60 Linux Commands system admin.txt

60 Linux Commands system admin.txt

The “lsblk” stands for (List Block Devices), print block devices by their assigned name (but not RAM) on the standard output in a tree-like fashion

lsblk is very useful and easiest way to know the name of New Usb Device you just plugged in, especially when you have to deal with disk/blocks in terminal.

3. Command: md5sum
The “md5sum” stands for (Compute and Check MD5 Message Digest), md5 checksum (commonly called hash) is used to match or verify integrity of files that may have changed as a result of a faulty file transfer, a disk error or non-malicious interference.

root@tecmint:~# md5sum teamviewer_linux.deb 
47790ed345a7b7970fc1f2ac50c97002  teamviewer_linux.deb
Note: The user can match the generated md5sum with the one provided officially. Md5sum is considered less secure than sha1sum, which we will discuss later.

4. Command: dd
Command “dd” stands for (Convert and Copy a file), Can be used to convert and copy a file and most of the times is used to copy a iso file (or any other file) to a usb device (or any other location), thus can be used to make a ‘Bootlable‘ Usb Stick.

root@tecmint:~# dd if=/home/user/Downloads/debian.iso of=/dev/sdb1 bs=512M; sync
Note: In the above example the usb device is supposed to be sdb1 (You should Verify it using command lsblk, otherwise you will overwrite your disk and OS), use name of disk very Cautiously!!!.

dd command takes some time ranging from a few seconds to several minutes in execution, depending on the size and type of file and read and write speed of Usb stick.

5. Command: uname
The “uname” command stands for (Unix Name), print detailed information about the machine name, Operating System and Kernel.

root@tecmint:~# uname -a
Linux tecmint 3.8.0-19-generic #30-Ubuntu SMP Wed May 1 16:36:13 UTC 2013 i686 i686 i686 GNU/Linux
Note: uname shows type of kernel. uname -a output detailed information. Elaborating the above output of uname -a.

“Linux“: The machine’s kernel name.
“tecmint“: The machine’s node name.
“3.8.0-19-generic“: The kernel release.
“#30-Ubuntu SMP“: The kernel version.
“i686“: The architecture of the processor.
“GNU/Linux“: The operating system name.

6. Command: history
The “history” command stands for History (Event) Record, it prints the history of long list of executed commands in terminal.

root@tecmint:~# history
1  sudo add-apt-repository ppa:tualatrix/ppa
2  sudo apt-get update
3  sudo apt-get install ubuntu-tweak
4  sudo add-apt-repository ppa:diesch/testing
5  sudo apt-get update
6  sudo apt-get install indicator-privacy
7  sudo add-apt-repository ppa:atareao/atareao
8  sudo apt-get update
9  sudo apt-get install my-weather-indicator
10 pwd
11 cd && sudo cp -r unity/6 /usr/share/unity/
12 cd /usr/share/unity/icons/
13 cd /usr/share/unity
Note: Pressing “Ctrl + R” and then search for already executed commands which lets your command to be completed with auto completion feature.

(reverse-i-search)`if': ifconfig

7. Command: sudo
The “sudo” (super user do) command allows a permitted user to execute a command as the superuser or another user, as specified by the security policy in the sudoers list.

root@tecmint:~# sudo add-apt-repository ppa:tualatrix/ppa
Note: sudo allows user to borrow superuser privileged, while a similar command ‘su‘ allows user to actually log in as superuser. Sudo is safer than su.
It is not advised to use sudo or su for day-to-day normal use, as it can result in serious error if accidentally you did something wrong

8. Command: mkdir
The “mkdir” (Make directory) command create a new directory with name path. However is the directory already exists, it will return an error message “cannot create folder, folder already exists”.

root@tecmint:~# mkdir tecmint
Note: Directory can only be created inside the folder, in which the user has write permission. mkdir: cannot create directory `tecmint‘: File exists
(Don’t confuse with file in the above output, you might remember what i said at the beginning – In Linux every file, folder, drive, command, scripts are treated as file).

9. Command: touch
The “touch” command stands for (Update the access and modification times of each FILE to the current time). touch command creates the file, only if it doesn’t exist. If the file already exists it will update the timestamp and not the contents of the file.

root@tecmint:~# touch tecmintfile
Note: touch can be used to create file under directory, on which the user has write permission, only if the file don’t exist there.

10. Command: chmod
The Linux “chmod” command stands for (change file mode bits). chmod changes the file mode (permission) of each given file, folder, script, etc.. according to mode asked for.

There exist 3 types of permission on a file (folder or anything but to keep things simple we will be using file).

Read (r)=4
Write(w)=2
Execute(x)=1
So if you want to give only read permission on a file it will be assigned a value of ‘4‘, for write permission only, a value of ‘2‘ and for execute permission only, a value of ‘1‘ is to be given. For read and write permission 4+2 = ‘6‘ is to be given, ans so on.

Now permission need to be set for 3 kinds of user and usergroup. The first is owner, then usergroup and finally world.

rwxr-x--x   abc.sh
Here the root’s permission is rwx (read, write and execute).
usergroup to which it belongs, is r-x (read and execute only, no write permission) and
for world is –x (only execute).

To change its permission and provide read, write and execute permission to owner, group and world.

root@tecmint:~# chmod 777 abc.sh
only read and write permission to all three.

root@tecmint:~# chmod 666 abc.sh
read, write and execute to owner and only execute to group and world.

root@tecmint:~# chmod 711 abc.sh
Note: one of the most important command useful for sysadmin and user both. On a multi-user environment or on a server, this command comes to rescue, setting wrong permission will either makes a file inaccessible or provide unauthorized access to someone.

11. Command: chown
The Linux “chown” command stands for (change file owner and group). Every file belongs to a group of user and a owner. It is used Do ‘ls -l‘ into your directory and you will see something like this.

root@tecmint:~# ls -l 
drwxr-xr-x 3 server root 4096 May 10 11:14 Binary 
drwxr-xr-x 2 server server 4096 May 13 09:42 Desktop
Here the directory Binary is owned by user “server” and it belongs to usergroup “root” where as directory “Desktop” is owned by user “server” and belongs to user group “server“.

This “chown” command is used to change the file ownership and thus is useful in managing and providing file to authorised user and usergroup only.

root@tecmint:~# chown server:server Binary
drwxr-xr-x 3 server server 4096 May 10 11:14 Binary 
drwxr-xr-x 2 server server 4096 May 13 09:42 Desktop
Note: “chown” changes the user and group ownership of each given FILE to NEW-OWNER or to the user and group of an existing reference file.

12. Command: apt
The Debian based “apt” command stands for (Advanced Package Tool). Apt is an advanced package manager for Debian based system (Ubuntu, Kubuntu, etc.), that automatically and intelligently search, install, update and resolves dependency of packages on Gnu/Linux system from command line.

root@tecmint:~# apt-get install mplayer
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following package was automatically installed and is no longer required:
java-wrappers
Use 'apt-get autoremove' to remove it.
The following extra packages will be installed:
esound-common libaudiofile1 libesd0 libopenal-data libopenal1 libsvga1 libvdpau1 libxvidcore4
Suggested packages:
pulseaudio-esound-compat libroar-compat2 nvidia-vdpau-driver vdpau-driver mplayer-doc netselect fping
The following NEW packages will be installed:
esound-common libaudiofile1 libesd0 libopenal-data libopenal1 libsvga1 libvdpau1 libxvidcore4 mplayer
0 upgraded, 9 newly installed, 0 to remove and 8 not upgraded.
Need to get 3,567 kB of archives.
After this operation, 7,772 kB of additional disk space will be used.
Do you want to continue [Y/n]? y
root@tecmint:~# apt-get update
Hit http://ppa.launchpad.net raring Release.gpg                                           
Hit http://ppa.launchpad.net raring Release.gpg                                           
Hit http://ppa.launchpad.net raring Release.gpg                      
Hit http://ppa.launchpad.net raring Release.gpg                      
Get:1 http://security.ubuntu.com raring-security Release.gpg [933 B] 
Hit http://in.archive.ubuntu.com raring Release.gpg                                                   
Hit http://ppa.launchpad.net raring Release.gpg                      
Get:2 http://security.ubuntu.com raring-security Release [40.8 kB]   
Ign http://ppa.launchpad.net raring Release.gpg                                                  
Get:3 http://in.archive.ubuntu.com raring-updates Release.gpg [933 B]                            
Hit http://ppa.launchpad.net raring Release.gpg                                                                
Hit http://in.archive.ubuntu.com raring-backports Release.gpg
Note: The above commands results into system-wide changes and hence requires root password (Check ‘#‘ and not ‘$’ as prompt). Apt is considered more advanced and intelligent as compared to yum command.

As the name suggest, apt-cache search for package containing sub package mpalyer. apt-get install, update all the packages, that are already installed, to the newest one.

Read more about apt-get and apt-cache commands at 25 APT-GET and APT-CACHE Commands

13. Command: tar
The “tar” command is a Tape Archive is useful in creation of archive, in a number of file format and their extraction.

root@tecmint:~# tar -zxvf abc.tar.gz (Remember 'z' for .tar.gz)
root@tecmint:~# tar -jxvf abc.tar.bz2 (Remember 'j' for .tar.bz2)
root@tecmint:~# tar -cvf archieve.tar.gz(.bz2) /path/to/folder/abc
Note: A ‘tar.gz‘ means gzipped. ‘tar.bz2‘ is compressed with bzip which uses a better but slower compression method.

Read more about “tar command” examples at 18 Tar Command Examples

14. Command: cal
The “cal” (Calendar), it is used to displays calendar of the present month or any other month of any year that is advancing or passed.

root@tecmint:~# cal 
May 2013        
Su Mo Tu We Th Fr Sa  
1  2  3  4  
5  6  7  8  9 10 11  
12 13 14 15 16 17 18  
19 20 21 22 23 24 25  
26 27 28 29 30 31
Show calendar of year 1835 for month February, that already has passed.

root@tecmint:~# cal 02 1835
February 1835      
Su Mo Tu We Th Fr Sa  
1  2  3  4  5  6  7  
8  9 10 11 12 13 14  
15 16 17 18 19 20 21  
22 23 24 25 26 27 28
Shows calendar of year 2145 for the month of July, that will advancing

root@tecmint:~# cal 07 2145
July 2145        
Su Mo Tu We Th Fr Sa  
1  2  3  
4  5  6  7  8  9 10  
11 12 13 14 15 16 17  
18 19 20 21 22 23 24  
25 26 27 28 29 30 31
Note: You need not to turn the calendar of 50 years back, neither you need to make complex mathematical calculation to know what day you were worn or your coming birthday will fall on which day.

15. Command: date
The “date” (Date) command print the current date and time on the standard output, and can further be set.

root@tecmint:~# date
Fri May 17 14:13:29 IST 2013
root@tecmint:~# date --set='14 may 2013 13:57' 
Mon May 13 13:57:00 IST 2013
Note: This Command will be very use-full in scripting, time and date based scripting, to be more perfect. Moreover changing date and time using terminal will make you feel GEEK!!!. (Obviously you need to be root to perform this operation, as it is a system wide change).

16. Command: cat
The “cat” stands for (Concatenation). Concatenate (join) two or more plain file and/or print contents of a file on standard output.

root@tecmint:~# cat a.txt b.txt c.txt d.txt >> abcd.txt
root@tecmint:~# cat abcd.txt
....
contents of file abcd 
...
Note: “>>” and “>” are called append symbol. They are used to append the output to a file and not on standard output. 
“>” symbol will delete a file already existed and create a new file hence for security reason it is advised to use “>>” that will write the output without overwriting or deleting the file.

Before Proceeding further, I must let you know about wildcards (you would be aware of wildcard entry, in most of the Television shows) Wildcards are a shell feature that makes the command line much more powerful than any GUI file managers. You see, if you want to select a big group of files in a graphical file manager, you usually have to select them with your mouse. This may seem simple, but in some cases it can be very frustrating.

For example, suppose you have a directory with a huge amount of all kinds of files and subdirectories, and you decide to move all the HTML files, that have the word “Linux” somewhere in the middle of their names, from that big directory into another directory. What’s a simple way to do this? If the directory contains a huge amount of differently named HTML files, your task is everything but simple!

In the Linux CLI that task is just as simple to perform as moving only one HTML file, and it’s so easy because of the shell wildcards. These are special characters that allow you to select file names that match certain patterns of characters. This helps you to select even a big group of files with typing just a few characters, and in most cases it’s easier than selecting the files with a mouse.

Here’s a list of the most commonly used wildcards :

Wildcard			Matches
*			zero or more characters
?			exactly one character
[abcde]			exactly one character listed
[a-e]			exactly one character in the given range
[!abcde]		any character that is not listed
[!a-e]			any character that is not in the given range
{debian,linux}		exactly one entire word in the options given
! is called not symbol, and the reverse of string attached with ‘!’ is true.

Read more examples of Linux “cat command” at 13 Cat Command Examples in Linux



21. Command: Find
Search for files in the given directory, hierarchically starting at the parent directory and moving to sub-directories.

root@tecmint:~# find -name *.sh 

Note: The `-name‘ option makes the search case sensitive. You can use the `-iname‘ option to find something regardless of case. (* is a wildcard and searches all the file having extension ‘.sh‘ you can use filename or a part of file name to customise the output).

root@tecmint:~# find -iname *.SH ( find -iname *.Sh /  find -iname *.sH)

root@tecmint:~# find -name *.tar.gz 

Note: The above command searches for all the file having extension ‘tar.gz‘ in root directory and all the sub-directories including mounted devices.

Read more examples of Linux ‘find‘ command at 35 Find Command Examples in Linux

22. Command: grep
The ‘grep‘ command searches the given file for lines containing a match to the given strings or words. Search ‘/etc/passwd‘ for ‘tecmint‘ user.

root@tecmint:~# grep tecmint /etc/passwd 

Ignore word case and all other combination with ‘-i‘ option.
root@tecmint:~# grep -i TECMINT /etc/passwd 

Search recursively (-r) i.e. read all files under each directory for a string “127.0.0.1“.

root@tecmint:~# grep -r "127.0.0.1" /etc/ 

Note: You can use these following options along with grep.

-w for word (egrep -w ‘word1|word2‘ /path/to/file).
-c for count (i.e., total number of times the pattern matched) (grep -c ‘word‘ /path/to/file).
–color for coloured output (grep –color server /etc/passwd).

23. Command: man
The ‘man‘ is the system’s manual pager. Man provides online documentation for all the possible options with a command and its usages. Almost all the command comes with their corresponding manual pages.


24. Command: ps
ps (Process) gives the status of running processes with a unique Id called PID.

root@tecmint:~# ps
PID TTY          TIME CMD
4170 pts/1    00:00:00 bash
9628 pts/1    00:00:00 ps
To list status of all the processes along with process id and PID, use option ‘-A‘.

root@tecmint:~# ps -A

Note: This command is very useful when you want to know which processes are running or may need PID sometimes, for process to be killed. You can use it with ‘grep‘ command to find customised output. For example,

root@tecmint:~# ps -A | grep -i ssh

Here ‘ps‘ is pipelined with ‘grep‘ command to find customised and relevant output of our need.

25. Command: kill
OK, you might have understood what this command is for, from the name of the command. This command is used to kill process which is not relevant now or is not responding. It is very useful command, rather a very very useful command. You might be familiar with frequent windows restarting because of the fact that most of the time a running process can’t be killed, and if killed it needs windows to get restart so that changes could be taken into effect but in the world of Linux, there is no such things. Here you can kill a process and start it without restarting the whole system.

You need a process’s pid (ps) to kill it.

Let suppose you want to kill program ‘apache2‘ that might not be responding. Run ‘ps -A‘ along with grep command.

root@tecmint:~# ps -A | grep -i apache2
1285 ?        00:00:00 apache2
Find process ‘apache2‘, note its pid and kill it. For example, in my case ‘apache2‘ pid is ‘1285‘.

root@tecmint:~# kill 1285 (to kill the process apache2)
Note: Every time you re-run a process or start a system, a new pid is generated for each process and you can know about the current running processes and its pid using command ‘ps‘.

Another way to kill the same process is.

root@tecmint:~# pkill apache2
Note: Kill requires job id / process id for sending signals, where as in pkill, you have an option of using pattern, specifying process owner, etc.

26. Command: whereis
The ‘whereis‘ command is used to locate the Binary, Sources and Manual Pages of the command. For example, to locate the Binary, Sources and Manual Pages of the command ‘ls‘ and ‘kill‘.

root@tecmint:~# whereis ls 
ls: /bin/ls /usr/share/man/man1/ls.1.gz
root@tecmint:~# whereis kill
kill: /bin/kill /usr/share/man/man2/kill.2.gz /usr/share/man/man1/kill.1.gz
Note: This is useful to know where the binaries are installed for manual editing sometimes.

27. Command: service
The ‘service‘ command controls the Starting, Stopping or Restarting of a ‘service‘. This command make it possible to start, restart or stop a service without restarting the system, for the changes to be taken into effect.

Startting an apache2 server on Ubuntu
root@tecmint:~# service apache2 start

Restarting a apache2 server on Ubuntu
root@tecmint:~# service apache2 restart

Stopping a apache2 server on Ubuntu

root@tecmint:~# service apache2 stop                                             
Note: All the process script lies in ‘/etc/init.d‘, and the path might needs to be included on certain system, i.e., in spite of running “service apache2 start” you would be asked to run “/etc/init.d/apache2 start”.

28. Command: alias
alias is a built in shell command that lets you assign name for a long command or frequently used command.

I uses ‘ls -l‘ command frequently, which includes 5 characters including space. Hence I created an alias for this to ‘l‘.

root@tecmint:~# alias l='ls -l'
check if it works or not.

root@tecmint:~# l

To remove alias ‘l‘, use the following ‘unalias‘ command.

root@tecmint:~# unalias l
check, if ‘l‘ still is alias or not.

root@tecmint:~# l
bash: l: command not found


29. Command: df
Report disk usages of file system. Useful for user as well as System Administrator to keep track of their disk usages. ‘df‘ works by examining directory entries, which generally are updated only when a file is closed.

root@tecmint:~# df
Filesystem     1K-blocks    Used Available Use% Mounted on
/dev/sda1       47929224 7811908  37675948  18% /


30. Command: du
Estimate file space usage. Output the summary of disk usages by ever file hierarchically, i.e., in recursive manner.

root@tecmint:~# du
8       ./Daily Pics/wp-polls/images/default_gradient
Note: ‘df‘ only reports usage statistics on file systems, while ‘du‘, on the other hand, measures directory contents.

31. Command: rm
The command ‘rm‘ stands for remove. rm is used to remove files (s) and directories.

Removing a directory

root@tecmint:~# rm PassportApplicationForm_Main_English_V1.0
rm: cannot remove `PassportApplicationForm_Main_English_V1.0': Is a directory
The directory can’t be removed simply by ‘rm‘ command, you have to use ‘-rf‘ switch along with ‘rm‘.

root@tecmint:~# rm -rf PassportApplicationForm_Main_English_V1.0
Warning: “rm -rf” command is a destructive command if accidently you make it to the wrong directory. Once you ‘rm -rf‘ a directory all the files and the directory itself is lost forever, all of a sudden. Use it with caution.

32. Command: echo
echo as the name suggest echoes a text on the standard output. It has nothing to do with shell, nor does shell reads the output of echo command. However in an interactive script, echo passes the message to the user through terminal. It is one of the command that is commonly used in scripting, interactive scripting.

root@tecmint:~# echo "Tecmint.com is a very good website" 
Tecmint.com is a very good website
creating a small interactive script

1. create a file, named ‘interactive_shell.sh‘ on desktop. (Remember ‘.sh‘ extension is must).
2. copy and paste the below script, exactly same, as below.

#!/bin/bash 
echo "Please enter your name:" 
read name 
echo "Welcome to Linux $name"
Next, set execute permission and run the script.

root@tecmint:~# chmod 777 interactive_shell.sh
root@tecmint:~# ./interactive_shell.sh
Please enter your name:
Ravi Saive
Welcome to Linux Ravi Saive
Note: ‘#!/bin/bash‘ tells the shell that it is an script an it is always a good idea to include it at the top of script. ‘read‘ reads the given input.

33. Command: passwd
This is an important command that is useful for changing own password in terminal. Obviously you need to know your current passowrd for Security reason.

root@tecmint:~# passwd 
Changing password for tecmint. 
(current) UNIX password: ******** 
Enter new UNIX password: ********
Retype new UNIX password: ********
Password unchanged   [Here was passowrd remians unchanged, i.e., new password=old password]
Enter new UNIX password: #####
Retype new UNIX password:#####

34. Command: lpr
This command print files named on command line, to named printer.

root@tecmint:~# lpr -P deskjet-4620-series 1-final.pdf
Note: The ‘lpq‘ command lets you view the status of a printer (whether it’s up or not), and the jobs (files) waiting to be printed.

35. Command: cmp
compare two files of any type and writes the results to the standard output. By default, ‘cmp‘ Returns 0 if the files are the same; if they differ, the byte and line number at which the first difference occurred is reported.

To provide examples for this command, lets consider two files:

file1.txt

root@tecmint:~# cat file1.txt
Hi My name is Tecmint
file2.txt

root@tecmint:~# cat file2.txt
Hi My name is tecmint [dot] com
Now, let’s compare two files and see output of the command.

root@tecmint:~# cmp file1.txt file2.txt 
file1.txt file2.txt differ: byte 15, line 1

36. Command: wget
Wget is a free utility for non-interactive (i.e., can work in background) download of files from the Web. It supports HTTP, HTTPS, FTP protocols and HTTP proxies.

Download ffmpeg using wget

root@tecmint:~# wget http://downloads.sourceforge.net/project/ffmpeg-php/ffmpeg-php/0.6.0/ffmpeg-php-0.6.0.tbz2
--2013-05-22 18:54:52--  http://downloads.sourceforge.net/project/ffmpeg-php/ffmpeg-php/0.6.0/ffmpeg-php-0.6.0.tbz2
Resolving downloads.sourceforge.net (downloads.sourceforge.net)... 216.34.181.59
Connecting to downloads.sourceforge.net (downloads.sourceforge.net)|216.34.181.59|:80... connected.
HTTP request sent, awaiting response... 302 Found
Location: http://kaz.dl.sourceforge.net/project/ffmpeg-php/ffmpeg-php/0.6.0/ffmpeg-php-0.6.0.tbz2 [following]
--2013-05-22 18:54:54--  http://kaz.dl.sourceforge.net/project/ffmpeg-php/ffmpeg-php/0.6.0/ffmpeg-php-0.6.0.tbz2
Resolving kaz.dl.sourceforge.net (kaz.dl.sourceforge.net)... 92.46.53.163
Connecting to kaz.dl.sourceforge.net (kaz.dl.sourceforge.net)|92.46.53.163|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 275557 (269K) [application/octet-stream]
Saving to: ‘ffmpeg-php-0.6.0.tbz2’
100%[===========================================================================>] 2,75,557    67.8KB/s   in 4.0s   
2013-05-22 18:55:00 (67.8 KB/s) - ‘ffmpeg-php-0.6.0.tbz2’ saved [275557/275557]

37. Command: mount
Mount is an important command which is used to mount a filesystem that don’t mount itself. You need root permission to mount a device.

First run ‘lsblk‘ after plugging-in your filesystem and identify your device and note down you device assigned name.

root@tecmint:~# lsblk 
NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT 
sda      8:0    0 931.5G  0 disk 
├─sda1   8:1    0 923.6G  0 part / 
├─sda2   8:2    0     1K  0 part 
└─sda5   8:5    0   7.9G  0 part [SWAP] 
sr0     11:0    1  1024M  0 rom  
sdb      8:16   1   3.7G  0 disk 
└─sdb1   8:17   1   3.7G  0 part
From this screen it was clear that I plugged in a 4 GB pendrive thus ‘sdb1‘ is my filesystem to be mounted. Become a root to perform this operation and change to /dev directory where all the file system is mounted.

root@tecmint:~# su
Password:
root@tecmint:~# cd /dev
Create a directory named anything but should be relevent for reference.

root@tecmint:~# mkdir usb
Now mount filesystem ‘sdb1‘ to directory ‘usb‘.

root@tecmint:~# mount /dev/sdb1 /dev/usb
Now you can navigate to /dev/usb from terminal or X-windows system and acess file from the mounted directory.

Time for Code Developer to know how rich Linux environment is

38. Command: gcc
gcc is the in-built compiler for ‘c‘ language in Linux Environment. A simple c program, save it on ur desktop as Hello.c (remember ‘.c‘ extension is must).

#include <stdio.h>
int main()
{
printf("Hello world\n");
return 0;
}
Compile it

root@tecmint:~# gcc Hello.c
Run it

root@tecmint:~# ./a.out 
Hello world
Note: On compiling a c program the output is automatically generated to a new file “a.out” and everytime you compile a c program same file “a.out” gets modified. Hence it is a good advice to define a output file during compile and thus there is no risk of overwrite to output file.

Compile it this way

root@tecmint:~# gcc -o Hello Hello.c
Here ‘-o‘ sends the output to ‘Hello‘ file and not ‘a.out‘. Run it again.

root@tecmint:~# ./Hello 
Hello world

39. Command: g++
g++ is the in-built compiler for ‘C++‘ , the first object oriented programming language. A simple c++ program, save it on ur desktop as Add.cpp (remember ‘.cpp‘ extension is must).

#include <iostream>
using namespace std;
int main() 
{
int a;
int b;
cout<<"Enter first number:\n";
cin >> a;
cout <<"Enter the second number:\n";
cin>> b;
cin.ignore();
int result = a + b;
cout<<"Result is"<<"  "<<result<<endl;
cin.get();
return 0;
}
Compile it

root@tecmint:~# g++ Add.cpp
Run it

root@tecmint:~# ./a.out
Enter first number: 
...
...
Note: On compiling a c++ program the output is automatically generated to a new file “a.out” and everytime you compile a c++ program same file “a.out” gets modified. Hence it is a good advice to define a output file during compile and thus there is no risk of overwrite to output file.

Compile it this way

root@tecmint:~# g++ -o Add Add.cpp
Run it

root@tecmint:~# ./Add 
Enter first number: 
...
...

40. Command: java
Java is one of the world’s highly used programming language and is considered fast, secure, and reliable. Most of the the web based service of today runs on java.

Create a simple java program by pasting the below test to a file, named tecmint.java (remember ‘.java‘ extension is must).

class tecmint {
public static void main(String[] arguments) {
System.out.println("Tecmint ");
}
}
compile it using javac

root@tecmint:~# javac tecmint.java
Run it

root@tecmint:~# java tecmint
Note: Almost every distribution comes packed with gcc compiler, major number of distros have inbuilt g++ and java compiler, while some may not have. You can apt or yum the required package.


41. Command: ifconfig
ifconfig is used to configure the kernel-resident network interfaces. It is used at boot time to set up interfaces as necessary. After that, it is usually only needed when debugging or when system tuning is needed.

Check Active Network Interfaces

[avishek@tecmint ~]$ ifconfig 
eth0      Link encap:Ethernet  HWaddr 40:2C:F4:EA:CF:0E  
inet addr:192.168.1.3  Bcast:192.168.1.255  Mask:255.255.255.0 
inet6 addr: fe80::422c:f4ff:feea:cf0e/64 Scope:Link 
UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1 
RX packets:163843 errors:0 dropped:0 overruns:0 frame:0 
TX packets:124990 errors:0 dropped:0 overruns:0 carrier:0 
collisions:0 txqueuelen:1000 
RX bytes:154389832 (147.2 MiB)  TX bytes:65085817 (62.0 MiB) 
Interrupt:20 Memory:f7100000-f7120000 
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:16436  Metric:1 
RX packets:78 errors:0 dropped:0 overruns:0 frame:0 
TX packets:78 errors:0 dropped:0 overruns:0 carrier:0 
collisions:0 txqueuelen:0 
RX bytes:4186 (4.0 KiB)  TX bytes:4186 (4.0 KiB)

Check All Network Interfaces
Display details of All interfaces including disabled interfaces using “-a” argument.

[avishek@tecmint ~]$ ifconfig -a
eth0      Link encap:Ethernet  HWaddr 40:2C:F4:EA:CF:0E  
inet addr:192.168.1.3  Bcast:192.168.1.255  Mask:255.255.255.0 
inet6 addr: fe80::422c:f4ff:feea:cf0e/64 Scope:Link 
UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1 
RX packets:163843 errors:0 dropped:0 overruns:0 frame:0 
TX packets:124990 errors:0 dropped:0 overruns:0 carrier:0 
collisions:0 txqueuelen:1000 
RX bytes:154389832 (147.2 MiB)  TX bytes:65085817 (62.0 MiB) 
Interrupt:20 Memory:f7100000-f7120000 
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:16436  Metric:1 
RX packets:78 errors:0 dropped:0 overruns:0 frame:0 
TX packets:78 errors:0 dropped:0 overruns:0 carrier:0 
collisions:0 txqueuelen:0 
RX bytes:4186 (4.0 KiB)  TX bytes:4186 (4.0 KiB) 
virbr0    Link encap:Ethernet  HWaddr 0e:30:a3:3a:bf:03  
inet addr:192.168.122.1  Bcast:192.168.122.255  Mask:255.255.255.0
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:0 
RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

Disable an Interface
[avishek@tecmint ~]$ ifconfig eth0 down

Enable an Interface
[avishek@tecmint ~]$ ifconfig eth0 up

Assign IP Address to an Interface
Assign “192.168.1.12” as the IP address for the interface eth0.
[avishek@tecmint ~]$ ifconfig eth0 192.168.1.12

Change Subnet Mask of Interface eth0
[avishek@tecmint ~]$ ifconfig eth0 netmask 255.255.255.

Change Broadcast Address of Interface eth0
[avishek@tecmint ~]$ ifconfig eth0 broadcast 192.168.1.255

Assign IP Address, Netmask and Broadcast to Interface eth0
[avishek@tecmint ~]$ ifconfig eth0 192.168.1.12 netmask 255.255.255.0 broadcast 192.168.1.255
Note: If using a wireless network you need to use command “iwconfig“.

42. Command: netstat
netstat command displays various network related information such as network connections, routing tables, interface statistics, masquerade connections, multicast memberships etc..,

List All Network Ports

[avishek@tecmint ~]$ netstat -a
Active UNIX domain sockets (servers and established)
Proto RefCnt Flags       Type       State         I-Node   Path
unix  2      [ ACC ]     STREAM     LISTENING     741379   /run/user/user1/keyring-I5cn1c/gpg

List All TCP Ports

[avishek@tecmint ~]$ netstat -at
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address           Foreign Address         State      
tcp        0      0 localhost:mysql         *:*                     LISTEN     

Show Statistics for All Ports

[avishek@tecmint ~]$ netstat -s
Ip:
4994239 total packets received
0 forwarded
0 incoming packets discarded
4165741 incoming packets delivered

OK! For some reason if you want not to resolve host, port and user name as a output of netstat.
[avishek@tecmint ~]$ netstat -an

Fine, you may need to get the output of netstat continuously till interrupt instruction is passed (ctrl+c).
[avishek@tecmint ~]$ netstat -c

43. Command: nslookup
A network utility program used to obtain information about Internet servers. As its name suggests, the utility finds name server information for domains by querying DNS.

[avishek@tecmint ~]$ nslookup tecmint.com 
Server:		192.168.1.1 
Address:	192.168.1.1#53 
Non-authoritative answer: 
Name:	tecmint.com 
Address: 50.16.67.239

Query Mail Exchanger Record
[avishek@tecmint ~]$ nslookup -query=mx tecmint.com 
Server:		192.168.1.1 
Address:	192.168.1.1#53 
Non-authoritative answer: 
tecmint.com	mail exchanger = 0 smtp.secureserver.net. 
tecmint.com	mail exchanger = 10 mailstore1.secureserver.net. 

Authoritative answers can be found from:
Query Name Server
[avishek@tecmint ~]$ nslookup -type=ns tecmint.com 
Server:		192.168.1.1 
Address:	192.168.1.1#53 
Non-authoritative answer: 
tecmint.com	nameserver = ns3404.com. 
tecmint.com	nameserver = ns3403.com. 

Authoritative answers can be found from:
Query DNS Record
[avishek@tecmint ~]$ nslookup -type=any tecmint.com 
Server:		192.168.1.1 
Address:	192.168.1.1#53 
Non-authoritative answer: 
tecmint.com	mail exchanger = 10 mailstore1.secureserver

Authoritative answers can be found from:
Query Start of Authority
[avishek@tecmint ~]$ nslookup -type=soa tecmint.com 

Authoritative answers can be found from:
Query Port Number
Change the port number using which you want to connect
[avishek@tecmint ~]$ nslookup -port 56 tecmint.com
Server:		tecmint.com
Address:	50.16.76.239#53
Name:	56
Address: 14.13.253.12

44. Command: dig
dig is a tool for querying DNS nameservers for information about host addresses, mail exchanges, nameservers, and related information. This tool can be used from any Linux (Unix) or Macintosh OS X operating system. The most typical use of dig is to simply query a single host.

[avishek@tecmint ~]$ dig tecmint.com
; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.17.rc1.el6 <<>> tecmint.com 
;; global options: +cmd 
;; Got answer: 
;; ->>HEADER<

Turn Off Comment Lines
[avishek@tecmint ~]$ dig tecmint.com +nocomments 

Turn Off Authority Section
[avishek@tecmint ~]$ dig tecmint.com +noauthority 

Turn Off Additional Section
[avishek@tecmint ~]$ dig  tecmint.com +noadditional 

Turn Off Stats Section
[avishek@tecmint ~]$ dig tecmint.com +nostats 

Turn Off Answer Section
[avishek@tecmint ~]$ dig tecmint.com +noanswer 

Disable All Section at Once
[avishek@tecmint ~]$ dig tecmint.com +noall 

45. Command: uptime
You have just connected to your Linux Server Machine and founds Something unusual or malicious, what you will do? Guessing…. NO, definitely not you could run uptime to verify what happened actually when the server was unattended.

[avishek@tecmint ~]$ uptime
14:37:10 up  4:21,  2 users,  load average: 0.00, 0.00, 0.04

46. Command: wall
one of the most important command for administrator, wall sends a message to everybody logged in with their mesg permission set to “yes“. The message can be given as an argument to wall, or it can be sent to wall’s standard input.

[avishek@tecmint ~]$ wall "we will be going down for maintenance for one hour sharply at 03:30 pm"
Broadcast message from root@localhost.localdomain (pts/0) (Sat Jun 29 14:44:02 2013): 
we will be going down for maintenance for one hour sharply at 03:30 pm

47. command: mesg
Lets you control if people can use the “write” command, to send text to you over the screen.

mesg [n|y]
n - prevents the message from others popping up on the screen.
y – Allows messages to appear on your screen.

48. Command: write
Let you send text directly to the screen of another Linux machine if ‘mesg’ is ‘y’.

[avishek@tecmint ~]$ write ravisaive

49. Command: talk
An enhancement to write command, talk command lets you talk to the logged in users.

[avishek@tecmint ~]$ talk ravisaive
Note: If talk command is not installed, you can always apt or yum the required packages.

[avishek@tecmint ~]$ yum install talk
OR
[avishek@tecmint ~]$ apt-get install talk

50. Command: w
The command “w” is a combination of uptime and who commands given one immediately after the other, in that order.

[avishek@tecmint ~]$ w
15:05:42 up  4:49,  3 users,  load average: 0.02, 0.01, 0.00 
USER     TTY      FROM              LOGIN@   IDLE   JCPU   PCPU WHAT 
server   tty7     :0               14:06    4:43m  1:42   0.08s pam: gdm-passwo 
server   pts/0    :0.0             14:18    0.00s  0.23s  1.65s gnome-terminal 
server   pts/1    :0.0             14:47    4:43   0.01s  0.01s bash

51. Command: rename
As the name suggests, this command rename files. rename will rename the specified files by replacing the first occurrence from the file name.

Give the file names a1, a2, a3, a4.....1213
Just type the command.

rename a1 a0 a?
rename a1 a0 a??

52. Command: top
Displays the processes of CPU. This command refresh automatically, by default and continues to show CPU processes unless interrupt-instruction is given.

[avishek@tecmint ~]$ top
top - 14:06:45 up 10 days, 20:57,  2 users,  load average: 0.10, 0.16, 0.21
Tasks: 240 total,   1 running, 235 sleeping,   0 stopped,   4 zombie
%Cpu(s):  2.0 us,  0.5 sy,  0.0 ni, 97.5 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem:   2028240 total,  1777848 used,   250392 free,    81804 buffers
KiB Swap:  3905532 total,   156748 used,  3748784 free,   381456 cached
PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+ COMMAND                                                                                                            
23768 ravisaiv  20   0 1428m 571m  41m S   2.3 28.9  14:27.52 firefox                                                                        

53. Command: mkfs.ext4
This command create a new ext4 file system on the specified device, if wrong device is followed after this command, the whole block will be wiped and formatted, hence it is suggested not to run this command unless and until you understand what you are doing.

Mkfs.ext4 /dev/sda1 (sda1 block will be formatted)
mkfs.ext4 /dev/sdb1 (sdb1 block will be formatted)

54. Command: vi/emacs/nano
vi (visual), emacs, nano are some of the most commonly used editors in Linux. They are used oftenly to edit text, configuration,… files. A quick guide to work around vi and nano is, emacs is a.

vi-editor

[avishek@tecmint ~]$ touch a.txt (creates a text file a.txt) 
[avishek@tecmint ~]$ vi a.txt (open a.txt with vi editor)
[press ‘i’ to enter insert mode, or you won’t be able to type-in anything]

echo "Hello"  (your text here for the file)
alt+x (exit insert mode, remember to keep some space between the last letter.
ctrl+x command or your last word will be deleted).
:wq! (saves the file, with the current text, remember ‘!’ is to override).

nano editor

[avishek@tecmint ~]$ nano a.txt (open a.txt file to be edited with nano)
edit, with the content, required
ctrl +x (to close the editor). It will show output as:

Save modified buffer (ANSWERING "No" WILL DESTROY CHANGES) ?                    
Y Yes 
N No           ^C Cancel
Click ‘y’ to yes and enter file name, and you are done.

55. Command: rsync
Rsync copies files and has a -P switch for a progress bar. So if you have rsync installed, you could use a simple alias.

alias cp='rsync -aP'
Now try to copy a large file in terminal and see the output with remaining items, similar to a progress bar.

Moreover, Keeping and Maintaining backup is one of the most important and boring work a system administrator, needs to perform. Rsync is a very nice tool (there exists, several other) to create and maintain backup, in terminal.

[avishek@tecmint ~]$ rsync -zvr IMG_5267\ copy\=33\ copy\=ok.jpg ~/Desktop/ 
sending incremental file list 
IMG_5267 copy=33 copy=ok.jpg 
sent 2883830 bytes  received 31 bytes  5767722.00 bytes/sec 
total size is 2882771  speedup is 1.00
Note: -z for compression, -v for verbose and -r for recursive.

56. Command: free
Keeping track of memory and resources is as much important, as any other task performed by an administrator, and ‘free‘ command comes to rescue here.

Current Usage Status of Memory

[avishek@tecmint ~]$ free
total       used       free     shared    buffers     cached
Mem:       2028240    1788272     239968          0      69468     363716
-/+ buffers/cache:    1355088     673152
Swap:      3905532     157076    3748456

Tuned Output in KB, or MB, or GB

[avishek@tecmint ~]$ free -b
total       used       free     shared    buffers     cached
Mem:    2076917760 1838272512  238645248          0   71348224  372670464
-/+ buffers/cache: 1394253824  682663936
Swap:   3999264768  160845824 3838418944

[avishek@tecmint ~]$ free -k
total       used       free     shared    buffers     cached
Mem:       2028240    1801484     226756          0      69948     363704
-/+ buffers/cache:    1367832     660408
Swap:      3905532     157076    3748456

[avishek@tecmint ~]$ free -m
total       used       free     shared    buffers     cached
Mem:          1980       1762        218          0         68        355
-/+ buffers/cache:       1338        641
Swap:         3813        153       3660

[avishek@tecmint ~]$ free -g
total       used       free     shared    buffers     cached
Mem:             1          1          0          0          0          0
-/+ buffers/cache:          1          0
Swap:            3          0          3

Check Current Usage in Human Readable Format
[avishek@tecmint ~]$ free -h
total       used       free     shared    buffers     cached
Mem:          1.9G       1.7G       208M         0B        68M       355M
-/+ buffers/cache:       1.3G       632M
Swap:         3.7G       153M       3.6G

Check Status Contineously After Regular Interval
[avishek@tecmint ~]$ free -s 3
total       used       free     shared    buffers     cached
Mem:       2028240    1824096     204144          0      70708     364180
-/+ buffers/cache:    1389208     639032
Swap:      3905532     157076    3748456
total       used       free     shared    buffers     cached
Mem:       2028240    1824192     204048          0      70716     364212
-/+ buffers/cache:    1389264     638976
Swap:      3905532     157076    3748456

57. Command: mysqldump
Ok till now you would have understood what this command actually stands for, from the name of this command.mysqldump commands dumps (backups) all or a particular database data into a given a file.For example,

[avishek@tecmint ~]$ mysqldump -u root -p --all-databases > /home/server/Desktop/backupfile.sql
Note: mysqldump requires mysql to be running and correct password for authorisation. We have covered some useful “mysqldump” commands at Database Backup with mysqldump Command

58. Command: mkpasswd
Make a hard-to-guess, random password of the length as specified.

[avishek@tecmint ~]$ mkpasswd -l 10
zI4+Ybqfx9
[avishek@tecmint ~]$ mkpasswd -l 20 
w0Pr7aqKk&hmbmqdrlmk
Note: -l 10 generates a random password of 10 characters while -l 20 generates a password of character 20, it could be set to anything to get desired result. This command is very useful and implemented in scripting language oftenly to generate random passwords. You might need to yum or apt the ‘expect’ package to use this command.

[root@tecmint ~]# yum install expect 
OR
[root@tecmint ~]# apt-get install expect

59. Command: paste
Merge two or more text files on lines using. Example. If the content of file1 was:

1 
2 
3 
and file2 was: 
a 
b 
c 
d 
the resulting file3 would be: 
1    a 
2    b 
3    c 
     d

60.Command: lsof
lsof stands for “list open files” and displays all the files that your system has currently opened. It’s very useful to figure out which processes uses a certain file, or to display all the files for a single process. Some useful 10 lsof Command examples, you might be interested in reading.

[avishek@tecmint ~]$ lsof 
COMMAND     PID   TID            USER   FD      TYPE     DEVICE SIZE/OFF       NODE NAME
init          1                  root  cwd       DIR        8,1     4096          2 /
init          1                  root  rtd       DIR        8,1     4096          2 /
init          1                  root  txt       REG        8,1   227432     395571 /sbin/init
init          1                  root  mem       REG        8,1    47080     263023 /lib/i386-linux-gnu/libnss_files-2.17.so