 0 64M 0 disk

ram8 1:8 0 64M 0 disk

ram9 1:9 0 64M 0 disk

loop0 7:0 0 0 loop

loop1 7:1 0 0 loop

loop2 7:2 0 0 loop

loop3 7:3 0 0 loop

loop4 7:4 0 0 loop

loop5 7:5 0 0 loop

loop6 7:6 0 0 loop

loop7 7:7 0 0 loop

ram10 1:10 0 64M 0 disk

ram11 1:11 0 64M 0 disk

ram12 1:12 0 64M 0 disk

ram13 1:13 0 64M 0 disk

ram14 1:14 0 64M 0 disk

ram15 1:15 0 64M 0 disk

List Device Permissions and Owner

To display information related to the owner, group and mode of the block device, use the -m option.

root@Linuxforfreshers ~ $ lsblk -m

NAME SIZE OWNER GROUP MODE

sda 465,8G root disk brw-rw----

├─sda1 100M root disk brw-rw----

├─sda2 80G root disk brw-rw----

├─sda3 297,9G root disk brw-rw----

├─sda4 1K root disk brw-rw----

├─sda5 28G root disk brw-rw----

├─sda6 3,7G root disk brw-rw----

└─sda7 56,2G root disk brw-rw----

sr0 1024M root cdrom brw-rw----

Print the SIZE column in bytes

Use the -b option to achieve this :

root@Linuxforfreshers ~ $ lsblk -b

NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT

sda 8:0 0 500107862016 0 disk

├─sda1 8:1 0 104857600 0 part

├─sda2 8:2 0 85878374400 0 part

├─sda3 8:3 0 319815680000 0 part

├─sda4 8:4 0 1024 0 part

├─sda5 8:5 0 29998710784 0 part /

├─sda6 8:6 0 3999268864 0 part [SWAP]

└─sda7 8:7 0 60298362880 0 part /home

sr0 11:0 1 1073741312 0 rom

Hide the information related to slaves

If you do not want to display slave related information, use the -d option.

root@Linuxforfreshers ~ $ lsblk -d

NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT

sda 8:0 0 465,8G 0 disk

sr0 11:0 1 1024M 0 rom

Saturday, October 1, 2016

How to Check the Connection to MySQL DB Server in linux?

I would share the way to check the connection to MySQL database server. We can use the telnet or nc command to connect to port 3306. This would be very useful when we do the troubleshooting or to test the connectivity between the application server and the database server. Sample outputs will show succeeded or connected as below :

Host A = 192.168.0.5

Host B = 192.168.0.6

1. Test connectivity using nc command :

[root@linuxforfreshers~]# nc -z -w1 192.168.0.6 3306

Connection to 192.168.0.6 3306 port [tcp/mysql] succeeded!

2. Test connectivity using telnet command :

syntax: telnet hostname portnumber

[root@linuxforfreshers~]# telnet 192.168.0.6 3306

Telnet escape character is 'X'.

Trying 192.168.0.6...

Connected to 192.168.0.6.

Escape character is 'X'.

telnet> Connection closed.